Herve Baudu - Ship Handling Dokmar 2017 STA OCR [PDF]

Zitiervorschau

SHIP HANDLING

1

AUTHOR: Herve Baudu

LAY-OUT: Klaas van Dokkum

PUBLISHED BY: DOKMAR Maritime Publishers BV P.O .Box 360 1600 AJ Enkhu izen, The Netherlands.

1st edition : 2014 .© Copyright 2014 , DOKMAR Enkhuizen, The Netherlands

ISBN 978-90-71500-27-5 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, including electronic, mechanical, by photocopy, through record ing or otherwise, without prior written perm ission of the publisher. Great care has been taken with the investigation of prior copyright . In case of omission the rightful claimant is requested to inform the publishers .

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LIBRARY OF THE FRENCH INSTITUTE FOR PROFESSIONAL MARITIME TRAI NING

SHIP HANDLING

HERVE BAUDU

1sT EDITION

DOKMAR - 2 014

3

TABLE OF CONTENTS

4

Preface Acknowledgements Foreword Introduction to manoeuvring

8 10 11 12

Chapter 1: Ship

14

4.

The propeller

46

Means of propu lsion Propeller pitch Propeller thrust Forces on a blade Operation of the propeller in manoeuvring The turning effect: Prop walk Fixed-pitch propeller (fpp) Turning effect of a fpp in forward motion Turning effect of a fpp in reverse motion Controllable (variable pitch) propellers (vpp) Operation Turning effect of a vpp in forward motion Turning effect of a vpp in reverse motion Benefits of a variable pitch propeller Drawbacks of a variable pitch propeller Ship fitted with two drive shafts Eccentricity effect Twisting effect Assembly counter-rotating outwards or inwards Ship fitted with 2 drive shafts with 2 fpp Ship fitted with 2 drive shafts w ith 2 v pp Vortex effect, cavitation problems Cavitation phenomenon Stall Aeration phenomenon Blade area ratio Skew

47 48 48 48 50 51 52 52 53 54 54 55 56 57 57 58 58 59 59 60 62 64 64 64 64 65 65

1.

Characte ri sti cs a nd defi nition of the ship

16

1 2 3 3.1 3 .2 4 5 5.1

Description of the ship Term s and descriptions of ship movements Shapes of the vessel Ratio of length I beam Draugh t Vessel inertia - power/displacement ratio Propulsion Different types of propulsion

17 17 18 18 19 19 19 20

2.

M a noe uv ring gear

22

1 1.1 1. 2 1.3 1.4 2 2 .1 2.2 2 .3

An chorag e gear - Anchors Forward manoeuvring deck Anchor The chain Equipment number Mooring gear - Mooring lines The different types of moorings Mooring wharfs Mooring equipment

23 23 23 24 24 24 24 26 27

1 1.1 2 2.1 2.2 2.3 3 3.1 3.2 4 4.1 4.2 4.3 4.4 4.5 5 5.1 5.2 5.3 5.4 5 .5 6 6.1 6.2 6 .3 6.4 6.5

3.

The helm

32

5.

Bow and stern thrusters

66

1. 1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 2 2 .1 2.2 3 3.1 3.2 3.3

The passive rudder Characteristics and properti es of the helm Lift and drag Rudder profiles Righting moment Fin rudder Swivelling nozzle Profiled rudder Flow controller rudders Rotor rudder Blade control surfaces Steerable rudder propel ler Steerable prope ller driven rudder Active, hydrojet rudder Cavitation problems Twisted Leading Edge profile rudder Promas profiled rudder Water current deflectors

33 33 35 37 37 38 39 40 41 42 42 43 43 43 44 44 45 45

1.1 1.2 2 3 3.1

Prope ll er thruster Operation Limits of use for tunnel thruster Sha llow draught thrusters Use of thruster Increasing turning moment

67 67 68 70 70 70

6.

Other methods of propulsion

72

1.1 1.2 1.3 2 2.1 2.2 2.3 3 3.1 3 .2 3 .3

Contro llab le thrusters Principle Electrical ly-driven azimuth thrusters in pod Mechanica ll y-driven azim uth thrusters Waterjet propu lsion Operating principle Performance specifications Use in manoeuvMng Epicycloid propulsion Voith-Schneider Operating prin.ciple Performance specification s Use in manoeuvring

73 73 73 77 79 79 80 80 81 81 83 84

TABLE OF CONTENTS Chapter 2: Ship underway 1.

1.1 1.2 1.3 2 2.1 2.2 2 .3 3 3.1 3.2 4 4.1

5 6 6.1 6.2 6.3 6.4 6 .5

7 8 8 .1

84

V essel underway: Basic concepts

86

Kinematics of ship motion Reference system for stud y Kinematics and trajectory Analysis of ship movements Pivot point concept Definition and principle Use of the pivot point concept Trajectory of centre of gravity G Ship dynamics - Effect of a force on a ship Basic principle of dynamics applied to the ship Effect of a force on the ship Concepts of inertia and added mass Example of the different kinds of inertia caused by a turn

88 89 89 90 91 91 92 93 94

Force applied to a solid by a fluid : thin airfoil theory Force applied by wind on the ship Apparent wind Effect of wind on ship Position of instant centre of windage Ca lculation of forces app lied to a sail Wind effect - Displacement of instant centre of windage - Neutral position

94 94 96 96

97 98 98 99 100 100 100

Force app lied by water on the ship Hull resistance Hull resistance to forward motion Hull resistance = wave resistance + viscous resistance

102

Resistance to oblique motion Effects of water on moving hull Action of current in confined water on the hull Combined effects of wind and hull resistance Heeling effects Drag, lift and safe speed Summary

106 106 108

102 102

2.

2 2.1 2 .2 2.3 2.4 3 3.1 3.2 3.3 4 4.1 4 .2 4.3 4.4 5 6 6.1 6.2 6.3 6.4 6 .5 7 8 8 .1 8 .2 8 .3 8.4 8.5 8.6

3.

9 9.1 9 .2 9.3 9.4 9 .5 10

108 112 112 113

1.1 2 2.1 2.2 3 3.1 3 .2

TABLE OF CONTENTS

Complementary concepts of ship hydrodynamics

114

Introduction to complem entary co ncepts of ship hydrodynam ics General rema rks on flo ws Definition of press ure Viscosity Boundary layer Quality of flow s : Re y nolds ex periment Friction resistan ce Shape effect Fouling of hull Ageing of hull Resistance to pressu re Continuity equations Bernoulli la w - Conservati on of energy Euler's theorem Demonstration of pressure resista nce Wa ve resistance Hull resistanc e Detachments Reference surfa ces Resistan ce of appendag es Summary To sum up Ship dynamics Hull test tank Hull resistance Similitude laws Extrapolation of test res ults Other types of tests Calculation of efforts exerted on hull Sample calculation of the limit of feasi bility of a manoeuvre

115 115 116 116 117 117 118 118 120 120 121 121 122 122 123 125 125 126 128 128 128 129 129 129 132 132 134 134 137 138

Wind effect on superstru cture

140

Wind effect on superstru cture Wind Ship Example of digital appl icati on Aerodynamic coefficients curves Influence of superstructures Example of aerodynamic coefficie nt Cy weighting Simplified formula

141 141 142 143 143 145 146 146

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TABLE OF CONTENTS 4.

The influence of waves

148

2 3 3 .1 3 .2 4 4.1 5 5.1 5.2

Formation and development of swell Ship's behaviour in a beam sea - Roll Ship 's behaviour in a heading sea - Pitch, heave /\: wavele ngth of swell, and L: length of ship Slamming effect Ship's behaviour in a following sea Yaw effects in swell Ship 's behaviou r in bad weather Hove to Runn ing before the wind

149 150 151 151 151 152 152 152 152 153

5.

Turnin g

154

Turning: the role of the rudder The rotation moment Tu r ning phases The manoeuvring phase The rotational phase The turning phase The turning curve The_turning point The rate of turn Factors affecting the turning curve Influence of speed Influence of helm angl e Influence of wind and current Influences caused by the ship's characteristics Influence of the ship's trim Roll/yaw coupling Plotting a turning point Stability of heading - Zigzag test Comparison betwee n turning capacity and stability of heading Use of turning in manoeuvring Effect of a force

155 155 156 156 156 157 158 158 159 159 160 160 161 161 161 162 162 162 165

1.1 2 2 .1 2.2 2 .3 3 3 .1 3.2 4 4.1 4.2 4. 3 4.4 4 .5 4 .6 5 6 6.1 7

Chapter 3 : Special manoeuvres

6

165

166

1.

Navigation in shallow water

168

1.1 1.2 2 2 .1 2.2 2.3 3 4

The squat effect The phenomenon of braking The squat phenomenon Squat calculation : Barras formula Barras formula National Physical Laboratory nomogram Digital modelling Influence on turning circle Influence on emergency stop distances

169 170 170 170 170 170 172 172 171

2.

Navigating in rivers and in chan ne ls

176

Particular features of navigating Regulating ships in rivers Particular features of the river Particular features of the dynami c behaviour Passing through a channel with or against the current Dynamic concepts Physical phenomena involved Interactions between ships Example of manoeuvring in the current Effects of current on hull Coming alongside against the current Moving off against the current Coming alongside with the current Moving off with the current Turning in the current on the anchor Turning with one part in slack water

177 177 178 181 181 181 181 184 186 187 187 187 188 188 189 189

2 2.1 2.2 3 3 .1 3.2 3.3 3.4 3.5 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4 .8 4.9 4.10 5

Mooring in open water Areas of use Holding at anchor Anchor holding on the bottom Forces exerted on the vessel Different mooring methods Mooring on one anchor Mooring on two anchors: forked mooring Mooring in bad weather Mooring with head and stern anchors Mooring at anchor in port manoeuvres Anchoring practice Choice of mooring Length of chain to pay out: warp Preparation and presentation Anchoring with no wind or current Anchoring with wind Anchoring head into the current Anchoring Monitoring anchors Specific issue of VLCCs Moving off using one anchor Glossary

190 191 191 191 192 193 193 193 193 193 194 196 196 196 196 196 197 198 199 200 202 202 203

4. 1 2 2.1 2.2 2 .3 3 4 4.1

Man overboard manoeuvres Recovery conditions Man overboard observed at once Planned actions Launch of lifeboat Rescue manoeuvres Recovery manoeuvres without life-raft Man overboard at time unknown Planned actions

204 205 205 205 206 206 208 209 209

1.1 1.2 2 2.1 2.2 2.3 2.4 3 3.1 3 .2 3.3 3.4 3.5 3.6 3 .7 3.

TABLE OF CONTENTS 5.

Ship st opping m anoeuvres

210

1.1 2 2 .1 2.2 2.3 2.4 3 3.1 3.2 3.3 4 4.1 5

Ship's stopping capability Types of propulsion Planned stopping manoeuvres Inertia for stopping Stopping with engine in re verse Zigzag manoeuvre Calibrating stopping distance Emergency procedure Crash stop procedure Stopping by turning alone Emergency stop with anchors Illustrating emergency stop distances Emergency stop distance on container ships Summary

211 212 213 213 213 213 214 214 214 215 215 216 216 217

6.

Towing

218

2 2.1 2.2 2.3 2.4 3 3.1 3.2 3.3 4 4.1 4.2 4.3

Description and general points Towing in port Rules Types of tugboa t Usage Special cases Escort towing Rules Types of tugboat Procedures and precautions Support towing Types of tugboat Equipment: ships and tugboats Procedure

2 19 219 219 222 227 232 234 234 234 234 235 235 235 236

7.

Cooperation with pilots

238

Involvement of the pilot on the bridge Bridge resource management Information to be given to the pilot Master pilot exchange General points Duties of master, bridge officers and pilot Procedures for requesting a pilot Master - pilot information exchange Communications language Reporting of incidents or accidents Embarkation and disembarkation of pilot Transfer of the pilot by helicopter

239

2 2.1 2.2 2.3 2.4 2 .5 2.6 3 3.1

239

8.

Docksid e ma noe uvring practice

244 245 245

2.1 2.2 2.3 2.4 2.5 2.6 2.7 3 3.1 3 .2 3 .3 3.4 3 .5

Preamble Resources available to t he ship - ha ndler on the bridge The engine I t hruster unit The engine speed I ship speed pairing Loss of helm co ntro l situations The rudder The bow thruster The stern thruster Use of mooring lines Ship underway Reminder about the pivot point Direction of the resultan t of the forces Position of pivot point in a man oe uvre Estimate of speed Disruptive effects : w in d

245 245 245 245 246 247 247 248 248 248 248 254 256

9.

Ma noeuvring training resources: simulation

258

2

Training on a manoeu v ri ng simulator Training on sailing mod el

259 261

10.

Regu latio ns

262

Regulations Safety regulations (Solas) IMO Recommendations on ship manoeuvrab ility IMO rules on manoeuvring defined in resolution A.751 (18) Manoeuvring tests Helm order Examples STCW 2010

263 263 263 264

2

1.1 1.2 1.3 1.4 2 2.1 3

Appendices Index Photo credits

265 266 266 267 268 276 279

239 239 240 240 240 240 240 242

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Preface Dear reader, Every single day, French marine pilots based in 32 pilot stations control the manoeuvring operations of ships arriving at, and leaving, ports in France and French overseas territories. Around the whole of France 's coastline, in all the commercial ports, in all weathers and throughout the year, the 350 marine pilots ensure the safe and free flow of maritime traffic by their presence on the bridge of the ships. They are the essential link in the protection of coastal and port zones and more generally of the marine environment of this beautiful country. They guard the living environment of their fellow citizens resident on the French coast. Marin e pilots have always ensured the success of this public service by giving training the ve ry highest priority in their professional practice . Its delivery has changed over time, as technology ha s advanced. As maritime traffic grows, and the size of vessels has increased at a spectacular rate along with ever greater demands in terms of environ mental pressures, pilot stations have developed ever more varied and effective training courses. One of the major compo nents used in delivering training nowadays is the use of simulators for practical work . The International Maritime Organization, in its resolution A960 on the training of marine pi lots, exp li citly recommends this method. One consta nt fa ctor remains, however: the most experienced pilots are still dedicated to passing on their knowledge and experience to their yo unger colleagues. This knowledge-transmission proce ss is the keystone in the marine pilots ' training structure. Continuing this lengthy tradition of transgenerational interchange, Messrs . Eric Baron, Pascal Oilier, Eric Veche, Benolt Sagot and David Toullalan, marine pilots at the ports of MarseilleFos, Le Havre, Dunkirk and Seine respectively, take an active part in writing a Ship Handling book with Herve Baudu , professor of marine education at the Marseille centre of ENSM. This close collaborative effort has led to the work you have before you, which I am certain will become a reference work in the subject. It offers a co mplete explanation of the subtleties of ship behaviour, as well as the difficulty of ship manoeuvres. With generous illustrations, practical and educational examples, this Ship Handling book will be favourably received by all seafarers who wish to improve their knowledge of ship manoeuvring .

8

After reading this book, I am proud to have agreed to write the preface to this edition. Manoeuvring a ship is teamwork. All those involved have to coordinate in order to bring the operation to a successful conclusion. As the "conductor" of this orchestra, the pilot will stimulate the essential synergy among the bridge crew, the captain, the tug boats and with all the links in the port system in general. The work of the tug boat crews and the pilot boats should be highlighted at this point, with their unhesitating bravery, no matter what the weather and the vulnerability of their small craft compared to the size of the ships they serve. Their vital role must be saluted! Let us hope that reading this book on manoeuvring also contributes to the continued improvement in the essential technical skills of all these professionals. All seafarers, from the novice to the most experienced, understand that manoeuvring a ship is not an exact science. Don't they often speak of the "art of manoeuvring" , or the "shiphandler's eye"? There is no one solution to handling a vessel in confined waters. Depending on circumstances, the pilot has to choose from a variety of scenarios for wharfing or casting off. This can be a difficult choice, but it helps to make the profession of harbour pilot particularly fascinating. The manoeuvres described here are theoretica l only, and are not to be treated as sacrosanct examples . In a particular situation , one manoeuvre very different from the recommendations of this work may be THE correct operation. Finally, before I leave you to some enthralling reading, I would like to offer my sincere thanks to the marine pilots involved in writing this work - Eric Baron , Pascal Oilier, Eric Veche, Benolt Sagot and David Toullalan - in close collaboration with Herve Baudu, professor of maritime education . In the current context of the demand for zero risk at sea, this treatise indicates the commitment of so many marine pilots to maritime training bodies, and in particular to the merchant navy officers. Good reading, and good manoeuvring ... F. Moncany de Sa int-Aignan President of the French Federation of Marine Pilots.

Dear reader,

The ' Guide' meets the latest statutory STCW requirements on shiphandling . The topics described are also in lin e w ith IMO reso-

It is my privilege to be invited to preface the first English trans-

lution A960 .

lation of the book 'SHIP HANDLING', by Herve Baudu from the National Maritime College of France (ENSM). This recent publica-

The 'Guide' is informed, up-to- date, precise, instructive and re l-

tion was brought to my attention by my distinguished colleague,

evant - an invaluable source of information for anyone about to

Captain Frederic Moncany de Saint-Aignan, the President of the

undertake shiphandling responsibiliti es .

FFPM (French Maritime Pilots' Association) and Vice President of IMPA (International Maritime Pilots ' Association) .

The ' Guide' strikes a good balance between the theory and the

The IMPA brings together over 8000 of the most experienced maritime shiphandlers in the world and I am proud to represent

a very easy - to- understand manner giving the reader a greater

practice of shiphandling: the theoreti cal aspects are exp lained in

them and to promote their interests worldwide .

insight and a better understanding of forces acting on the sh ip

I have been honored to serve as President of IMPA since first

while maneuvering . The practical 'hands-on ' approach with nu -

being elected at our 2006 Congress in Havana Cuba, and now in

merous illustrated examples is equally valuable and edifying . This

my second term after re-election in 2010 . Today IMPA represents

unique publication is required reading for all mariners , no matter

over 60 nations world-wide and plays an essential role in the

where they work: on rivers , in open seas, in narrow passages or

advancement of pilots' professional qualifications through the or-

in limited draft conditions .

ganization of further education courses and professional training with simulation and practice. That is why as Presi dent of IMPA, I

I am sure you will all join with me in than king Herve Baudu and the members of the FFPM for their dedication and fo r producing

would like to commend all the Pilot Stations of the FFPM and the

this valuable 'G uide' for both novice and experienced mariners .

individual pilots who have worked with Herve BAUDU in develop-

It will be welcomed by everyone in the maritime community who

ing this Guide . 'S HIPHANDLING' is the fruit of their cooperation.

seek to improve their shiphandling abilities . Captain Michael Watson, President Internation al Maritime Pilots Association American Pilots Association

PREFACE

9

Acknowledgements I would first like to express my gratitude to the President of the French Federation of Marine Pilots, Frederic Moncany de SaintAignan, who did me the honour of sponsoring this project, and together with the community of marine pilots, gave his support for the writing of this book. My sincere thanks go also to Yves Richard, Secretary General of the Federation, marine pilot at the port of Marseille-Fos, who gave his time generously throughout the project. The successful completion and the consistency of this book are the result of close collaboration with a team of marine pilots whom I thank most sincerely for their dedication: - Eric Baron, marine pilot at the Port of Marseille-Fos and former professor at the Marseille merchant navy college; his perfectionism, his vast experience and extraordinary ability to explain the theory of ship manoeuvres in great detail, especially in the field of hydrodynamics, have made him a valuable and diligent contributor throughout the writing of this book; - Pascal Oilier, marine pilot at the port of Le Havre, especially for his clear explanations of towing; - Eric Veche, marine pilot at Dunkerque , for his practical approach to wharfing manoeuvres; - Beno1t Sagot and David Toullalan, marine pilots on the Seine, for their insight into navigation in rivers and shallow waters; I want to thank all those who responded to my requests and gave their time generously to share their knowledge, including: -

-

Patrick Payan, president of, and all the marine pilots at, the Marseille-Fos station, for their hospitality at L'Estaque, Fos and Frioul stations; Franck Maleco, ship's husband for the harbour tugs from Boluda to Marseille and his crews, for their enthusiasm in sharing with me their fascinating work .

Container ship, 13, 880 TEU

10

There have been numerous others who helped me in my research, among whom I would like to thank in particular : - Christine de Jouette, hydrodynamics engineer with Principia and her staff, for their well-informed, expert opinions on digitisation tools for hydrodynamic flow; - Erwan Jacquin, chief executive of the HydrOcean laboratory, specialising in the field of computational fluid mechanics, for allowing me to reproduce documents from their work. - Barry Roberts, Liverpool, United Kingdom This book includes numerous illustrations and photos from marine companies Bourbon, CMA-CGM, La Meridionale and SNCM; from manufacturers, especially Transas, Becker Marine Systems, Rolls-Royce, Schottel, Voith Schneider, Wartsila, ABB; from laboratories including HVSA, Onera, Friendship Systems and DGA Towing tanks . My sincere thanks to all of them . The author should not fail to mention Pierre Bertran , president of IFPM and former Inspector General of maritime education and Commander Serge Bethoux , the Secretary-General , who, through their association, have facilitated the publication of this work. I must also thank the management of the Marseille centre of the National Maritime College of France for their support and encouragement, and my colleagues who have given me relevant advice on particular aspects of the work. Finally, I offer sincere thanks to my family and loved ones, for their encouragement and support in this work at the expense of valuable time that I could have spent with them.

Foreword Acting quickly, without looking ahead, often leads to trouble. On the other hand, with careful attention and a willingness to learn, we can gain valuable insights that enhance our level of competence, even if we still might make mistakes . This book aims to share experience with seafarers who wish to begin to learn ship handling, or to improve their skills: students in maritime training institutions, bridge watch officers, commanders - whether just beginning or with long service - as well as those sitting the marine pilot competitive examinations . In other words, anyone wishing to acquire a theoretical approach to the general prin ciples of ship manoeuvring. Each chapter attempts to meet th e various needs of the read ers by starting with a basic section ex plaining the manoeu v ring pri nciples, moving on to a second part that sets out further concepts in more detail. This staged learning process is facilitated by dividing the book into three parts : - Part 1 focuses on describing the elements of the ship that will allow the ship handler to develop skills; a sort of "toolbox". - Part 2 provides an explanation of the mechanisms that put the -

In following this plan , the book com plies with the recommendations of STCW 20 1 0 (Sta nda rds of Tra ining, Ce rtificati o n and Watchkeeping) regardin g t he manoeuvring skills required of deck officers . All of these principles as descri bed in this book Treatise on ma noeuvring are explained on t he associated website - www.traitedemanoeuvre .f r - in on lin e animations reco rded from the manoeuvring simulator at th e Marseil le ENSM centre . The author disclaims all responsibility consequent on the incorrect use of information and data provided, and may not be held responsible for any errors or omissions or for the consequences of the incorrect use of information and diagrams contained in this work, particularly with regard to the simulator examples.

Any comments readers may ha ve can be sent by emai l to the following address: contact@traitedeman oeuvre .fr.

ship in motion. Part 3 describes the specific manoeuvres for a particular en vironment (sailing in confined waters, mooring, etc.) as well as emergency handling . The chapter on "Principles of wharfing manoeuvres" provides an overview of the principles set out in this work with concrete example s of the most common manoeuvres.

Gigan tic ships

11

Introduction to manoeuvring Introduction to manoeuvring Ships have always remained fundamentally unchanged , subject as they are to the same physical laws . However, technological advances allow them to evolve continually. Especially in recent years, the explosive growth of international commerce and the expansio n of the cruising market has led to ships becoming larger, faster and in general, more powerful. Greater awareness of the need for safety at sea and the risks of po llution have also led to international developments in regulations to make ships safer, especially in coastal waters. Ports have also responded to this need for safety, by incorporating VTS into their structures. They are not always suitable, however, for the huge size and bulk of ships . Sailing through narrow straits, passing through locks and channels, then manoeuvring in confined, shallow basins that subject to the wind and current, is a very complex exercise. Port manoeuvres are thus more than ever one of the major challenges for the movement of vessels. They invo!ve fast decision-making, dependent upon rational knowledge of the sea, and intuition . Intuitive understanding and reason, also known as sea sense, complement each other, the one tempering the extremes of the other. Ship manoeuvring may be understood as a physics problem. The process actually consists of a quantitative and conceptual analysis of forces, and the effects that act upon the ship. There is a very large quantity of data to be considered. These data are associated with the vessel and its loading , as well as the port environment, the wind and the sea. The significant number of parameters involved, some unstable, with greater or lesser influence on the situation, make each manoeuvre unique and difficult to reproduce. Thus it is very difficult to develop a precise theory about these forces , with which the shiphandler still has to be entirely familiar. The basic principles of dynamics imply that such forces cause moving solids to accelerate .

Vessel Traffic Service (VTS) is a body linked to the port authorities to manage shipping traffic at port approaches .

12

The resulting speed and acceleration are physically associated with the vessel's mass, calculated as several tens of thousand tonnes . It is therefore vital to take account of the principles of inertia, since they demand precise control of speed and trajectory when manoeuvring . "The art of manoeuvring" thus involves continually adapting the course and speed of the vessel with the help of these forces, whose scale varies and is often impossible to measure. Sailors have lea rned by experience to assess their relative importance according to circumstances. For the most experienced pilots, this assessment is often instant and almost instinctive, since the speed at which the problems arise during a manoeuvre leaves no time for calculation and little time , if any, for reflection. The various forces acting on the vessel as it manoeuvres fall into two categories: - Those which the ship handler applies and which can then be modified as required . These are mainly the thrust of

-

the propellers, the turning moment of the rudder, the tension of hawsers and anchor chains, the traction applied by the tugboats, and so on. The forces the shiphandler encounters but which ca n be used to his advantage: effects of wind, current, hull and swell resistance, turning effects of propellers, etc.

In order to enhance theoretical knowledge, the ship's interaction with the fluids around it may be better understood through experiments carried out using hull models in tanks and in wind-tunnels. Studies of the thrust of the ship's propeller, the efficiency of the rudder blade, the effects of wind or analysis of the turning trajectory provide information helpful in understanding a vessel's reactions. They also help in developing mathematical models for perfecting simulators This work is useful as a basis on which to improve experience. Analysis of the reality of hydrodynamic and aerodynamic phenomena that come into play during port manoeuvres is made difficult by the very complex water and air flows involved. Nor is it realistic to expect to be able to simulate by modelling all possible real world situations that might be enco untered.

There are in fact far too many interactions between the ship and the port environment, in both water and air. It is therefore very difficult to accurately

decode all the forces acting on the vessel's equilibrium and to understand how it behaves. Even the most powerful computers struggle to model all physical phenomena individually, especially those associated with water and air flow around the ship . The latest ships are larger and proportion ately lighter than their predecessors . This means they drift more easily. It is therefore difficult to control their trajectories and their movements . The high risk of accident means that improvisation cannot be tolerated and a professional, rigorous approach is necessary. It is formalised by a close cooperation between the ship's captain and the pilot, whose particular training and ability to anticipate are so va luabl e. The pilot has followed an initiatory course combining several years' sailing experience and theoretical knowledge, leading to national competitive examinations and a lengthy, peer-led period of training. Captains and pilots jointly take responsibility for manoeuvring the vessel, favouring an approach based on observation, practice and experience . "First study the science, then proceed through practice born of the science." This method, firmly anchored in maritime tradition , enables the ship's reactions to be anticipated and "felt" through empiri cal knowledge of the forces applied to the underwater hull or the topsides . It invokes the sailor's "sensitivity " . This intuitive approach is all the more effective when it is based on theoretical knowledge, and uses the ship-handler's varied experience . Nowadays, it is also supported by accurate, high-performance digital positioning tools. It also teaches humility and sea sense in general, leading ultimately, after long years of practical experience, to a thorough understanding of how a ship moves within its environment, with the associated expertise in ship speed, inertia and handling qualities, and with the competence of their crews.

"The art of manoeuvring" improves day by day. It takes years to learn, requiring a significant personal commitment. It is achieved through theoretical training, nautical knowledge and a great deal of practice work on real ships, in order to gain solid experience. Most marine training schools, many ship owners and the majority of pilot stations have therefore supplemented the theoretical training they offer with simulator exercises to encourage the sharing of experience. The ship handler must be certain of his or her decisions, as well as continu ally ques tioning them in order to adjust quickly to fast-changing situations . The human factor in the management of the ship and its crew is therefore also included in teaching, since stress - often born out of ignorance - can have adverse effects on safety when manoeuvring . While technical skills are necessary for proper understand ing of the re lationship between the vessel's movements and their causes, a successful manoeuvre requires many other qualities.

Knowledge of a ship begins with a description of its physical and technical properties . Hull resistance and the various forces applied to the ship are then linked to basic concepts of hydrodynamics, using experimental data gathered from hull tank tests . The ship's behaviour can then be understood in combination with the changing effects of wind, sea, the environment, the tugboats and other forces which act on it during manoeuvring . Navigation along rivers and canals is also described in detail . Some examples from pilots themsel ves are also given to bring theory and practice together, emphasising the involvement needed for success in un favourable sailing conditions. In bad weather, the courage of the sailors and their ability to adapt is expressed by the saying: "Start moving, then observe". In a port environment, which is both protective and hazardous for ships, this watchword could also apply to manoeuvring.

Safety does, of course, impose a limit on this . Setting this limit, and taking responsibility for it, often requires greater courage than facing stormy weather, because of the very demandin g regulatory and financial constraints that app ly today. The zero-risk culture is imposed on everyone, captain and pilot alike. Sometimes, it's important to know when to give up. This understa ndi ng of the process allows for the technical difficulties of manoeuvring, the seriousness of the risks involved, and the scale of the environmental and commercial challenges. We old hands have learned this, and we constantly rem ind ourselves of the saying "The Tarpeian Rock is close to the Capitol" (Arx tarpeia Capitoli proxima); simply put, 'pride comes before a fall' . So day by day, sailors undertaking difficu lt ship manoeu vres prove their skill through their courage and bravery, and through their mastery of th e art of compromise .

First of all, "the right questions" have to be asked, analysing the situation, its likely development and the risks involved . The crew must then be informed clearly of the resources to be imp lemented. The vessel's reactions also need to be anticipated, in order to engage in a coherent scenario which allows for the vessel's particular features and those of the port environment. Finally, rapid adjustments are needed in relation to all the different traffic situations associated with the port's operation. Ship manoeuvring is therefore a discipline that requires the mastery of commands, responsiveness and "sea sense". It is clearly impossible to summarise a vessel's behaviour in a few formulae that are more or less consistent with each oth-

Cap tain and pilot in tandem

er. The "practical" sailor may be tempted to do this, but it is damaging if it leaves no room for professional development. The ship handler must build on experience by making a sensible assessment of each manoeuvre . Hence the purpose of this work is not to provide a few simple recipes for manoeuvring, but rather to give each person the necessary tools to understand how ships respond, in order that they construct their own style of ship manoeuvring.

Sophisticated tools - Bridge on a 360-metre container ship

13

I

•

--.

·'

_ SCHENKER _:·:_:_ -_ __ .J _

---:-:-""' -..,.____~- SCHENKER .\ SCHENKER ....

Chapter 1. The ship

-

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;

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0

Characteristics and definition of the ship

1

Description of the ship

Any assessment of the manoeuvring qualities of a ship begins with an appraisal of its overall appearance. It is often impressive in its size and bulk, but its hull shape and waterline evoke purity and delicacy. Sailors often say that a good ship is also a beautiful ship. Despite seeming subjective, this aesthetic quality actually expresses how well-balanced are the vessel's shapes, and how well suited it is to its marine environment. Greater specialisation in ship design has led to sign ificant differences in their shapes and the power of their engines . Nonetheless, the forces of sea and wind constrain the ship's manoeuvrability, or its safety in general , imposing rules of balance and harmony on naval architects. International rules and regulations on safety and manoeuvrability, governed by good sea sense, reinforce this approach by restricting the freedom of shipbuilders . Above all, a vessel must be entirely suited to its environment. What is most striking as one becomes familiar with ships is their common behaviour. Whatever their type , size or power, their resistan ce to forward movement through the water and air comes from the same causes . The "ship" as an entity is therefore a re ality wait ing to be discovered . It is nonetheless undeniable that they

The following criteria characterise a ship 1. Its hull: length, beam, draft, trim , block coefficient (in ertia) also ca ll ed coefficient of fineness, windward surfaces (longitudinal and transve rse) , 2. Propulsion : type of engine , power, propeller, 3 . Rudder : type, surface area, 4 . Special equipment: Transverse and azimuthal thrusters . 5. Windward surface.

Charact eristics of a ship

3 2

2

A ship's under water body consist of the part of the hull unde rwater. Th e topsides is the part above th e waterlin e. The hull can also be called th e 'quickwork' because it is the major compon ent in th e smooth running of the vessel. The ship is actually surrounded by t wo fluids: water an d air.

each have a set of issues in manoeuv ring specific to their particular dimensions, load ing, engines and appendages. It is therefore vital to gauge the ship's capacity by observation . In this way, its reactions can be anticipated, depending on circumstances, and the manoeuvre undertaken with the minimum of energy outlay (minimum action by engine, propellers or tugs). A "good" manoeuvre therefore is one in which the ship handler is assisted by the forces exerted on th e vessel by wind and sea, while demanding a minimum of external actions. This simple approach helps to improve safety since it always allows for a safety margin .

Terms and descriptions of ship movements

is about eight hunThe water dred and fifty times denser and a hundred tim es more viscous t han air, so it qui ck ly has a much more sig nificant effect on the ship's performance. Th e und erworks allow t he ship to float and displace water, but ex perience sig nificant resi st ance to forward movement. The t opsi des of the ship present a significan t w ind surface area lead ing to th e appearance of aerodynamic forces that interfere w ith the ship's motion, and create drift .

y

X

A ship 's six degr ees of freedom

z

The six degrees of freedom are defined by t he f ollowing t e rms : 1. Heave is a vertical displacement movement of the ship. 2. 3. 4. 5.

6.

Sway is a transverse movement . Surge is a longitudinal movement. Yaw is a rotation movement around the vertical axis. It involves a chang e of headi ng . Pitch is a rotation movement around th e tra nsvers e ax is of t he ship. It is normally a periodic movement around a median position (the trim ) caused by t he sh ip passing through a swell coming from the front or rear. Roll is an alternating rotation movement around th e longitud ina l axis of t he shi p. Generally it is caused by swell or sea taking the ship from th e re ar. I f the ship is angled to one side only, it is said to list. Some ships, such as cont ainer vessel s, can start periodic, large amplitude (parametric) rolling, caused by seas coming from in front. This phenomenon occurs when the rolling period is close t o t hat of t he waves, and the roll and pitch movements begin to resonate.

Under Keel Clearance: an expression that has entered into current speech that signifies taking a marg in of safety du ring a manoeuvre and by

Inertia : Typical finen ess coefficient of a sh ip's hull , whi ch also shows its mass and inertia; concept expla ined in m ore detail in the "Sh ip in motion"

extension in any action undertaken.

chapter.

17

3

Shapes of the vessel

The shapes of a ship, its underside and topsides, are arrived at through many compromises . The ship 's volume is determined by the type , quantity and packaging of the goods transported, while its shape is imposed by its speed and stability. The ship's manoeuvring characteristics, of most interest to us, depend on its shape, displacement (volume of water displaced ), the power of its engine and the efficiency of its manoeuvring equipment (rudder, transverse thrusters, and so on). These qualities are all the more subtly concealed since they often conflict with other seakeeping qualities. For instance , turning capabi lity when ma noeuvring (large rudder blade, flat stern) conflicts with the need for course holding stability and the low resistance to forward movement at sea (low drag rudder, narrow stern shapes). Observing a ship therefore teaches many lessons about the behaviour to expect when manoeuvring (inertia, turning capacity, sensitivity to drift, course holding, etc.). The forces the sea exerts on a turning hull are concentrated mainly on the bows and the stern (slender body theory) . These are the parts of the ship on which we need to concentrate, therefore. Generally speaking, fairly - round and bluff- bowed shapes turn better. Shapes that are tapered in the

The forward res istance of a ship is proportional to its beam . The ratio between its

bows and stern have greater directional stability. These also allow the ship to continue its manoeuvre, responding to the helm , while it retains a little forward way with engines stopped. The fineness of the

length Land its beam B is therefore a characteristic of speed for a given power. Fast ships are generally, long, narrow and light. The length to beam ratio also greatly affects the turning qual ity. Given the limita-

rear, creating few eddies, allows the water flow to continue to act on the rudder. On the other hand, solid shapes create eddies even with very little way on, thus reducing the rudder's action when the

tions, it is easy to see that an extremely narrow ship behaves as a thin, vertical plane, naturally turning less well than a ship with rounded shapes comparable to an ellipsoid. In reality, a narrow ship more easily counters the drift which its rudder

engine is stopped. This makes it difficult to maintain a heading . The ship becomes unstable and tends to sway, based on its shapes. The shapes of the ship also influence its turning capability. During a turn, the stern of the ship drags on the outside of the turning circle, while the bow pushes against the water. Stern shapes that are flat (car ferry, ro-ro, etc.) or round (oil tanker) which limit the vertical plane surface areas encourage the stern to slip through the water. Sim ilarly, deep and straight bow shapes (fast ships, container vessels, etc.) counter drift, and thus assist turning. They often make it difficult to maintain a heading however.

18

The vessel's stern shapes are designed to improve propeller and rudder efficiency, and limit eddies, in other words resistance to forward movement . The shape of the vessel's stern panel is indicative of the hull resistance, especially when it is straight and submerged and creates eddies (oil tankers). Bow shapes thrust the water aside as the ship moves forward , and their fineness reduces resistance to forward movement. Adding a ram bow also reduces resistance caused by the formation of the waves that accompany the ship. Nonetheless, the latter works properly for a trim and draft of a fully-laden ship. The shapes of the ship are thus determined by many scientific and empirical factors which ship builders use in their designs . For instance, hydrodynamics is the study of water flow around the vessel's hull and the hull resistance . These have an essential influence on the ship's speed and manoeuvrability. These resistance forces are studied in the hull basin, and in general vessel behaviour has many lessons to teach on their seagoing qualities. Ship-builders have many possible solutions to choose from, selected according to parti cular data and ratios which highl ight various turning characteristics .

3.1

Ratio of length I beam

tries to create, and turns less well than a broad ship. The currents of water find it harder to flow round most of the surface of the sides of the hull, it is easier to maintain a heading, and the ship turns less easily. A narrow ship, for the same length, therefore has a wider turning diameter than a broad ship.

Example oil tanker: L = 330 meters, B = 58 meters LIB=5.7. This ship has an average turning diameter of 850 meters (2.6 * L).

Example container ship: L= 300 meters, B = 40 meters LIB=7.5 . This ship has an average turning diameter that can reach up to 1,490 meters (S*L) . Example Ferry: L= 145 meters, B = 25 meters LIB=S.S This ship has an average turning diameter that can reach up to 410 meters (2.8*L). Improvements in naval architecture, especially the design of rudders, non etheless tend to overcome these problems and improve ship turning capabilities ( 1 to 3 times length L). Out at sea, or in port transits, turning qualities affect directional stability. A narrow ship easily maintains its heading, while a broad ship can only do so by repeated adjustments of the rudde r. The former follows a straight line, the second yaws as it moves, thus reducing its speed. Finally a narrow ship stops turning once the rudder is realigned while a wide ship takes a fairly long time to stabilise once the helm is shifted ahead. Older ships have a length I beam ratio (L I B) of 8 or more. Increased loading capacity and changes to regulations (dualhull) ensure that this ratio now varies from 5 to 7.

3.2

Draft

Draft is one of the essential factors since given the various depths it determines where the ship can sail. For a given length, draft also affects the area of the hull underwater, and thus the vertical surface area countering drift. At the same time, all parts of the hull no longer underwater become topsides . Depending on the draft, the drift of the ship therefore varies more or less according to the effect of wind or a turn. Operations such as passing through a channel and manoeuvring with a light ship are thus made more difficult by crosswinds . With a shallow draft, a light ship turns quickly, is sensitive to wind and easily loses way. A loaded ship, with its maximum draft, has greater inertia when turning , and less drift. The draft has to be combined with the ship's trim, the difference between its draft fore and aft. If the trim is too great, whether positive or negative 1 , this indicates the ship is no longer positioned within its draft lines. Its hydrodynamic qualities may thus be sharply degraded , reducing the performance of the propulsion unit and the rudder. With a positive or negative trim , the qualities of the control surface are also severely affected. In the former case, normally occurring with a smaller load , the ship turns easily but tends to drift as it turns. In the second case, with a larger load, the ship drifts less, but sways and has difficulty maintaining its heading. If the hull is close to the sea bed, this also affects the ship's behaviour adversely. The flow and pressure of water around the hull are affected by confinement. This changes the balance of the ship. Resistance to forward movement increases, and the control surface becomes sensitive (the squat phenomenon)2.

4

Vessel inertia- power/ displacement ratio

Manoeuvring a ship requires accurate control of its movements, and thus of its inertia. The mass, or displacement, of the ship is thus also an essential feature, since it directly influences its inertia. Strictly speaking the shape of the ship is also im portant, because it determines the moment of inertia 3 . The displacement over drive power (D/P) ratio is an important factor for assessing a ship's manoeuvrability. Clearly, the higher this ratio, the harder it is to manoeuvre the ship . This is a very variable ratio; for instance, it is ten tonnes per horsepower for a large crude carrier; a tonne and a half per horsepower for a large container ship and one tonne per horsepower for a passenger liner. The "block coefficient or coefficient of Fineness" 4 , equal to the ratio of the hull volume over the cuboid containing it

V LX

5

Propul sio n

The manoeuvrability of a ship depend s on the performance of t he propu lsio n unit and the rudder. The latter norma ll y li es behind the propel ler, for increased efficiency, and its surface area is often lim ited by the ship's draft. There are many different propul sion and control systems. Their performances vary tremendo usly, and help to strength en the particu lar na ture of each ship, as we ll as its suita bili ty for its function and use. The most co mm on propulsion dev ice fo r sh ips is the prope ll er. It creates a propulsive force of about one tonne per 100 hp of the engine (0.6 tonne per 100 hp in reve rse). The rudder blade diverts the flo w of water as it leaves t he propeller. This creates a transverse force of about 30 % to 50% of t he propell er's thrust, to turn the ship . Ships with a high bl ock coefficient no rmally have lowe r propulsive perfo rm ance, since the flo w of wa t er thro ugh the prope ller are disrupted by the so lid stern shapes that generate eddi es .

B X Te

allows displacement of a given ship to be calculated . It expresses the fullness of the hull. It varies from 0.5 for a fast ship (frigate-type naval vessels) to 0.85 for a VLCC (very large crude carrier) . The block coefficient for a given ship may vary with its draft. The hull fineness also affects inertia of the

1.

2. 3.

ship (idea of added mass) 5 . The slimmer the hull, the easier it is to accelerate and conversely the harder it is to slow down. Finally, the efficiency of the propulsion unit, especially its reversing power, affects the inertia .

A ship has a posit ive t ri m if its ste rn is sunk down , and negative trim whe n the bows are dow n. This subject is ex plai ned more tho roughly in the chapter on squa t . The moment of inertia represe nts t he resistance of a body subject t o rotation , or to angula r accele ration , a concept explained in more detail in the chapter, " Vessel in motion , com plementary aspects on ship hydrodynamics" .

4.

5.

It is normally shown on th e Wheelhouse Poster displayed on th e bridge for a loaded ship. The principle is expanded in the chap ter "Vessel in motion, compl ementary aspects on ship hydrodynamics" .

B Te

Block coefficient Cb

19

5.1

Different types of propulsion

There are different types of engine, each with its own manoeuvring characteristics. The term used is propulsion assembly since the type of drive used determines the type of propeller and vice versa. There is a difference therefore between driving engines with a fixed pitch propeller and those with gear units or reverse drive.

5.1.1 Slow diesel engine pitch propeller

I fixed

Most cargo vessels over 20,000 deadweight tonnes are equipped with this drive unit combination . It can be started and stopped very quickly. It has the advantage of a powerful starter thrust. This combination gives the best overall propulsion performance . On the other hand, the minimum "very slow" forward movement speed is often high, around 6 to 7 knots . The switch to reverse movement is unpredictable above a certain speed (forward drive of the propeller caused by the wake). The number of consecutive starter operations is often limited to around ten, because of the compressed air capacity reserves needed for starting up. The latest motors have an electronic injector control unit which reduces the minimum rotation speed giving a lower "very slow" forward speed.

5.1.2 Medium-speed diesel engine

I

clutch I gear unit I variable pitch propeller This propulsion assembly is used on most Combustion engine propulsion unit

cargo ships below 20,000 tonnes, such as ferries , ro-ro vessels, small passenger ships, etc. The engine is put in gear at the last moment to manoeuvre the vessel. Reverse motion is provided by inverting the propeller pitch . This arrangement gives the greatest flexibility, thus ensuring reliable reverse motion . Low speed control is possible . Ships with two rudders and two shafts are among the most manoeuvrable . The constant speed of the engine means it is possible to couple an alternator to the engine in order to increase the useful electricity production output. Continuous rotation of the propeller makes control difficult at neutral pitch, and there is a high risk of tangling with mooring ropes while manoeuvring at the dockside. Compared to a slow diesel engine/fixed propeller pitch combination, the starting thrust is less efficient and the propulsion performance often lower, especially in reverse.

Electric propulsion unit

5.1.3 Medium (or high) speed diesel engine I gear unit 1 reversing drive I fixed pitch propeller For vessels of less than 5000 deadweight tonnes, a high-speed or medium-speed diesel engine with a gear unit, fixed pitch propeller and reve rsing drive is a reliable system. The reversing drive makes backward movement quick and unrestricted when manoeuvring . There is a powerful starter thrust and good propulsion performance . However, the minimum "very slow" forward movement speed is often high, around 5 to 6 knots . Reverse motion is unpredictabl e at high speed . 5 . 1.4 High (or medium) speed diesel engine I gear un it ble propeller

I controlla-

A normal ly streamlined propeller, mechanically driven by a diesel engine, can turn through 360° to give equal power no matter what direction it takes. The additional cost of this type of propulsion unit means it is only used for ships that require very good manoeuvrability (tugs, fast ferries , liners, offshore ships, etc .) .

5.1.5 Steam boiler I turbine-gear units 1 fixed pitch propellers This type of propulsion unit is still fitted to some large ships (crude carriers, aircraft carriers), and liquefied natural gas carriers . This system has the advantage of delivering very high power ( 45,000 hp) . Forward "very slow" speed is indeed very slow, ensuring good control when manoeuvring. Power and operating time are however limited in reverse motion. The starter thrust is inefficient, since acceleration time is too slow. Finally, overall propulsion efficiency is moderate in comparison with standard propulsion systems . This type of propulsion unit, on methane carri ers, is gradually disappearing, in favour of a diesel-electric un it fuelled partly by the evaporated gases from the tanks of the vessel.

5. 1 .6 Diesel e ngine I alternator I variable speed controller I synchronous motor I fixed pitch propeller Electric propulsi on is widely used on la rge liners, and is beco m ing more popu lar for som e types of cargo ships in order to comply with carbon-reduction targets . There are many advantages to these. They have very good propulsion performance, especiall y with t he coupling of generators used as required . The installatio ns are compact, with low noise and vi brati on, since t he diesel motors used are lower power. Manoeuvrability is improved , wi t h unlimited reversing speed. "Very slo w" forwa rd speed is low, and acceleration is gra dual to ensure good control at low speed . The pod-mou nt ed electric motor is steerable, so the vesse l's performance is especially good . There are nonet he less construction costs associated wit h this propulsion unit and comparatively high operatin g co sts compared to conventional units, especially for t he pods.

1. 2. 3.

Clu tch transfo rmers synchrono us motors

4.

converters

Conventional electric propulsion un it

Electric prop l!lsion unit

21

I

1

Anchorage gear Anchors

Anchors are desi gned to immobilise ships in an outer an chorage. In particular cases, as described below, they can also be very useful in port manoeuvres. The constantly-increasing size of ships, together with improvements in ship-build ing, especially development of bow thrusters and efficient rudders, reduce the need for these practices which must nonethe less be part of the ship-handler's range of skills . In an emergency, the anchors may be the last resort for losing way or otherwise immobilising ships, in order to escape a dangerous situation. Before entering the port environment, it is therefore essential to prepare and hold the anchors ready for mooring . As an example, there is an account of a 300,000 tonnes crude carrier approaching the port, which lost the use of all its con trols and propulsion, but still managed to Jose way and avoid a serious accident by using its anchors.

1.1

Forward manoeuvring range

Ships have two anchors and cables, one on each side . Each side has an anchor with multiple lengths of cable, usually joined by Kenter shackles. The anchor chain is stored in the chain locker where its end is secured to a fitting in the chain locker bulkhead. This is known as the anchor chain's "bitter end" and must be capable of quick release (or should be of weaker construction than the surrounding materials, which should then fail if the anchor cable runs away, preventing damage to the ship's structure). Anchor chains are connected to the

Forward manoeuvring deck

1.

The average, regulation speed for winding or releasing an anchor line is about 10

2. 3. 4.

metres a minute, or one shackle every 3 minutes . On some windlasses, there may be a warping end with the winch to haul in the ropes. A sprocket/winch coupling and clutch lever also enables transfer between the anchoring and mooring functions. The stopper is the device which takes up the effort of the anchor once it is lowered, thus relieving the force applied to the windlass . A solid hinged part locks into a link . The anchor is secured ready for sea by a cable called a lashing. For medium-tonnage ships, the anchor line tension can also been taken up by a single cable or a quick-release clip attached by a turnbuckle called a Guerigny stopper. Finally, the hawse pipe is a cast metal support piece which links the deck and the plating, as well as the housing for the anchor once it is hoisted into its storage posi-

Hawse pipe Chain stopper Anchoring chain Kente r shackle

1.2

5. 6. 7. 8.

Gypsy w hee l Brake Drum Warping head

Anchor

Hinged stockless anchors are the most common. Hall type anchors are the usua l type found on merchant sh ips, but there are other varian t s, Spek, Baldt, etc. The anchor consists of a shank at the end of which the chain is attached. The flukes hinge at around 45° for better penetration into the seabed . The we ight of the anchor is determined according to the equipment number (see section 1.3) . ULCC anchors may wei ght over 20 tonnes. W eight in kg 3 x equipment number

=

Anchor holding capacities are covered in th e cha pter on "Anchoring". Count 4 to 10 times the weight of the anchor fo r good holding bottoms . Ultra La rge Crude Carrier : crude carrier over 300,000 tonnes

tion . When anchoring, a removable grille, the hawse pipe cover, blocks the opening to prevent personnel from boarding. A safety rail stops people passing across the deck and falling into the hawse pipe .

windlass, or if vertically mounted, to the capstan . The capstan/windlass has a drive wheel called a gypsy, which is notched to suit the forged steel chain links of the cable . The chain comes out of the chain locker through the spurling pipe to the windlass gypsy above. The chain is fed from the capstan/windlass along the fo'c'sle deck through a pawl/chain stopper and down through a hawse pipe in the deck, exiting at the ship's bow. From here, the chain drops downwards and is connected to the anchor using a shackle whose hardened steel pin passes through a hole drilled in the anchor central shank." A band brake controls the travel of the anchor line and blocks it as necessary.

..____ o 1. 1.

Hinged part

2.

To sprocket

3. 4.

Anchoring line Turn buckle

2. 3.

Base Fluke Hinged palm

4. 5.

Shank Tip

23

1.3

The chain

The anchor chain is measured in lengths of about 30 metres , each length consisting of links. All the links have a stud in the middle, so they do not distort under extreme tension, and are perpendicular to each other as they arrive at the notches on the sprocket, and finally do not form kinks. The chain diameter, gauge "d" is also calculated according to the ship's equipment number, and depends on the quality of steel used in its manufacture (mild, high-strength or very high-strength steel). For a high-strength steel: d = 1.60. v'equipment number. The wei ght in kg per linear metre of chain is given by the following formula: p = 0.0218.d 2 All lengths are linked together by a removable link called a Kenter shackle. At the end of the first length, which passes through a ring called a coupling, at the bottom of the chain locker, the last link is attached to a bitter end remote release hook . The position of the remote release control must be marked and known to all the personnel concerned, so that the anchor line can be quickly released in an emergency. At the other end , the final link is attached to the anchor. The anchor clinch consists of removable links, a swivel so the anchor can turn on itself, studless links and a shackle assembly. The removable link which joins the anchor clinch and the chain length must be accessible from the deck so the anchor can be detached safely if necessary.

1.4

Equipment number

-

The equipment number defined by classification companies is used to calculate the weight of the anchors and gauge of the chain. It is often expressed with the formula: NA= 1:!.. 2 13 + 2.0 X h X B + A I 10 - D. = ship's displacement in tonnes at -

-

the summer load waterline. h = effective height above waterline, under summer load, of the highest deckhouse, not including camber and sheer. B = ship's beam in metres. A = lateral surface area of the hull in square metres, superstructures, deckhouses whose width is more than V4 B above load waterline under summer load, and between uprights.

2

2.1.2 Mooring line materials Synthetic cables are most popular. New, high-strength materials are fast replacing steel wire for mooring lines. Synthetic mooring lines: - Polyethylene and propylene ropes are sensitive to abrasion, and have low strain resistance. These ropes are buoyant and do not absorb much water, are light and fairly low-cost, used mainly for tripping lines and tow-lines , though more rarely for hawsers.

Mooring gear and - lines -

2.1

The different types of moorings

2.1.1 The structure of mooring lines Mooring lines are divided into two categories, synthetic mooring lines and steel wire lines. - Synthetic mooring lines: These mooring lines are assembled in

tional to the desired strength. A section of 48mm is standard for mooring most cargo ships. Some mooring cable materials are very sensitive to abrasion and to ultraviolet light. A braided sheath-

a single shaft, having a fixed pitch righthand propeller, the port anchor line is the most used, since the ship's swinging to the right when moving in reverse prevents the anchor line from chafing on the ram bow.

ing is therefore needed to protect the mooring line against such attacks. These sheaths can affect buoyancy.

Polyamide (nylon) towing lines are strong and have a good elongation under stretch loading. They are very absorbent, and do not float. These hawsers are used in mooring configurations under significant strain, on buoys, where there is significant swell , or for ship-to-ship couplings .

-

several braids, themselves then combined into several strands, in order to achiev~ the properties required. The more the line is braided and stranded, the greater the lengthening, for one material. The hawser section is propor-

A ship carries between seven and fifteen chain lengths. The port side anchor line is normally longer than that on the starboard side . Because of the unfavourable prop walk in reverse motion for a ship with

Steel wires: Steel mooring cables are normally composed of twisted steel wires with a steel core . They have higher breaking strain and very low stretch. They are very difficult to handle, because of their composition. They are being generally replaced by high-strength, synthetic fibre cables .

-

Polyester ropes are very strong, durable and resistant to abrasion. Although they do not float, and are relatively expensive, they are used for demanding mooring operations, on crude carriers for instance, and also for towing, or on mooring posts . Aramid fibre ropes, more commonly known as Kevlar, have very high traction resistance, and low elasticity. They do not float, are sensitive to ultraviolet light and low resistance to friction and abrasion. For this reason, these ropes need to be sheathed. Despite being expensive, they are preferable to polyester ropes since they are twice as strong, but weigh the same. These are used on large crude carriers to replace steel cables for mooring posts.

One (shackle) length is equivalent to 15 fathoms or 27.3 metres.

1. 2. 3. 4.

Removable link Chain shackle Swivel Links

Kink : a knot formed by the chain when it twists round on itself. The weight of one shackle is approximately equivalent to around half the weight of the anchor. Sheated cable

Steel cable

Tow-lines: low cross-section rope attached to the mooring line eye, so that the latter can be hauled more easily.

_ HMPE (High Modulus Polyethylene) ropes, are lighter than aramid, with the same strength and elasticity characteris- _ tics. Although they tolerate abrasion better, they also need to be sheathed for protection against ultraviolet light. They are preferred for use in towing because of their buoyancy despite the higher cost . They are easy to recognise as they are usually yellow. Steel cables: - Although only having half the strength of high-performance synthetic ropes, steel Winch on crude carrier

Towing cable

cables are much cheaper. They are very resistant to abrasion and ultraviolet light. Steel cables twist easily and quickly lose their tensile strength when used with very small radii of curvature . Special care must be taken when they are stored on winches or attached to a bollard (figure 9) . They are mainly used for mixed mooring lines on large ships (crude carriers), as mooring lines, for mooring posts and buoys, or for towing. 2.1.3 Risks associated with use of mooring lines Many accidents, some very serious, are caused when handling mooring lines on manoeuvring decks. This is why these tasks must be carried out by trained, qualified personnel. The main hazards when handling mooring lines are associated with their elasticity, flexibility, weight and degree of wear. The elasticity of mooring lines which reach breaking point and snap

Shackle on mixed towline

cause them to whip round . This is an especially serious risk with polyester and polyamide mooring . lines. Synthetic ropes can give way with no prior warning. Mooring lines must not be put under excessive strain, and personnel Shackle on mixed towline

1. 2.

Shackle pin Nylon end

must remain in safety zones while directing operations involving tension on the line. Material

Tensile

Elongation

Resistance

Resistance

(commercial name)

strength

(at SO% of

to abrasion

to UV

(kg/mm 2 )

breaking strain)

Density *

Melting T 0

Loss of strength ( with knots )

Polyethylene

so

0.97

6%

poor

average

150°C

55 to 60%

Polypropylene

60

0.91

7%

poor

poor

165°C

55 to 65 %

Polyamide (Nylon)

90

1.14

12 to 30%

average

average

220°C

60 to 65%

Polyester (Dacron, Terylene)

110

1.38

7 to 11%

good

very good

260°C

50 to 60%

Steel

180

7.8

0 .5%

excellent

excellent

2000°C

I

Aramid (Kevlar)

300

1.44

1.5%

average

average

560°C

60 to 65%

HMPE (Dyneema, Spectra)

300

0.97

1.5%

good

average

1500C

30 t o 50 %

*Shown in bold, buoyant mooring lines

CHAPTER 1 •

PA~T

2.2 •

2.2

Mooring wharfs

Construction of wharfs where ships come alongside varies according to the place or type of cargo to be loaded or unloaded. Most wharfs have a solid, closed construetion, built on foundations. In some environments, rivers for instance, the wharf may consist of a concrete platform resting on pillars or piles, generally built against the river bank to absorb horizontal traction forces . For jetties where ships cannot come directly alongside, such as those for oil or gas loading, mooring points are sometimes provided by firm, isolated structures called dolphins .

In the docking stage, the movement of the water mass displaced by the ship along the wharfside varies depending on whether the wharf is solid or piled (hollow wharf) . The vertical walls of the wharf, known as dock faces, have to have defences to absorb the ship's residual kinetic energy during berthing phases, and prevent wear on hulls caused by friction against the stone surface of the wharf. These fittings are designed to distort and absorb kinetic energy from the largest ships berthing at the terminals . Where more rudimentary defenc-

Cylindrical defences

es are still found in some places, such as tyres, these are generally of two kinds, either floating or solid cylinders in synthetic materials, or neoprene panels . The ship is held at the wharf using mooring lines tied to bollards or samson post, cast iron or steel fittings. Bollards, spaced about twenty metres apart and anchored into the wharf's foundations, can tolerate traction forces of 50 to 300 tonnes. The forces from the moored vessel are distributed among at least four to six bollards. Several ropes are normally atta ched

Solid wharf

to one samson post. If these are lines from different ships, the latest arrival makes its lines fast by passing them through the eye of the ropes already attached . Thus the first ship at the dockside can normally depart without having to cast off the other mooring lines. Bollards are set back slightly from the edge of the dock face, so that the counter of the ship's hull does not strike against them during docking operations .

Bollard set back from dockside . On wharfs for gas, crude oil and chemi cal tankers, traditional mooring posts are replaced by quick release hooks (QRH) . Th ese may include a capstan on the top, to haul in the rope before tying it off. On some new types of equipment, a central control device is used to cast off mooring lines remotely. These hooks may have a strain gauge to control how the ship is held at the dockside. Automatic mooring systems are now appearing in some terminals, in order to remove the need for lengthy and risk han-

Quick release hooks QR H

dling of ropes and minimize the use of coasting pilots. The most popular of these is the vacuum type. A hinged plate which compensates for the ship's vertical movement, is applied to the flat side of the hull. The vacuum created between support plate and wall holds the ship alongside the wharf.

Dolphins

26

Vacuum-based mooring

2.3

Mooring equipment

Mooring lines keep the ship alongside the wharf so that commercial operations can proceed in optimal conditions of safety. A good mooring must resist the various forces to be applied to the ship during its stopover. These forces are caused mainly by the effects of: - wind, - tide and current, - variations in trim, draught and list -

caused by commercial operations, eddies caused by other ships passing nearby, swell and waves, ice.

In some circumstances, mooring lines are also used to assist in manoeuvring when coming alongside or getting under way. This particular operation is covered in the chapter on practical manoeuvring situations .

2.3.1 Manoeuvring decks Most new ships have drums on which ropes are stored . They are driven by electric or hydraulic motor, for winding and unwinding the rope as required. The drum has a jaw-brake to maintain tension on the rope .

1. 2. 3. 4.

Winch motor Clearance cable drum End cable drum Fairlead

5. 6. 7.

Samson post I bollard End cable drum Winch motor

8.

Breast line

Mooring plan

Some drums have a tensioning device to maintain constant traction on the rope no matter how the tide or freeboard changes. Each drum holds one rope . The rope is fed outwards, passing through roller fairleads. Drums are placed on the manoeuvring deck in order to limit the radii of curva-

1. 2.

ture of the ropes, so as not to affect their strength. Samson posts are arranged on the decks for tying off additional mooring

3. 4.

and tow ropes.

2.3.2 Arrangement of mooring lines The mooring plan is designed to hold the ship alongside the dock with ropes arranged forward, across and to the stern of the ship, each with their own particular function. - Bow or stern springs: These keep the ship alongside the dock, reducing longitudinal movements. They are duplicated to distribute forces. In order to reduce tension applied towards the dock, caused by a crosswind for instance, the second line is held away by the fairlead. If there are not enough bollards at the bow end on the wharfside, the waist line is fed by a fairlead to the rear of the manoeuvring deck then carried towards the end of the wharf.

stern spring line stern breast line waist line

5. 6. 7. 8.

stern spring

Head ropes

bow spring waist line forward breast line bow spring line

-

Breast lines:

Arranged perpendicular to the dockside, breast lines prevent the ship moving away · from the wharf, especially when pushed by an offshore wind or strong backwash. If the ship has a high freeboard, breastlines will be pulled downwards and lose efficiency. This is also affected by the short distance between bollard and mooring point which prevents these ropes using their natural elasticity. There is a risk of breaking caused by too much tension. Extra head lines are favoured in such configurations . The breastline is no longer considered effective if the angle to the wharfside exceeds 30°. Breast lines

-

•

Spr ings:

In addition to bow lines, bow and stern springs are fastened longitudinally as much as possible, in order to prevent the ship approaching or retreating from the wharfside.

Typical coefficients of different ships are given in the chapters on "The ship in motion" and "Towing". They are used to assess these constraints.

Springs

Analysing the constraints imposed by the environment, along the ship's vertical, longitudinal and transverse axes allows mooring plans to be drawn up and managed to balance forces on the various mooring lines during the stopover. Mooring lines do not tolerate vertical forces, which are very powerful when the ship rises with the tide or variations in its draught. Their length therefore has to be adjusted regula rly. A mooring line is at its maximum efficiency when it is aligned so that it directly counters the force it has to overcome. Wind and current forces are approximately horizontal. It is therefore important to take account of the angle to the horizontal made by the rope, and the direction of the force applied when assessing its effectiveness. These forces may be very high (several hundred tonnes), especially transverse forces in high winds . The simplest mooring plan consists of cancelling out longitudinal forces with springs and head lines, and transverse forces with breast lines.

This is the preferred configuration at oil, gas and chemical tanker terminals . If possible, the following provisions must be respected when drawing up the mooring plan : - the mooring plan must be as symmetrical as possible with

-

respect to the central perpendicular, breast lines must be as far as possible from the central perpendicular, and be perpendicular to the ship's longitudinal axis . springs must be as parallel as possible to the longitudinal axis

-

of the ship (see next section), the angle of each mooring line with the horizontal must be

-

-

-

small, mooring lines with the same task (springs,'breast lines, etc.) must have similar characteristics (diameter, length, elasticity, etc.), so that stresses are evenly distributed among them, the mooring plan must consider the particular features of the port (wind, current, depth, traffic, etc.) .

28

2.3.3 Vertical distribution of forces The mooring plan must be designed so that the breaking strain limits on the m oori ng lines are not exceeded. For a force F applied to the ship, tensi on on the mooring line is cal culated by dividing this force by the cosine of angle a as follows: F

T=

cos a

This phenomenon becomes more noticeable as th e ship's load is li ghtened. The pressure of an offshore wind on the longitudinal surfa ce increases as t he freeboa rd area increases . The angle a increases at the same time and th e effect iveness of the tension applied to ea ch rope falls . The same reasoning applies for a ship moored to the wharf affected by the rise and fall of the tide. During t hese criti cal pha ses, special vigilance is needed , adjusting ropes and strengthening mooring lines if necessary. The major constraints on the ship can be evalu ated using th e data given in the chapters on the action of wind and current. These constraints are ex pressed by longitudinal and transverse forces, and yaw moments that show how important the breast lines are in comp ensating for th e transve rse forces and th e yaw moments.

The angle a with th e horizontal is th erefore kept as low as possible (below 30 ° for loaded ships) , since the tension T applied to the rope increases rapidly as thi s angl e increases for an unladen ship . For a force of 25 tonnes, th e force actuall y appli ed t o the rope is : - T = 28.9 tonnes for a = 30° , - T = 35 .3 tonnes for a = 45° , - T = 50 tonnes for a = 60° ,

2.3.4 Various mooring configurations The most commonly used mooring configuration is alongsid e t he wharf, since it is t he safest for commercial operations . There are pl enty of othe r ways to moor a ship, depending on th e t y pe of carg o, layout of the port, loadi ng site, river or open sea, su ch as mooring to a bu oy or post. The latter are mainly used for oi l or gas operations . - Mooring alongside a w harf : The ship is moored parallel to the wharf. Th e whole length of the deck is accessible for handling equipment .

25 tons

~ Force considered horizontal

Mooring alongside dock

The principle is identical for the longitudinal plane. The spring has to be as horizontal as possible since the strength of the rope falls with the angle it forms with the

Mooring alongside a wharf: This type of mooring is used especially on a river, or for oil or ga s load ing and di scharge operations . The ship is alongside a fairly short solid or piled dock, on w hich th e loading arms are installed . Only the springs are tied to the bollards on the docks ide, with the other mooring lines taken out to dolphins or tied up on the land .

wharfside.

Force considered horizontal

For large ships, subject to particularl y st rong forces from wind and current (ve ry confined nav igation in rivers, for instance) , "mooring post" atOblique force Mooring to wharf and dolph in

tachments of steel cable and chain are added to conventional moorings .

29

-

Mooring stern to wharf: Ships are moored perpendicular to the wharf, with the bows held by a mooring rope attached to a post or moored with one or two anchors, or even to a dolphin. This type of mooring is kept for ships where there is reduced space available on a wharf, with only the end of the wharf free to attach the mooring rope. Ships with a stern ramp, such as ro-ros, often use this mooring method. Holding on this position is tricky with a strong crosswind .

. :@__

Mooring stern to wharf

-

Sh ip-t o-sh ip mooring: In particular situations, where there is insufficient space in a port, for commercial loading or discharging of bulk liquids, two ships may have to be moored together. This manoeuvre is normally used for crude carrier lightening operations, with both ships hove to or moving at low speed . The mother ship has Yokohama type fenders along the freeboard to protect the hulls of both ships during the operation.

Ship -to-ship mooring

Yokoham a type fenders

2.3.5 Mooring to a buoy This method is mainly used for taking on oil or gas, with the refuelling ship moored to a single- point mooring (SPM) buoy. A floating, flexible collector is then connected on board from the buoy. The SPM buoy is anchored to the bottom, for a CALM (Catenary Anchor Leg Mooring) , or else attached to a rigid structure fixed to the bottom.

CA LM buo y

30

SPM buoy

2.3.6 Electronic mooring aids The ship handler has to be able to quantify movement continually. Heading and speed information normally comes from the gyrocompass and the log . In some cases, the accuracy of this information may be considered inadequate. The pilot is helped in manoeuvring by several effective electronic aids . The most popular and easily available is a portable computer connected to a DGPS (PPU, Portable Pilot Unit). The pilot brings the laptop on board, and connects it to a pair of DGPS antennas fitted on the bridge wing . The position obtained using this method is accurate to a few tens of centimetres, ± 0 .2° for heading and ± 0.5° per minute in turning rate. A specific map is provided to show the ship within the port plan, during the various stages of the manoeuvre . If greater ac-

PPU et unite portable DGPS

curacy is needed, the real-time kinematic system (RTK) combines the DGPS signals with real time corrections made by a reference land station. It is then accurate to a few centimetres. Nonetheless, on an ordinary computer screen, one pi xel is the equivalent of a few tens of centimetres for a scale appropriate for a mooring. The laser sight measurement system is the most accurate at the moment; a few centimetres at 500 metres from the wharf. Sensors at the ends of the quay berth measure the distance from the wharf, as well as the approach speed in centimetres per second . This information is displayed at the dockside. It is possible only to take a perpendicular measurement at the wharf in the line of sight, and the ship handler has to assess longitudinal speed. The sight can be ad justed for poor visibility.

Smart Dock DAS laser sight

31

1

The passive rudder

1.1

Characteristics and properties of the helm

The helm is one of the most important components in the ship's design. It allows it to maintain a heading and to turn. Manoeuvrability depends on the effectiveness of the helm. In general the rudder blade is behind the propeller. On the one hand, this increases the propulsive effect of the propeller, using some of the rotational energy contained in the flow, on the other to generate significant lateral force so the ship can turn even when it is stopped. Directing it one side or the other diverts the water flow behind the ship, to create a turning moment ( MF/G= F x d) whose transverse component (lift P ) allows the ship to turn . The helm's effectiveness increases with the angle of the rudder blade (a) and the speed at which the water currents flow along its profile until it reaches a maximum value . Beyond that, lift force P drops sharply when the rudder blade "stalls" . The force applied to the rudder blade also causes a drag component to appear T . The direction of the rudder blade is therefore described as a lift force which causes the ship to turn, and which also causes it to drift (initiating an oblique motion) together with a drag force which slows the ship .

Semi-suspended rudder

Turning moment of the rudder

The helm system consists of the following components: 1. the rudder blade, which is the profiled, moving part underwater, in contact with the water currents, 2. the stock, which forms the rotation shaft, 3. 4. 5.

the bearings, which support the stock/blade assembly, the steering gear mechanism (normally hydraulic and electrical), placed vertically above the rudder blade in a "steering gear compartment", the command station or information transmission chain between bridge and helm motors.

The criteria for easily defining the rudder blade are: - its surface area (the larger it is, the greater the manoeuvrability and drag), - its profile, especially its capacity for displacing the water flow separation which appears when it is at an angle, reducing its efficiency. -

its aspect ratio. Like a wing, lift is proportional to the aspect ratio of the profile and must comply with proportionality rules for the depth/width ratio . Unfortunately, the aspect ratio is often limited by the ship's draught and mechanical constraints , especially the bending force applied to the shank and bearings.

The first element in the design of the rudder assembly is its plane surface, marked AR. Design rules normally indicate respect for particular values for the ratio AR/AHL linking the plane surface of the rudder AR to the angled lateral surface area of the underworks of the ship AHL. This ratio varies from 1.5% to 5% depending on the different types of ship.

Suspended rudder

Lift coefficient The DNV (Det Norske Veritas) offers a formula for defining the rudder's surface area as a function of the ship's main geometric properties: AR

=~X 100

(1 + 0.25 (

~ )2) L

where: AR = surface area of the rudder blade, L = length of ship, B = beam of ship. T =draught.

Schilling

-0,5 When the rudder is not placed directly behind the propeller, it is recommended that the proposed surface area should be increased by 30% using this formula . Many scientific studies have been carried out of the force and turning moment applied to the rudder. The formula given by

Forward 50°

Joessel (1873) for conventional rudders can be used. The results from this formula, despite its age, are sufficiently accurate when the speed of the ship is less

30°

10° 10° 30° Helm angles

Comparison of the lift coefficient between the conventional rudder blade and the Schilling

than 8 knots : k

F=

X

S

X

V2

X

The forces on the rudder are very large.

sin i

0 .2 + 0.3 x sin i where: S = AR surface area in m2, V = velocity in m/s K = coefficient whose value in seawater is 41.35. The value of the turning moment created by the rudder is therefore:

Mt =

k

X

S

X

V2 X sin i

X COS

0.2 + 0.2 x sin i

i

L

x2

On a conventional rudder the maximum theoretical value of this moment is achieved at an angle of 36° The Solas convention and the classification organisations use this value, rounded to 35°, to determine the minimum value for the helm when hard over. Many manufacturers have also specialised in design of rudder blades, each developing their own technologies and profiles . They can provide graphs quantifying the efficiency of rudders using non-dimension-

The rudder is balanced so that the steering gear does not have to deliver too much force in order to move the helm. This means that the stock is placed close to the vertical from the point where the hydrodynamic forces acting on the rudder blade are applied . In fact, it is slightly forward of this point, so that if the steering gear fails, the rudder blade naturally returns to zero as soon as the ship starts to move forward. It must also be borne in mind that, once the sh ip begins to move astern, the point where hydrodynamic forces are applied moves towards the rear of the rudder (the trailing edge becomes the leading edge), causing an increase in the moment and therefore the mechanical force on the stock. Hence as soon as the ship starts moving astern, the helm must be returned to zero, . or else the speed must be kept low (the force applied to the rudder blade is a func-

There are two groups of rudders, the semisuspended and suspended classifications . The first type are hinged around a rudder skeg, a part attached to the hull which supports most of the bending forces applied to the rudder blade stock. This type is found on large vessels such as crude or bulk carriers, and have greater drag than suspended rudders. The latter have less dynamic drag because they are smaller. They are normally found on faster ships such as ferries and container ships.

Regulations require the rudder should be tested to ensure it can re sist forces applied to the rudder blade and its mechanisms, at a maximum speed in reverse. Nonetheless, these provisions are not validated by mandatory tests.

tion of the velocity squared).

al lift coefficients. These graphs highl ight the angle of inclination giving the greatest turning capacity, clearly showing how the profile stalls. Simple rudder

Suspended rudder

The various types of rudder

34

Semi-suspended rudder

Fin rudder

1.2

Lift and drag

The rudder as such is similar to a wing submerged in a fluid flow. The flow lines \j that flow along the profile are modified when an angle a is given to the rudder blade. The result of this is a force F ( pressure + friction), practically normal in profile, which divides into two forces : drag T, which slows the ship, and P, lift which helps to turn it.

-

fluid V

The lift coefficient is also very important. It depends on the rudd er geo metry. The physicist Prandtl demonstrated that a theoretical ell ipti cal win g, of infinit e span, in a perfect fluid, has a lift coefficient of 2n (Cp = 2n . a) . In practice, ship's rudders have a smaller aspect ratio than t hose of Prandtl's hypothesis. The lift coefficient can be quantified using various formulae de ri ved from model tests. One such is the Fujii formula: Where : br 2 6 .13 A A = aspect ratio of rudder A = - AR x a Cp(a) = (A+ 2 .25) br = rudder height AR = flat su rface area More specifically, with an angle of incidence below about 15° for a slim profil e, and 25° for a thick profile, the water flow observed is laminar. Acceleration of the water currents over the outer surface encourages lift. The pressu re zo ne contributes one t hird of the lift force and the suction zone provides the other two -t hirds. Drag remains limited. If the speed of the water flow is increased as the eng ine power rises, for instance, lift increases, and the ship turns more easily.

Forces applied to the rudder blade

The point at which the resultant of these pressure forces (force F ) is applied , retreats gradually with the angle a. Rudder profiles are constructed especially to ensure this point stays close to the stock, giving optimal lift. The lift force directly influencing the sh ip's turning capacity is ex pressed as: P( a) = 1/2.p. AR. V 2 .Cp(a) AR = surface area of the rudder blade, V = water flow velocity, = density of fluid, Cp (a) = lift coefficient (variable as a function of a). Laminar current flow

This formula highlights the importance of flow velocity to create a lift fo rce . In practice, rudders are placed behind the propeller, at a distance equal to the diameter of the propeller, to benefit from the acceleration of the resulting flow (around three times the ship's velocity). When manoeuvring, with the sh ip often practically stopped, a temporary burst in engine speed allows it to turn ahead , without picking up too much speed (the manoeuvre known as a whiplash). Related to this, ships with variable pitch propellers lose their steering capacity when the propeller is at zero pitch (the propeller acts as a screen placed in front of the rudder). Similarly, when the flow near the rudder is very turbulent (very dirty hull, ship not within its draught lines, sea breaking from astern, etc .), the ship's manoeuvrability is reduced by these wake effects.

CHAPTER 1 • PART 3.1 •

For an angle of incidence greater than 25° (thick profile), the water flow is no longer laminar, becom ing more and more turbulent as the helm angle increases. This facilitates drag .

Turbulent current flow

35

The force applied by the water on the rudder continues to rise, but is directed towards the stern of the ship more and more, while the depression zone on the outer surface gradually falls . Lift continues to rise, nonetheless, and with it the turning efficiency of the rudder, as the angle of incidence increases. It drops abruptly for an angle of about 35°, with a conventional rudder. This is the stall angle. Drag (the longitud ina l compo nent of the force applied to the rudder by the water) then increases quickly, facilitated by the separation of the water currents and the increased angle of incid ence. We should observe that the drag generat-

Rudder profiles are carefully designed for maximum lift and the lowest possible drag. "Thick" profiles encourage laminar flow and generate greater lift. They are therefore more efficient for manoeuvring . However they cause greater drag with the helm at zero, and thus greater resistance to forward movement . The choice of surface area and profile of the rudder blade is thus a compromise between manoeuvrability and resistance to forward movement. Similarly, the rudder blade is placed just behind the "discharge wake" for water flow caused by the vortex from the propeller, in order to increase its efficiency significantly. Systems such as the Kart nozzle or Horn type, profiled , hinged rudder optimise this principle.

ed by a steep helm angle helps to slow the ship, a method sometimes used deliberately to reduce forward movement without having to reverse the engines. This technique, known as zig-zag, consists of alternately swinging the helm from one tack to another; it is described in the chapter, "Emergency manoeuvres" .

36°

a

Stall angle of a profiled rudder

Lift

P

is expressed in the form:

P = 1fz • p . 5 1 . V 2 . Cy Cy is the lift coefficient, Sl is the projected surface area of the profile in a plane perpendicular to the water flow (lateral surface). Drag

E

E

is expressed in th e form:

= Vz • p . 52 • V

2 •

ex

Cx is the drag coefficient, 5 2 is the wetted surface area of the profile (double the lateral surface area) .

Horn Type r udder

36

1.3

Rudder profiles

There are two main famil ies of rudder profile: - convex profiles, which comply with NACA and HSVA standards - concave profiles, known as Schilling, compliant with IFS standards .

We have seen that the maximum permitted value of "i" is 35° . The forces applied to the rudder are very strong. In order to reduces these forces on the rudder spindle, the lever arm "d" is reduced by shifting the leading edge spindle to the rear, and offsetting the rudder blade. This effectively creates a surface ahead of the spindle wh ich generates a force F that facilitates the inclination of the rudder blade.

-

NACA: National Advisory Committee for Aeronautics.

w

HVSA : Hamburgische Schiffbau Versuchsanstalt GmbH . Non -offset rudder

Rudder offset in fo rward motion

----Rudder compliant with NACA standards

Schilling rudder

The value of the righting moment thus becomes : M; = F x (d- d') (see figures below). For conventional rudders, the offset ratio varies between 0.2 and 0.3 ( 20 to 30 % of the surface area is in front of the spindle), so that the resultant of the pressures F is still applied behind the axis of the spindle. Thus, if the steering gear should fail, the rudder will return to its centre line, and is easily immobilized , so it does not remain feathered , thus making it impossible to rig an emergency steering system . In reverse motion, the water flow first strikes the trailing edge of the rudder blade, with the offset part reversed, so becoming the part behind the rudder stock .

These profiles are designed to increase lift and concentrate the resultant from forces applied to the rudder at a point close to the stock. Optimising the profiles also reduces cavitation, vibration and dynamic drag which helps to save energy.

-

w

Shapes of the rudder and its associated fittings are designed to improve rudder efficiency, especially in manoeuvring, by increasing lift and extending the stall angle. The following rudder types generally meet these needs: - the Becker® fin rudder, - the controllable Kortnozzle® - the profiled Schilling® rudder blade, - the Schilling® Vectwin flow controller rudders, - the Jastram® rotor rudder blade .

1.4

I

Value of righting moment

Rudder offset in reverse motion

The value of the righting moment is: M; = F x (L- (d- d')) . The moment applied in reverse movement becomes much greater than in forward movement, whether or not the rudder blade is offset (see figures below) .

Righting moment

The water flow generates a force F which, applied to the rudder blade, tends to draw it back in line. This force, supported by the steering gear, depends on the distance "d" between the leading edge and the point C where the resultant of the pressure forces is applied, for a rudder blade placed at the end of the keep. This is the righting moment : M; = F x d. It represents the value of the moment to overcome so that the rudder blade can maintain the angle required .

'

Rudder not offset in re verse motion

Value of righ ting moment in reverse movement

Regulations do require that the rudder at maximum angle on one tack should tolerate the forces applied when the ship is reversing at its maximum speed , but it is still sensible to limit the angle of incidence to overcome the forces applied to the various components of the steering gear. Finally, in reverse movement, the rudder at maximum angle on one tack masks the propeller disk and interferes with the water flow, thus significantly reducing the propeller's performance .

CHAPTER 1- PART 3.1 •

37

1.5

Fin rudder

For ships that need a very effi cient rud der, high-lift systems have been designed whose purpose is to increase the lateral force applied by t he rudder. The simplest and most commonl y used is the trailingedg e flap rudde r. When the rudder is pointed at an angle a, th e flap is placed at an angle /5, equal to around 2a . The direction of the flap has the effect of adding camber to the angle of incidence . Thi s gives increased lift and drag . Th ese rudders ca n also be used with greater helm angl es (70°) . For instance, the Becker fl ap rudder blade consists of a main, offset bl ade and a fl ap hinged at the end of its tra ili ng edge. The flap represe nts one quarter of the t otal surface area of the rudder blade . The flap has a m echan ical arti culation,

I

I I I Maximum angle of incidence of rudder and flap at over 4 knots.

that ca nnot be di sconnected , although in some syst em s it can be locke d in the ce ntral position . Th is syst em increases th e pressure zo ne, t hough th e depression zo ne is not altered . Th e lift is increased by 60% . In some mod ul es, w hen at full sea speed in open water, only th e "secondary" fla p is used to co ntro l th e ship . When the rudder is at incid ence, t he overpressure zo ne in creases, but w ith out reducing t he stall angle . Th e turning radius for a particular ship is halved w hen it is fitted with a Becker rudder. I n port manoeuvring, or at speeds below 4 knots, t he rudd er angl e on some ships m ay be increased up to 65 ° , and the fin ang le exceed the perpendicular with the traject ory of the water current flow.

Becker flap rudder

I n t hi s co nfigurat io n, efficiency compara bl e to that of a stern thruster is possible. In this case, the ship's turning capacity is act ually the result of the rudder's lift, on the one hand, and its capacity to divert th e flow from the propeller on the other, th us mod ifying the pressure zone at the stern of the ship. It is suitable for use on

..... ..... -

fast sh ips, and those that often have to manoeuvre, su ch as f erries with a separate Becker rudder for each shaft line .

Used at extreme angles, they can generate way forward and in reverse. With the maximum rudder angle, together with the bow thruster, the ship moves out from the wharf sideways. In this configuration, the Becker rudder can be as effective as a stern thruster.

38

Maximum angle of incidence of rudder and flap below 4 knots

1.6

Swivelling nozzle

The Kort nozzle is suitable for a ship whose speed is under 12 knots . Above this speed, the increased drag caused by the nozzle is too great. The nozzle, fi xed or swivelling around the propeller, channels the water flow and acts as a stream tube that improves propeller performance . It is combin ed with a fixed or controllable rudder fin, on the Becker principle . The controllable nozzle is fitted mainly on ships that need good propulsion efficiency, together with good manoeuvrability at low speed , such as sea-going tug s or fi shing boats. For these, th e nozzle also provides protection for the propell er.

Kort Nozzle

Nozzle angles

Fixed nozzle with a rudder and fin Contro llable r udder nozzle with a fixed fin

It takes longer to align the helm at a given angle because of t he considerable mechanism involved. The propel ler has to be turning for control to be effective. Compared to a conventional propulsion system, in reverse, the ship has less power (the stream tube is "inverted", the propeller is less efficient), but better control, since the prop walk effect is less (the propeller is "shel tered" from wake effects) .

Contro llable rudder nozzle with an articulated fin

39

1.7

Profiled rudder

The particular Schilling rudder profile, designed to increase the rudder lift, by extending the stall angle. There is a further advantage over the fin rudder principle, in not having any moving parts, a source of mechanical weakness (striking ice for instance).

The leading edge is very wide and rounded to increase the stall angle, the trailing edge is thin to encourage depression of the water flow currents, and help directional stability. Two plates at the top and bottom channel water flow from the propeller, preventing it passing above and below the rudder.

Distribution of pressure forces on a Schilling profile at 15° of helm

Schilling rudder profile.

This max imum angle of incidence is used to move the ship's stern away from the wharf or swing it on the spot. Wh en making headway, a wide helm angle will lead to a sharp change of heading , causing the ship to lose way very quickly. Combined with a forward wh iplash action, the ship makes practically no headway. For rudder types with an angle of incidence greater t han 35° , the desired helm angle has to be accurately set. The helm hard over (70°) is actually effective only at very low speed.

Suspen ded Schilling r udder

At full speed, the ship's stability is improved, despite greater dynamic drag. The design of this type of rudder means that the pressure forces applied to the rudder blad e at speeds over 7 knots are greater than for a conventional rudder blade profile. The helm angle also needs to be limited above this speed range. If the speed rises too far, a protection system maintains or returns the helm angle to values below 35°. At port speeds, the maximum helm angle may reach 70°, which diverts water flow by 90°, thus making the rudder's action as effective as that of a bow thruster.

-,---

Maximum angle of incidence of the Schilling rudder.

40

DER

1.8

_'~(~

Flow controller rudders

The system consists of two separate, skewed Schilling profile rudders placed right behind the propeller. With a single, variable or fixed pitch propeller which turns continually forward during any manoeuvre, the ship can move in all directions. Each rudder has a range of movement over 145°: 105° inwards and 35° outwards. At running speed, the rudders are operated in parallel with a conventional helm, or from the automatic pilot. At manoeuvring speed, steering of the two rudders is synchronized from a joystick. There is a particular direction for each position. Because of the bulkier appendages, this system has the drawback of causing greater dynamic drag than conventional rudders.

~)

,-L~

=

Ship making headway

Ship turning to port

The joystick is pulled back to stop the ship . The two rudders are then set at 75° outwards. The space between the leading edges of each rud der is fi lled, so the water flow is then diverted outwards to the side. There is normally no resid ual movement either forward or in reverse . If the ship is close to the wharf, th e direction may be altered slightly. During a Crash Stop emergency manoeuvre, th e rudd ers automatical ly go to the same position . The speed at which the rudders feath er redu ces stopping distance by SO % with respect to a conventional, reverse engines stop . It is also possib le to co ntrol its direction during the deceleration phase . Reverse movement in the desired direction is the gre atest advantage of t his syst em, even at low speed, whether the ship is stopped , mov ing in reverse or even forwa rd . Directing the joystick towards the intended bearing adjust s th e ang le of each rudder. The following diagrams show the rudder configurations as a function of the inte nded direction, as well as the position of the joystick .

~~ ---~ ~ ~~D-

~

i

:.~

1

Ship stopped

~

J

~

f

f

I

Ship dropping astern to poft.

:2

The following diagrams sho w the rudder configu rations as a function of the intended direction, as well as the position of the joystick. It has excelle nt turn ing capabiliti es at low speeds. Together with a bow thruster, the ship's movement is very f lex ible and precise, in any direction .

t

~ Suspended Vectwin flow controller rudders

Both rudders are aligned and parallel with each other. The joystick is pushed all the way forward. The helm or joystick is turned in the required direction, to make the ship turn one way or the other. The inner rudder is at an angle of 70° in the turn, and the outer rudder at 35°. The engine speed falls, to reduce water current flow which rebounds off the walls of the wharf or the bottom, and could interfere with manoeuvring. This system overcomes the prop walk caused by a reversing propeller. Although it is unusual to keep the engine moving forward throughout the whole manoeuvre, it is easy to learn how to operate the single joystick to turn the ship, and it is flexible and simple to use.

CHAPTER 1 • PART 3.1 ·THE

(

T::s:rt

:~:

To::.@::.b'"'

ID

~~

\\

~~:~ t ,:~:: ~~~~

J75. J~ ~

Revers; to port

Revr rse

T

Combinations of Schilling Vectwin r udders

41

1.9

1.10

The rotor rudder

In order to maintain the low pressure zone providing two-thirds of the lift for as long as possible, a rotor is fitted on the rudder's leading edge. Th is rotor turns in the direction of flow of the water, to create an imbalance in pressure. When the angle of the helm is greater than 15°, the rotor begins turning in order to accelerate the water passing along the outer surface. The rotor turns at its maximum speed from an angle of 25° . At very low speed, when the rudder is angled at 65°, it practically never stalls, giving a high degree of lift. A Seeker type fin can also be added to the trailing edge to increase its capacity. 1

Blade control surfaces

For fast ships such as megayachts or speed boats, conventional rudder blades cause very significant drag. It causes vibration and greater directional instability. Humphree type trim tabs, operating together with the platform movement stabilisation system, reduce these constraints at sea . Port manoeuvres use differentiation of the conventional eng ines or hydrojet control to compensate for th eir lower efficiency at slow speed. Moving the joystick to the right controls the lowering of the starboard tab, whi ch extends beyond the hull and slo ws the flow of water, creating a force on this side of the hull. A turning moment is form ed, causing the ship to turn to starboard. The forces created by the tab control surfaces, the flaps and the lateral fins are integrated into the open sea roll and pitch movement compensation system by its computer.

Jastram rotor rudder blade

Trim tab on a h ydrojet

Trim tab on a hydrojet yacht

42

2

Steerable rudder propeller

:·

2 .2

St eera ble waterjet rudde r

Th e waterj et prop ulsion prin ciple is described in th e chapter on "Other prop ulsion method s". Ships normally have two lateral hydrojets used both for propulsion and also f or control. There ma y be a third , booster hyd roj et which simply provides additional power .

The steerable rudder propeller is used mainly for guidance and for au x iliary or main propulsion . Unlike the passive control device, its efficiency decreases w ith speed . Steerable rud der propellers are divided into three families : propulsion by propeller, cycloid drive and hydrojet.

2. 1

Steerable propeller driven rud der

A streamlined propell er which can turn 360° replaces the conventional rudder. This fixed - blade propeller is driven by a diesel or electric motor inside the ship via a mechanical Z-transmission. Often installed in pairs, these active steering devices provide main propulsion and steering control.

Turbine hydrojet Steerable r udder propeller (SRP) Combined only with a conventional main propul sion unit, th e st eerabl e rudd er propeller acts as a stern thruster and an auxiliary propulsion unit at low speeds . I t s 360 ° rotation means it provides the same power whatever its direction . Its flex ibility means thi s t y pe of configuration can be used on suppl y ships, dy nami c position ing ships and tugboats.

Th e chan ge of directio n is achie ved by deflecting t he water jet as it leaves t he nozzle by up to 30° either side of the exit t raj ect ory. The ship goes into reverse by loweri ng t he blade in front of the wate r jet. The water jets are t hus dive rted forward. These characteristics give ships fitt ed wi t h the hydrojet exr.e ll ent manoeu vrability, eve n when sto pped or at low speed . Dy nami c drag is re du ced by el iminating th e append ages below the hull . Nonetheless fins/co ntrol surfaces may be added on som e vessels such as fast catamarans, to gain directional stabil it y and to ensure better heading maintenance .

43

3

Cavitation problems

The rudder is placed just behind and in line with the propeller to gain the best possible lift by picking up the water flow discharge currents. The water flow from the propeller rotation, the vortex, is carried directly over the leading edge of the rudder.

All these random phenomena and their effects are associated with the friction applied by the water, generating surface erosion as well as vibration. All the discontinuities in shape, such the parts of the rudder skeg supporting the rudder blade are especially vulnerable .

Vortex of a right-handed propeller

Because of this uneven flow of discharge currents across the rudder, the pressure the water applies over the lower part is greater than over the upper part.

Erosion on a rudder blade at the bearing and the skeg

On some high speed ships, horizontal fences are fixed to the rudder blade's leading edge, in the direction of flow, to reduce air suction (aeration) caused by low pressure on the outer surface. This also channels the water flow more effectively over the rudder's surface, improving its efficiency. In the long-term , erosion caused by cavitation can damage the rudder blade. Dynamic drag linked to the thickness of the boundary layer which surrounds it, and its turbulent wake can also cause further waste of energy for driving the ship. The design of the rudder system is therefore a very important stage in its production.

3.1

Twisted Leading Edge profile rudder

These harmful effects from cavitation can be reduced by use of specially-adapted rudder profiles, such as the Twisted Leading Edge profile, from Becker.

Pressure distribution over the rudder blade

The aeration phenomenon, air mix and effect of water molecules associated with the rotation of the propeller all help to main tain an area of uneven density around it. As the ship's speed increases, the water speed also increases over the rudder sides. The flow is increasingly turbulent (Reynolds) and on the outer surfaces es-

The water flow vortex caused by the propeller's rotation creates a pressure imbalance on the rudder blade. This imbalance, which interferes with laminar flow, is overcome by using an asymmetrical profile. For a right-handed propeller, the upper part of the rudder is more rounded on the port side, and slightly off-centre, and conversely on the lower part of the rudder.

Twisted Leading Edge profile

pecially it encourages the appearance of micro-depressions which can cause unstable bubbles of water vapour to appear locally (cavitation). When these bubbles burst in contact with areas of higher pressure, they cause a physical shock able to erode the rudder's steel. These cavitation phenomena are facilitated by the continuous, variable angle of the rudder, which maintains the ship's heading.

44

Section of a Twisted Leading Edge profile for a right-handed propeller.

3.2

Promas profiled rudder

The Rolls-Royce Promas rudder has a profiled bulb in the middle of the leading edge . It is placed in line with the propeller, itself streamlined in its setting. This consistent assembly facilitates water flow over the rudder, even with sharp helm angles. Lift is improved and vibrations reduced .

3.3

Water current deflectors

Although placed upstream of the propeller, and not an integral part of the rudder blade, the Becker Mewis Duct nozzle, still ca ll ed a propell er with distributor upstream, helps to direct the water currents to wards the propeller, then to the rudder. Fi xed, pierced fins are arranged and directed in the nozzle so that the water currents passing over the propeller are rotated counter to the direction of the propeller. The departing water current flow therefore have a less pronounced vortex created a better flo w over the profile and thus increase lift. These devices fitted on some ships have provided improve ments in yield of 4 to 8% . This principle can be applied by placing fixed fin s onto the hub of the shaft line behind the propeller.

Mewis Duct nozzle

Fins on propeller hub

45

1

Means of propulsion

propellers are divided into three famili es : - propellers, - hydropropellers - Voith Schneider epicycloid-geared propellers. propellers remain the most commonlyused means of propulsion . Contrary to what one might think, a propeller does not act as a screw . It accelerates the water particles as they pass through it, and gives them a rotary motion . The thrust whi ch drives the ship co mes from thi s acceleration . It is produced by the blades, whos e extremely complex operation can be compared to that of rotary wings . A propeller's main characteristics are its diameter, number and type of blades (shape, surface area) and its geometric pitch (the angle of its blades) . The number of blades normally varies from two to five. Some propellers, such as those on naval ships, can have up to eight blades in order to minimise the noise gen -

Depending on the technique used , blades may be fixed on the hub (FP, fixed propeller) or controllable (CPP, Controllable pitch propeller). Blades thus turn at the same time around a shaft perpendicular to the hub, at a specific angle . The thrust (T) of the propeller is created by accelerating a mass of water from a velocity V 1 to a velocity V2 . It is the result of suction in front of the propeller and overpressure behind it. It

Hence, each ship constitutes a particula r case as determined by its en gin e and its specific operating cond itio ns, and therefore requires a su ita ble propeller. Port manoeuvres, especially t he efficiency of reverse motion , are included in the operating conditions t o be conside red when choosing a prope ller. It can be sho w n t hat :

v,

may be quantified by:

T This equation shows that to generate a particular thrust, a large quantity of wa ter may be slightly accelerated - a large propeller turning slowly - or conversely, strongly accelerating a small quantity of water - a small propeller turning qui ckly. The difference between these two solutions leads intuitively to the idea of propulsion performance and adaptation to a particular programme .

=

V2 - 3-

vp = - 2-

V 1 = V213 = Vpl2 defines the ship's optimal velocity for optimal motor power (where vp is th rust vel ocity) . The speed of the flow leaving the propeller and arri vi ng at the rudder is therefore three times the speed of the ship. As th e figu re sh ows, each specific propulsion requiremen t has its own particular t y pe of propelle r.

erated by ca vi tati on.

v1 Incoming flow Sl Vl

Outgoing flow 52 V2 Thrust force

Differ ent types of propeller

47

1.1

Propeller pitch

The geometric pitch of a propeller is the distance H travelled by a theoretical profile located at a distance r from the hub after one full rotation. _ _....,..

2 2.1

Propeller thrust Forces on a blade

The propeller blades use the rotation and torque from the engine to accelerate the water and drive the ship. The section of the propeller blade profile and its hydrodynamic operation are similar to the aerodynamic operation of a wing. The moving blade can enter on the water approaching its leading edge with an angle of incidence a. This starts the water moving and creates thrust (action = reaction; Newton's third law). A large mass of water is thus diverted by the propeller's operation . It extends a long way around and forms flow lines which divide in two to pass over the blades. The water currents flowing over the outer edge of each blade, the exterior surface, are accelerated in comparison with those that flow over the inner surface. The shape of the blade profile encourages laminar flow, thus contributing to maintaining this difference in velocity.

Propeller pitch This is the distance travelled by the propeller if it operated as a screw. In a fluid, the

0

1. 2. 3. 4. 5.

Inner surface Outer surface Leading edge Trailing edge Flow lines

propeller does not turn like a screw into a nut, but it rests on the water. The effective pitch is the distance actually travelled by the propeller after one full turn. The difference between these two concepts is called slippage . A propeller is said to be right-handed when an observer from the stern of the ship sees the propeller turning to the right (right-turning or clockwise turning) . . The ship is then moving forward . Each blade has the same angle a with respect to the axis of the hub of the shaft line.

Profile of a blade section The difference in the speed of flow of the water currents either side of the blade creates a pressure differential between its inner and outer surface, which supports

a

Right-handed propeller 1. 2.

Blade Propeller shaft

Bernoulli principles, concepts developed in the chapter "Vessel in motion, complementary aspects on ship hydrodynamics".

The operation of a propeller is very complex. Nonetheless, referring to simplifying theories, such as the disk actuator theory, half of the thrust force comes

the propulsive force. The thrust therefore varies with the angle of the blades, the viscosity of the water, the propeller's rotation speed and the quality of the profile. Rotation of blades leads to increased linear speed of the various blade sections proportional to the distance from the cen tre of the hub. The blade's angle of incidence is therefore reduced moving away from the axis of the propellers, and as the linear speed increases. The pressure differential is therefore similar in each blade section. This is what gives the propeller this particular profile: the twist. from downstream overpressure, and half from upstream low pressure. These are therefore the same in theory.

Pressure

I I I I I

I Zones of pressure applied to a blade

- PROPELLER THRUST

The propeller transforms the torque produced by the engine into propulsive force :

thrust. In order to study the forces operating on the propeller in more detail, we assume the ship and its propeller are fixed, and it is the water flow that circulates around the propeller at velocity V (V is composed of the rotation speed of the propeller, the speed from the ship's movement and the speed induced by the propeller itself).

The sum of all thes e forces RL between the hub and th e end of the bl ade, exerted on the various secti ons, quantifies t he propulsive force t hat moves t he ship forward; the sum of th e forces R, gives t he resisting torque, whi ch co unters t he rotation of the propell er.

Let us look at the action of the water on one cross-section of the blade . The water flow around the profile at a speed V creates a force R - mainly a pressure force perpendicular to the profile, but also a viscous friction force tangent to the profile. Th is force R can be divided into the lift force RL similar to th rust along Ox - longitudinal axis of the ship - and a drag force RN - perpendicular to Ox - which counters the rotation of the propeller shaft.

A

A

blade A-A

a

Section of a blade

Forces exerted on a blade

The cross-section of the profile of the blade is thicker near the hub in order to resist the mechanical constraints generated by the pressure forces . At the end, the blades are thinner to minimise drag . Thrust and torque ( RL and RN) vary as a function of the propeller rotation speed and the speed of the water flow through it (V), as well as the shape, the surface area and the angle - pitch - of all the blades .

Propeller disk

particles occurs upstream, and half down stream of the disk. The shape of the hull at the stern of the ship is optimised to direct the water flow towards the propeller or propellers . The turbulence -free quality of this flow partly depends on the performance of the propellers. The speed of the water flow drawn in by the propeller is actually affected by the ship 's wake (pressure variation caused by velocity and the boundary layer phenomenon).

Disk showing distribution of water flow velocities over the propeller

The propeller's operating principle may be simplified by replacing the propeller with a disk which transmits a pressure variation L'.P to the water that passes through it. Half of the acceleration of the water

Wate r flow currents con verging on propellers

Propellers undergo testing in a hydrodynamic tunnel so that they can be studied and characterised with dimensionless coefficients. There are many of these coefficients, indicative of the complex nature of propeller design, which is always a matter of compromise and therefore one for the experts. Some simple formulae using these numbers provide a better assessment of th eir function, highlighting especially the effect of rotation speed (n) and diameter (D) on thrust (T), resisting torque (Q) and efficiency (rJ). (1)

Jo

=

nD

(2) (3)

Kr (p .n2. Q4)

Kr : thrust coefficient

Q

KQ (p.n 2. Q S)

p : density of the water Kq : torque coefficient

the

propeller's

efficiency

in

manoeuvring is very difficult. Propeller performances are significantly affected by the speed of the engine (n) and the limited depth in ports and the structure of the flow around the hull when it is turbulent. The separation of boundary layers propeller's performance and affect thrust. The presence of unstable turbulent flows is often also revealed by serious mechanical vibration . The diagram (left) shows fo r instance that the propeller does not per-

fl o : effici ency

fl o

Assessing

and formation of vortices interact with the

T

(4)

Operation of the propeller in manoeuvring

velocity of the ship (V 0 ), as well as the

J 0 : advance coefficient of the propeller V0 : speed at which the water arrives to the propeller (does not include effects of the wake)

Y..2_

2.2

form at its most efficient during acceleration phases.

For instance, it seems that with constant coefficients, doubling the number of propeller rotations leads to the thrust (T) and torque (Q) being multiplied by four (equations 2 and 3) .

~\

Propellers are classified in families of identical profiles . Within each family, they are distinguished by size (p itch/diameter ratio) . Coefficients Kr and Kq are associated with the typical pitch

I diameter ratio of each propeller, allowing a compari son of their opera -

tion in the open sea. They are presented as curves (as a function of J 0 ) highlighting the

10 Ka Kret ljo

Fixed blade propeller in forward motion

~ V Fixed blade propeller in reverse motion

0,2

0,4

- - - Ka

0,6

0,8

- - - KT

1,2 -

-

-

llo

1. 2. 3. 4.

Outer surface Inner surface Trailing edge Leading edge

Typical curves of a family of propellers {3] importance of the P I D ratio and of J 0 on

In open sea, overall propulsion efficiency 3 in forward motion is around 0. 5 to 0.6.

Rotation speed and pitch of a fixed-blade

In other words, the propulsion power is

tangential

propeller are optimised for optimal veloc-

around 0.5 to 0 .6 times the power sup-

develop on the surface of a solid

shear

stresses

that

ity and movement of the ship, which nor-

plied by the engine - losses due to the pro-

in contact with a viscous fluid in

mally means a transit speed using the full

peller drive, rotation of the water, viscous

movement.

power of the engine. The fixed-pitch pro-

friction 4

peller's efficiency thus falls in line with the

fall to 0.25 and rarely exceeds 0.7.

,

etc . In difficult conditions, it may

fall in speed of the engine . 3) The perfect performance of the propulsion chain assembly would have a coefficient of 1.

50

4) The viscous friction is caused by

efficiency r]O.

5) Physical properties (speed, pressure, etc .) of unstable flows vary over time; see chapter "Complementary aspects on ship hydrodynamics".

2.3

The turning effect: Thrust effect

Efficiency is much reduced in reverse movement. Since the propeller blade profile is no longer optimised for forward movement, flow quality is degraded . For a fixed - pitch propeller, the trailing edge becomes leading edge and the outer surface becomes the inner surface .

When the ship is underway, the water does not flow evenly over its stern where the propeller sits. The closer it comes to the hull and the surface 6 , the more turbulent it is - turbulent flow and boundary layer to the rear of the ship. In rotation, the operation of the blades - their ability to accelerate the surrounding water particles - is also affected by the various flow

For a variable - pitch propeller, the outer

pressures and speeds. Thrust RL- lift - and resisting torque

surface becomes the inner surface (inverted camber). Since the profile shape is no longer optimised, the thrust - lift - is reduced, while th e resisting torque - drag

RN

- drag - forces therefore increase in proportion to squat.

The sum of th e trans verse com ponen t s R';; leads in particular to a res ultant whose di rection depends on the direction of rota tion of the prope lle r and which ha s a turning effect. For instance , for a propeller with 4 fixed blades, right- hand ed, the resultant of the resisting compon ents of each blade R;l + R;2+ R;3+ R;4 is applied at 0, the centre of th e hub . Their sum is RN. It is directed to t he right for a right- hand ed propeller movin g forwa rd, and to the left for the same pro peller in re ve rse .

- is increased. The assessment of reverse motion effi ciency, although vital in terms of manoeuvrability, is consequently difficult to assess since it has to take account of the type of propeller and its profile, extension, shape of the blades and the nature of the flow around the hull. For instance, a Ropax type ship (roll-on/ roll-off passenger ship), with two diesel engines (2x16,700 kW) and two vari -

80% 90%

Pressure field lin es on the stern of the vessel upstream of the propeller Prop walk of a right-handed propeller

able pitch propellers, manoeuvring at zero speed, develops maximum thrust of 1780 kN (10.7 kN per100 kW) on the propeller in forward motion and 1125 kN on the propeller in reverse (6.7 kN per 100 kW) [4] . A number of methods have been developed to improve propeller propulsion efficiency. They are placed upstream to divert the flow through the propeller or around the blades (nozzle) to create a stream tube, thus increasing the speed differential (V 2 - V 1 ), as well as downstream to recover energy by righting the rotary flow exiting the propeller. These solutions all generally save energy, but do not necessarily improve the propeller's efficiency when manoeuvring . In some circumstances they may even reduce it (for instance, the nozzle in reverse motion, see section "The helm").

When the machine is used in reverse to cause the ship to lose way, which

Velocities of water flow on the rotating p ropeller

This lateral force is called transverse thrust (prop walk) . This effect is increased the larger the diameter of the propeller and the faster it turns .

is common when manoeuvring, the ship's speed and the effects of wake must be taken into account, which re-

6. At the end of this chapter we will show that the phenomenon of aeration caused by the propeller's rotation close to the surface contributes

duce the propeller's efficiency. When in doubt, the overall reversing effi-

to the turbulent flow of water currents.

ciency is assumed to be low (around 0.25), and it is thus always sensible to manoeuvre at the minimum speed compatible with drift and with good helm control. This basic concept is particularly difficult to evaluate.

7. At 100% nominal velocity of water flow we observe a loss of over half this flow on the upper blade - data for a container ship with a fixed - pitch shaft line.

It is fundamentall y different, howeve r, depending on the type of propeller, the di rection of movement (forward or reverse) and interactions with the shapes of th e ship's stern .

In practice, when manoeuvring, the direction of the prop walk must be taken into account. It is noted on the Pilot Card handed to the pilot or on the Wheelhouse Poster displayed in the bridge . It is often marked by the side to wi1ich the ship moves when the engine is put into reverse (for instance, to show a right-handed propeller : acting righthanded effect when coming astern) .

51

3 3.1

Fixed-pitch propeller Turning effect of a fixed-pitch propeller in forward motion

For a fixed-pitch, right-handed propeller, the drag force R;; pushes the stern of the vessel to the right. This transverse force, to the rear of the centre of gravity "G" 1, causes the ship to drift and creates a turning moment to port .

Moving forward, the water flow is sucked from front to the stern by the propeller and accelerated to the rudder. The rudder efficiency is improved by this acceleration of the water flow.

This makes it easier to control the turning effect to port caused by the prop walk effect, slightly compensating from the helm 2 •

Prop walk compensated with helm

1. The centre of gravity, "G", the ship's

centre of mass, is the identifiable point used to study the forces acting on the ship's equilibrium when stationary. This concept is explained in more detail in the "Ship in motion. Basic concepts" chapter.

Turning effect in forward motion

52

Turning to port is made easier

2. Transverse thrust of PC in forward motion

3.2

Turning effect of a fixed-pitch propeller in reverse motion

Reversing the direction of rotation of th e shaft line to run astern, the propeller no

They strik e t he rear sta rboard curve of

longer operates at optimal efficiency. Its

The force R;:; is now greater and directed towards the port for a fixed-pitch, right-

profile is no longer properly adapted (lead-

the ship, gen erat ing hydrodynam ic pres -

handed propeller. As this transverse force

sure forces . These forces, t he deadwood effect, are add ed t o those generated by

ing edge becomes trailing edge, camber

is behind the centre of gravity "G", the

the transverse th rust 3 .

is inverted) . Deterioration in the quality

stern therefore turns to port .

of water flow over the blades reduces lift

This deadwood effect is significant for

and increases drag. This increased drag,

In addition, the water currents generated

together with the difference in efficiency

by the propeller are now flowing from the

ships with on e shaft line , where the kee l, towards the stern , acts as a wall .

of the blades and the proximity of the hull

stern forward.

It is less apparent fo r ships w ith two shaft

noted above, once more leads to the ap-

lines fitted o n a bracket, without an anti-

pearance of a turning effect.

drift effect . 3 . Transverse t hrust of PC in reverse m otion

d

Deadwood effects

Turning effects in reverse m otion

Large dea dwood effect

The ship's stern moves to port

Small deadwood effect

53

Ky

0,12

4

Controllable (variable pitch) propellers

0,1

4.1 0,08

Operation

A variable-pitch propeller is not cast from one piece as a fixed-pitch propeller is . It consists of a propeller shaft hub and controllable blades that can turn around an axis perpendicular to the shaft line .

0,06 0,04 0,02 0 ·0,02 0,2 0,4 0,6 ·0,4 -0 ,2 0 Transverse thrust coefficient

0,8

1,2

Experi ence indicates the importance of the coefficient KY of the prop walk effect as a function of the advance coefficient

Va nD (Va is the speed of the water flow through

the propeller4) . It differs from V0 because it includes the ship's wake effects Va = V0 .( 1-w). The tra nsverse t hrust force is given by the formula : FP= Ky.( p.n 2 .D 4 ) Ky : thrust coefficient p : density of the water The KY gr.aph referred to does not apply for all types of propeller. It does, however, clearly show that in manoeuvring it is better to treat prop walk as a random factor to make helm control more complicated.

Controllable-pitch propeller

Controllable-pitch propeller hub

The blades are controlled by a hydraulic mechanism. The motor always rotates in the same direction, so the propeller thrust direction is inverted by altering the orientation of the blades. The pitch is said to be "positive " for forward motion, "negative" for reverse motion, and "zero" for no thrust when the ship is stopped. Ma ximum thrust is reached when the pitch is at maximum (specific operational point defined by the manufacturer) and the revolutions per minute of the engine are also at their highest. The pitch of the propeller and the engine speed are often linked by a law. On some ships, however, the engine is connected to an alternator. This means the rotation rate must be kept constant, and the ship's velocity varied by changing only the angle of incidence of the blades (less efficient). The controllable-pitch propeller on a ship thus gives great flexibility when altering speed, without the limitations associated with engine control.

Studying the graph of the coefficient KY actually shows us that this is a significant effect (around 15% of thrust) and that it fluctuates with speed, one of the essential components in manoeuvring, therefore the variations in heading and drift that cause the transverse thrust effect are uncertain , or at least difficult to predict. Nonetheless, this effect is real, and therefore must be incorporated into the manoeuvring scenario. Water flow created by the propeller no longer passes over the rudder, so the system is less efficient. Accumulation of these lateral forces to port makes it almost impossible to hold a straight heading astern using only the rudder control.

Hydraulic mechanism for changing the pitch of blades

The turning effect to port when moving in reverse can be very marked, so the port side is often considered the best for berthing. On the other hand it is not so good when getting underway. If the ship has a single fixed-pitch , left-handed propeller, the effects are reversed . The stern turns to starboard when in reverse. Starboard then becomes the best side for berthing.

The propeller's profile is designed so that the angle of incidence of each blade creates a force proportional to the intended power and direction . At zero pitch, the propulsion component of the lower part of the blade counters that of the upper part . There is therefore considerable water disturbance at zero pitch .

54

Conversely, in both reverse and forward movement, the angle of incidence of the blades is enough to ensure that all the forces applied to each section of the blade are in the intended direction of movement. 4 . See

equations in "Propeller thrust" .

section

2.1.

Forward

Zero pitch

Reverse

Propulsion forces as a function of pitch angle of incidence

The importance of the blade profile in order to ensure optimum efficiency, whate ver the ship's speed, is now evident. In particular, it offers greater blade area ratio than that of a fixed - pitch propeller. Since efficiency in forward movement is clearly preferred, the complex profile of the blades often limits efficiency in reverse movement. Compared to a fixed-pitch propeller, the controllable-pitch propeller gives greater flexibility in use, but is often less efficient for manoeuvring . The direction of the blades varies the propeller's pitch angle, so it is better to call them variable-pitch propellers, rather than controllable-blade propellers .

4.2

Turning effect of a variable-pitch propeller in forward motion

For a right-handed controllable-blade propeller, the turning effect is similar to that of a fixed-pitch propeller. The resultant of the drag forces

R;: moves the

rear of the ship to the right.

As this transverse force is behind the centre of gravity "G", the effect is to turn to port.

Turning effect in forward motion

Turning to portis made easier

The turning effect is proportional to the pitch angle and the rotation speed of the propeller- it is at maximum for maximum pitch. As with the fixed-pitch propeller, the turning effect is easy to control with the helm.

55

4.3

Turning effect of a variable-pitch propeller in reverse motion

Reverse motion is obtained - with the shaft line always turning in the same direction (right-handed propeller in our example) - by inverting the angle of incidence of the blades (negative pitch). The propeller thus behaves like a left-handed, fixed-pitch propeller in reverse (righthanded rotation). The drag force R;: remains to starboard, but is greatly increased since the blade profile is no longer optimised (camber inverted). As this transverse force is behind the centre of gravity "G", the stern therefore turns to starboard. As with the left-handed, fixed blade propeller, the water flow generated by the rotating propeller is directed to the port counter. The deadwood effect is added to the transverse thrust.

d

Turning effects on starboard side

The ship's stern moves to starboard

In both forward and reverse movement, turning effects therefore remain the same with a controllable-pitch propeller. For a right-handed propeller, the stern always moves to starboard. The effects are reversed for the left-handed propeller. Most ships with a single, controllable-pitch shaft line normally have a left-handed propeller to gain the same turning effect as that of a right-handed fixed-pitch propeller in reverse motion .

56

4.4 • CONTROLLABLE PROPELLERS

4.4

Benefits of a variable pitch propeller

There are many benefits from a variable-pitch propeller: The main advantage of the controllab le-pitch propeller over the fixed-pitch type in mechanical terms is that fast changes of speed can be reliably obtained, with no risk of starter problems or clutch lag, and no need to engage an inverter. - The engine torque is used more effectively since it is operating in a range that gives better performance. - There is no limit to the number of reverse or forward movements the ship can make, as is the case with slowstart engines. - For a fixed-pitch propeller ship, the minimum engine speed (idle) often gives a fairly high minimum speed of movement (5 to 6 knots) while on the other hand, there is no minimum speed with a variable-pitch propeller, since the idle rate can be overcome by reducing pitch. - With a fixed-pitch propeller, thrust variation from forward to reverse depends on the time needed to reverse propeller rotation. The time needed to achieve this thrust variation is reduced with a variable-pitch propeller. - The turning effect, always to the same side, in forward and reverse movement, assists turning on the spot (only in the direction preferred by the transverse thrust). - In the open sea, there is often a reduction in vibration, since pressure variation on the propeller is reduced with blade profiles having a high skew angle, with greater blade area ratio . - If a blade is damaged, only that blade has to be changed, not the entire propeller. - A controllable-pitch propeller, with Becker or Schilling rudders, forms an excellent combination, often as effective as a stern thruster.

4.5

Drawbacks of a variable pitch propeller

There are still some drawbacks, including: - The mechanism for controlling the direction of the propeller blad es is complex, making the controllable-blade propeller more vulnerable and expensive than a fixed -pi tch propeller. - Outside the conditions for which the blades were designed, controllable-pitch propellers are slightly less efficient than fixed-pitch propellers. - When running in reverse, they are not so efficient (50% to 60% less) for comparable diamete r and pitch, as well as having a greater turning effect on the sh ip than that for a fixed-pitch propeller. With poorer perfo rmance, t he ship has to reverse for longer, or at high er power, t hus tending to increase the prop walk effect. - When the controllable-pitch propeller is at zero pitch, it continues to turn before the rudder, forming a screen that blocks the flow of water over the rudder blade. The ship often becomes difficult to control without a minimum amount of forward pitch on. Speed managemen t is therefore different when manoeuvring. Reduction or variation in speed therefore must be done very graduall y. - The propeller continues to turn whil e berthing or getting underway, with the associated risk of fouling ropes that fall into the water after casting off. Floating mooring ropes are -

-

-

recommended . When the ship is stopped , moored to the wharf at zero pitch, there may be a residual effect from maladjustment, which can be corrected by resetting the blade angle. A ship with only one variable pitch shaft line should not be left to move ahead slowly with the propeller at zero pitch, because it becomes more difficult to control. If the water at or near the wharf becomes stirred up, this may reduce the efficiency of the propeller wh ile berthing or

getting under way ("water wedge" effect) . - The turning effect is always in the same direction , wh ether moving ahead, in reverse or stopped, which is a problem when trying to hold a straight course. - Some remote pitch control systems do not reset the pitch to zero, if the control source is lost. There are propellers "blocked" in full reverse, following a loss of hydraulic con trol, or propeller pitch reverses at speed in the open sea , with the hazards and breakdowns that are consequent on this. The solution in such circumstances is to stop the drive engine .

CHAPTER 1 • PART 4.4 • CONTROLLABLE PROI!ELLER

57

5

Ship fitted with two drive shafts

For regulatory demands, such as those applicable to ships carrying passengers 1 , or to increase speed and manoeuvrability - increased swing capabilities, straightline reversing capability - ships may have two shaft lines. They are fitted equidistant from the median line of the ship.

5.1

Eccentricity effect

The eccentricity effect is the torque created on the ship around the centre of gravity by the action of the propulsion unit on a single shaft line. If the starboard engine is operating alone, a turning moment M:'v appears, equivalent to d x P. The ship then begins to turn to port. Conversely, if the same engine is started in reverse, the turning moment M;',d x - P causes the stern of the ship to turn to port.

The resulting torque is greater the further the propeller shafts are placed from the longitudinal axis of the ship. In general, the eccentricity effect takes precedence over the prop walk effect of each propeller. It is also independent of the type of propeller, whether fixed or controllable pitch. In addition, unlike the turning effect of the rudder blade (lift) and the propeller (transverse thrust), the eccentricity effect does not cause the ship to move sideways.

1. Rudders may be actuated independently in order to avoid a rudder I I

Gl

+

placed hard over masking suction of water flow from the shaft line running in reverse. They are said to be in asynchronous mode.

Catamarans, fast boats, for instance, are very good at turning on the spot, because of the distance between the floats. This gives them maximum eccentricity effect. Conversely, some ships have converging shaft lines towards the bows, reducing their twisting capability.

Starboard forward

58

Starboard reverse

Since a propeller's efficiency is better in forward motion than in reverse, the effectiveness of the twist is limited by the power from the propeller running in reverse.

5.2

Twisting effect

The eccentricity effect on each shaft line may be used to swing the ship on the spot. For instance a starboard turn can be made with the port engine started in forward motion, and the starboard engine running in reverse. The eccentricity effect of each of the shaft lines is added to create a pair of forces that initiate turning in the desired direction without drift. Conversely, with a small amount of forward way, the twisting effect of the propellers will only be fully effective if combined with the angle of the rudder behind the propeller in forward motion (helm to the right before the port shaft line in our example) . The rudder lift (Cy) creates a further turning moment, but also causes the ship to drift and therefore leads to an increased lift from the hull, from which the turning moment is also added to the twisting effect. See chapters on "The helm", "Turning" and "Vessel

I I

G l

•

Turning to sta rboard, twisting on 2 shaft lines: no drift

in motion, complementary aspects on ship hydrodynamics" .

1. 2.

Port forward Starboard in reverse

Twist with rudders in asynchronous mode 1

• sans bilrre Ro Ro Passcnecr Ferry (D1s 11001)

t J ''"'" '"" ""' lee Cbtt

Twist with helm to p ort, Twist with helm

35° to right: drift

Vow

R ~s

Conll'ol

f\i2

APM

Anchor

Tug

n MF

______. Drift

p t

Heeling under the effect of wind

~

c

:&::

~- F - ··--_ -;_ ~_ ____,_ ..~ ;; F' _

}

·()

Wine' effect on trim negligible

On the longitud inal plane, the wind effect has a negl igible impact on trim , because of th e length of the vessel and the ve1-y slight lever arm between C and G.

CHAPTER 2 - PART 1.6 -

99

6.3

Position of instant centre of windage

The resultant F of forces exerted by the wind to the vessel is applied at point C (instant centre of windage). This is a virtual point. It moves with the wind direction, and may be outside the vessel. It is easier to assess this force and its point of application if it is broken down into several forces applied to the representative surfaces the vessel presents to the wind . For instance, for a crude carrier in a crosswind : ~ force on aft superstructure force on hull on f orecastl e

F; F;

Th e resu ltant of the forces applied to the hul l:

F

=

c~ +

F;

+

i=;) .

6.4

Calculation of forces applied to windage

The force F the wind exerts on the vessel is carried parallel to it at the vessel 's centre of gravity. It is then broken down into a longitudinal component and a

f\, .

F;.

transverse component The force accelerates longitudinal displacement of the vessel. It is possible to compensate for its effect by running the engine in forward or reverse, depending on the direction of thrust . The force on the other hand, is more "restricting" since it causes drift. A simple empirical formula from a study by the British National Ports Council can be used to make an approximate calcu lation when there is a crosswind and the acceleration from drift is at a maximum. Using this assessment helps to determine the difficulty of the manoeuvre, and quantify the assistance that may be required from the tugs:

F;.

f\, ,

Wind effectDisplacement of instant centre of windage Neutral position

The force exerted by the wind on the vessel causes a turning effect that vari es with the profile the vessel presents to t he wind. It is often this effect that is the most sensitive in manoeuvring , since it is usually easier to make a vessel turn than to make it drift. Like any other solid exposed to stress, th e vessel tends to return to its neutral posi tion, that is to say, a position that allows a uniform movement. The simpl est of th ese movements is translational movement. A vessel when stopped and exposed to t he effect of wind will turn and drift until its instant centre of windage moves and t he vessel reaches its neutral position, so it can drift without changing its heading . In order to find its neutral position, t he

V2

stopped vessel will fall off or luff until t he resultant F from th e wind passes approx imately through its centre of gravity

S: surface area of transv erse wind surface

G and its moment is cancelled out. This is only a theoretical position. As previously seen, the action of the w ind on a vessel leads to two combined

2

X

Fy

S

X

100

ex pressed in thousands of m 2 ( mean aero dynamic coefficient Cy = 1.2) . V: windspeed in knots . F: in tonnes-force. The incidence (i) of the wind with respect to the longitudinal axis of the vessel can be incorporated empirically by repla cing S in the formula with the projected surface (S x sin i) , that is the windage surface of the vessel : Resultant of forces caused by wind applied to superstructures.

6.5

2 Fy

X

S

X

sin i

X

V2

turning movements : the vessel luffs or falls off, and drifts and moves forward , or drifts and moves back . These movements imply the appearance of hull resistances. Nonetheless, when the movement of a vessel can be likened to simple drift, t he point of application of the hull resista nce is close to midships and its lever arm with respect to G (thus its moment) is weak . It is therefore the centre position of win dage that mainly affects the neutral position of the stopped vessel when it drifts.

100

The effect of wind on a vessel can therefore be summarised as a force which causes drift, a force which makes it move forward or back, and an effect which turns it until it reaches a neutral position . Wind tun-

f\,

F;.

nel tests show that in most cases the turn ing effect is at its maximum when the angle between the wind and the vessel's lubber line is around 45°, i.e. when there is a quartering wind blowing across the bows or stern.

1 quarter represents 1/32nd of 360°, or 11.25° ; 4 quarters is equivalent to 45°.

6.5.1 - Neutral position -vessel stopped- C to stern -crosswind The vessel's neutral position depends on the position of the instant centre of windage C. The further astern it is, the smaller the angle with the wind direction will be once this neutral position is reached . There is a significant difference in this, depending on whether the vessel is laden or unladen.

c astern on a loa ded vessel

C on a light vessel

Luffing

\ \ 0

\

. Ffr ~drift Vessel stopped beam on to the wind

Neutral position winda g e astern

Let us allow the vessel to be stopped beam to the wind . With the crosswind, the vessel will luff (MF;G ) until C moves forward and F passes through G.

The force

F

from the wind divides into a driftin g force

F:.

F:,

and

a braking force The vessel then stabilises in the balance position . As a first approximation, ignoring the effect of water on th e hull , it falls off astern and drifts in the direction of th e wind.

6.5.2 - Neutral position - vessel stopped - C forward - crosswind The wind age centre is forward: tug boats and supply vessels.

Centre of windage C towards the bows

c \

G

earing away

Tug boat stopped

Neut ral position

beam to the wind

windage forward

The vessel is stopped beam to the wind. With the crosswind, the vessel will fall off (MF;G ) until C moves astern and passes through G.

F

The force

F

from the wind divides into a drifting force

F:.

F:,

and

a thrust force The vessel then stabilises in the balance position. In this case, ignoring the action of the water, the vessel also moves forward and drifts in the direction of the wind.

101

Force applied by water on the vessel - Hull resistance

7

The water exerts a vertical force on a vessel, generally known as buoyancy, which enables it to float . It is equal to the weight of the volume of water displaced by the hull. It is a hydrostatic force. When the vessel moves, the water also exerts a hydrodynamic force that counters the movement: this is the hull resistance . It is important to evaluate this force in ord er to make a useful assessment of the vesse l's dy namic balance. It is a very co mp lex ca lculation, however, and has been studi ed extensi v ely in testing tanks, since it invol ve s the combined phenomena of fricti on, hydrodynamic pressure and wa ve s. As water is a "heavy" fluid, hull resistance is significant enough justify vessel's hull s being profiled carefully to reduce the energy needed to drive them. As a first approximation , th e hull resistance varies with the square of the vessel's speed as fo ll ows : R K X S x yz

8

Hull resistance to forward motion

A vessel underway with the helm midships is subject to hull resistance on the parts underwater. The vessel pushes back the water close to its bows, creating an overpressure and a wave . This water moves aside then around the vessel, accelerating and forming further waves. Close to the stern, it slows down and rises to form another wave which pushes back the ves sel . The energy expended to push back the water and form these waves is proportional to the pressure the water exerts on the hull, or more visually at the top of the waves: this is called wave resistance .

8.1.1 Wave resistance As the vessel moves forward, it distort s the "free" surface area of the water. It "pulls" the water, forming lines of waves. These waves give a visual and physi cal expression to the difficulty of movement. The bigger the waves, the greater the resistance the water exerts on the hull. As the vessel moves, it generates two systems of waves: - The first is divergent, and open at an

-

angle of 19°, starting mainly from th e bows and to a lesser extent from th e stern. The second , transverse to the ves se l's longitudinal axis and often smaller, li es within the first system.

This effect can be observed in water flo wing along a bridge pillar (the pillar opposes the flow of water, which forms a wave) .

Resistance to forward motion -

=

R: hull resi stance S : wetted surface of the hull in m2

The flow of water along the hull also causes friction forces to appear. These forces ,

V: velocity of the vessel K : experimental coefficient characterising

known as viscous resistance, increase when the quality of the flow degrades and turbulence appears . The water then becomes more "viscous" in a way. A simple but still rigorous approach con sists of comprehending the hull resistanc-

the shape the hull presents to the water flow . This coefficient varies with the velocity of the vessel. When underway 1 and at manoeuvring speed (less than 8 knots), K may be treated as a constant. In this case, multiplying the speed by 2 will multiply the hull resistance by 4, and therefore also the propeller thrust .

es in forward movement (straight course), and studying separately the resistances from water viscosity and those induced by

System of waves

water pressure and the formation of the

the vessel and the hull fineness, which can be characterised by the immersed mid -

waves that accompany the vessel.

This wave resistance assumes an expendi ture of energy. It depends on the shape of

ships frame . 1

When underway forward, the vessel follows a straight course and the pressure forces either side of the vessel are balanced . Conversely, when moving obliquely, under the effect of drift, the pressure forces are not identical (this concept is explained in more detail in section 9 of this chapter) .

8.1

Hull resistance

Hull resistance = wave resistance + viscous resistance Wave resistance is linked to the volume of the hull. Viscous resistance is linked to the wetted surface area of this hull.

The immersed midship section is the maximum transverse section of the vessel below the waterline. It also depends on the square of its velocity and the density of the liquid. A length Lv can be deduced from this resistance, this is the distance separating the succes sive peaks of the waves generated, whose value is: Lv = 0.169 V 2 (V in m/s) .

If the speed increases, the wave resistance also increases . A single wave remains . In order to get past it, the vess el has to level off and provide a significan t amount of energ y. For vessel s with a large deadweight, this is the limiting speed.

102

wa ve created at a velocity below Limiting speed reached In order to reduce wave resistance, a system of artificial waves is overlaid on the existing system of waves, generated by the bulb . The bow alone generates a system of waves that pushes the water up-

An underwater sphere, as it moves, creates an overpressure on the forward side, like the bow, and a lower pressure on its

If the sphere is carefully attached to the bows, the t wo wave systems are overlaid, with the crest of the bo w wave filling in

rear side.

the hollow of th e sph ere. The resultant bo w wave formed by the vessel is reduced .

Other principles exist to reduce the vessel 's resistance to forward movement .

vessels, with the advantages of reducing pitch and generating less v ibration, or sli m hulls, known as wave piercing, suitable for some high-speed vessels.

wards.

The energy needed to move the vessel forward at its maximum speed is less, because there is less resistance, giving a fuel saving. The bulb is only effective however for a given draft, trim and velocity.

Ferry bulb

For instance, the inverted hull shape, such as X-Bow or Ulstein ® , fitted to supply

In verted bow

Wave piercing bow

103

8.1.2 Pressure resistance A moving vessel disturbs the mass of water around it in a zone once or twice its width and about one and a half times deeper than its draft. The pressure forces exerted over the whole hull are the sum of the hydrostatic pressures caused by immersion and hydrodynamic pressures proportional to the speed, owing to the flow of water currents over the vessel's hull. Distribution of these pressures over the hull show: - a greater relative overpressure zone on the bows than on the stern. The suction created by the screw reduces the size of the ove rpressure zone to the stern (aspiration and wave effect). - a relative low pressure zone on the centre.

Pressure and lower pressure zones As a first approximation, these diagrams show the effect of the shapes of the vessel on hull resistances, especially the shapes of the bow, stern and midships, which determine the intensity of the pressure zones.

Zone of resistance to

Overpressure zone ahead of a crude carrier

forward movement

Hydrodynamics provide a way of quantifying the pressure forces exerted by the water on the hull, by studying the impact of the hul l on the velocity of a "potential" flow in which the vessel is immersed. The vessel in this case is immobile, and the water flows arou nd the hull (like the currents of water around the pillar of the bridge). The phenomena linked to hydrodynamic pressure have a number of consequences on vessel manoeuvres, especially in confined and shallow waters: (All these points are explored in the chapter "Navigation in shallow waters" .) effects of interaction when crossing or passing other vessels, sinkage phenomenon, squat, reduction in speed, increased turning radius, bank effect.

~ Veers off

~

voocooffr~

Acceleration water flows Sinkage effects

Interactions between vesse ls

8.1.3 Friction resistance The flow of water along the hull is not uniform: ahead of the boat, the water currents flow in parallel, following the line of the hull; the flow is laminar; -

in the middle the water flows take more space as they flow; the flow becomes turbulent; astern, the turbulence increases; the flow becomes a vortex.

Boundary layer

turbulent

104

Pilot boat in the zone disturbed by the bow wave and the boundary layer

Looking at the vessel more closely, it can be seen that a liquid envelope spontaneously surrounds the hull. The water molecules in contact with the hull are drawn along at the same speed as the vessel. Further out, at some distance from the hull, the water is stationary. Between these two extremes, the complex movements of the water particles generate friction forces, caused by the difference in the speed of these water particles within the zone. This kind of shell, the boundary layer, follows the vessel as it turns, being constantly renewed by the eddies (See photo pilot boat) .

The depth of this space varies along the vessel until it breaks at a separation point. A transition zone then forms, where the water particles take up more space to move around, until they finally become entirely detached and thus turbulent. The water molecules are excited and agitated in every direction. Vortices are therefore formed to the rear of the separation point, significantly increasing energy expenditure and friction resistance - also called viscous resistance - since all these phenomena are caused by water viscosity. Friction resistance therefore depends on the qua lity of the hull and its capac-

ity to limit eddies, on t he foulin g of th e hull and essentiall y on t he speed of th e vessel, which en courages instab il ity in the flow. At a certain speed , t he bou ndary layer deepens . Th e water particles fol low their initial traj ectory and thus detach from the vesse l's wate rlines. Th is creates a lower pressure zone, f illed by th e water from the stern . Th e drag caused is called eddy resistan ce . The more cambered the stern shapes, the greater the drag and the more they encourag e the detachme nt of the bounda ry layer.

Boundary layer

Vortices

turbulent

Eddies increasing hull resistance

La m inar Boundary la yer and eddy zone

While hull resistances in forward move-

constraints caused by the formation of

When the resistan ces of t he hu ll

ment are hard to quantify (especially while accelerating), their direction is easy to identify, however. The perfect symmetry of hulls implies a resultant of the friction and pressure

waves directed along the vessel's longitudinal axis and directly countering the thrust of the propulsion unit.

balance the t hrust of the propel ler, th e vessel maintains a co nst an t speed ( uniform movement) . The hull resi stan ce in fo rwa rd movement is a drag force

Uniform curren t flow

R:.

Resistance to forward motion

105

Resistance to oblique motion

9

9.1

Effects of water on a moving hull

When turning, or manoeuvring in the port, any movement of a vessel combines a longitudinal displacement, drift and rotation. Observing the hydrodynamic phenomena in a reference mark placed on the vessel's centre of gravity, the hull no longer shows a perfectly symmetrical shape, profiled for the flow of the water. The hull resistan ce increases and becomes m ore difficult to comprehend . Its intensity, direction and point of application on the hull change with the variations in trajectory of "oblique motion". A turning effect (yaw moment) appears, which tends to cause the vessel to turn.

Th e water reacts by exerting a force on the vesse l (resistance to oblique motion) whose moment with respect to the centre of gravity causes a typical turning tendency to appear. The transverse forces and the yaw moment occupy all the vessel handler's attention. They express the vessel's capacity to counter drift, while creating a turning torque which tends to change its heading . The hull's lift is therefore very significa nt in terms of manoeuvrability and port manoeuvring. This hull resistance is always proportional to the square of the speed, but it is very difficult to quantify since the hydrodynamic coefficient (K) also varies . For a better experimenta l eva lu ation , it is investigated on models,in tow testing tanks, breaking up the vessel's movements . Simple drift, turni ng without drift, and turning with drift are all tested. This breakdown of the movements shows up a drag force (Rx) proportional to the longitudinal component of the speed, a lift force (Ry) proportional to the transverse

9.1.1 Vessel drifting in forward motion

Intuitively, the water can be imagined as passing beneath the keel, creating a hull resistance (thin aerofoil theory). This resistance force R that the hull exerts on the undersides is applied at a point Cd, centre of drift. This po int of applicati on moves longitudinally with the speed of t he vessel and the angle of drift. Th e ma ximum turning moment of the hull resi stance is reached at a drift angle of abo ut 45° (variable as a function of the hull and its draft). For instance, with a wind from port th at causes a drift to starboard, the ves se l moves forward along its surface cou rse

R:. Heading Luffing

~

Drift

---. Rs

speed (drift) and a so-called dampening force proportional to the angu lar speed (turning).

Ferry in a turning motion

Luffing of vessel in forward motion with a wind from port.

A vessel's motion is said to be oblique when the flow of water along either side of its hull is not symmetrical. This sailing trim is adopted when the vessel is exposed to the following external actions : - wind causing drift,

-

-

current (in confined waters), turning,

-

-

external forces (tug boat, etc.) .

Hydrodynamic pressure leeward causes a resistant force R to appear, applied at Cd, the instant centre of drift, forward of the centre of gravity G. The vessel will : luff under the effect of the moment MR/G (yaw moment), counter the drift and heel in the tran sverse vertical plane with (Cd above

f\,

In these situations, one of the sides of the vessel (gunwale) "presses" on the water. On the other side of the hull, vortices form, mainly at the bow and at the end of the keel.

f\,

the waterline). is a lift force, - slow with the effect of ~ which is a drag force . As a first approximation, with the effect of the wind or any other cause of drift, a vessel making headway is slowed dow n, it luffs and heels (Cd below the centre of gravity of the waterplane). For a given speed, the turning effect (yaw mome nt) or luff effect is normally greatest when th e drift angle is close to 45°.

Water flow currents turning

X

Cha nges to

F as a function

Speed (knots)

Drift

Position "a" de Cd /Ox

Direction "a" of R/Ox

6.6

so

0 .36 L

go

6.6

10°

0 .25 L

22°

6.6

15°

0.21 L

35° 48°

6.6

20°

0 . 18 L

4.5

30°

0 . 16 L

79°

3 .0

50°

0.10 L

80°

2.4

70°

0.09 L

87°

A simulation of simple drift on a model representing a "crude carrier" vesse l, clearly shows how th e point of application moves and the direction of hull resistance changes. This simulation can be compared to the trajectory the vessel would follow approaching its berth while drifting with a strong wind . As th e vessel slows, or the wind increases, the drift increa ses, the centre of drift approach es the centre of gravity, and the hull resistan ce becomes perpendicular to the lubber line.

of the speed and drift angle

9.1.2 Vessel drifting in reverse motion

9.1.3 Vessel turning (w ith out drift)

With wind from th e starboard quarter, the vessel falls off along its surface course Hydrodynamic pressure leeward causes a resistant force F to appear, whose re-

R:.

In reality, simple turning is very rare, but it is still investigated in manoeuvring tank tests that break down the trajectory components and enable a better analysis of the forces that oppose a rotation movement of the hull . These experiments show that the water flows along the port and starboard sides of the boat at different speeds.

sultant is applied at Cd, astern of the centre of gravity G. The vessel will: - move upwind, under the effect of the yaw moment MR; e; - slow with (drag) and counter the drift with (lift).

R:

1\,

Mo'e cpwiod ~

~

Heading

t

Drift

7~ ----.. Rs

This difference in speed causes a difference in pressure In specific terms, the water counters the movement by creating a resistance force , called damping w hich is inversely proportional to the turning radius (MR;G ). In fact, the turning motion is normally accompanied by drift to the outside of the turn, caused by inertial and lift forces from the rudder blad e. The moments and the lift forces from the resistances caused by drift and turning counter

----.. R'

each other. The behaviour of a turning vessel is therefore co mpl icated, and depends on the predominance of two effects, caused by turning and drift. Turning tests on vessels or on "free" self-propelled models va lidate the turning qu alities of hulls, and the design of the rudder system.

The stern of the vessel moves upwind in reverse movement

Similarly, with the effect of wind or any other cause of drifting , a vessel making sternway is slowed, heels and falls off (the stern moves into the wind).

R 2 • PART 1.9 • RESISTANCE

Vessel turning only

107

9.2

Action of current in confined water on the hull

When the vessel is underway with an angle with respect to the current (of a river, for instance), the relative flow of the water on. the hull has an angle of incidence to the Iongitudinal ax is of the vessel (same reasoning as for the apparent wind). The water then exerts a force R, which tends to alter the vessel's heading. This effect increases as the under keel clearance reduces.

Vessel making headway with current

9.3

Vessel in reverse with current

Combined effects of wind and hull resistance - Neutral positions

The vesse l under way is therefore subject to two external actions: - the action of the wind on the superstructure (w ind surface), the resultant of which F is applied to the instant centre of windage C, -

which counters this luffing M R; G and t his reverse motion (Rx) and the drift (Ry). As the vessel changes heading, C and Cd move, and the vesse l find s a neutral pos ition .

F and R com-

pensate their actions, or to be exact their respective moment : M R/G hull resistance changes with the square of the vessel's speed and the angle of drift. It causes luffing . Its point of application Cd moves as the vessel

Vessel in neutral position

makes way and with the angle of drift (thin aerofoil theory) .

For most vessels, there are three neutral positions of stability: the vessel will tend

MF/G the resultant of the wind action changes with the square of the speed and the angle of incidence of the apparent wind. It causes falling off. Its point

to return to them whenever it moves away from them : - vessel stopped, - vessel moving forward, sailing into the

of application moves with the direction of the apparent wind.

wind, vessel moving in reverse, sai ling with a tailwind, There are two unstabl e neutral positions: the vessel will tend to move away from these whenever it turns off its course:

The neutral position depends on: - the type of vessel (wind surface, shape of hull), -

the speed of the vessel, the force of the wind.

These various parameters demonstrate the randomness of the concept of the neutral position. It is nonetheless a really important concept, especially in port manoeuvring, which takes place at slow speed. It expresses a natural and recurrent tendency of the vessel, which is useful to understand .

108

Superstructures on the stern: A vessel whose instant centre of wi ndage is to the rear of the centre of gravity, and which is stopped beam on to t he wind, encounters a force F caused by t he wind action on the superstructures whi ch causes it to luff (MF), drift ( Fy) and fa ll back (Fx). The reaction of the wa ter on the hull t o these movements generates a force R

the action of the water on the undersides(hull), the resulta nt of which R is applied to the instant centre of drift Cd .

Balan ce is achieved when

9.3 .1 Vessel stopped The stopped neutral position is certainly the most important to understand , sin ce in order to come to a halt, the vessel goes astern and therefore no longer has rudder control. It is exposed to the effects of wind and water, and tends to return to its neutral position. It is a function of the distribution of the wind surfaces of the superstructures. The angle 1 of the vessel t o the wind is expressed as a bearing 1 or a rhumb 2 . 1 ) Angle expressed in degrees with resp ect to the lubber line, or the vessel's med ia n axis. 2 l 1/4 is equivalent to 11.25°.

-

-

vessel moving forward, sailing with a tailwind ,

-

vessel moving in reverse, with a head-

wind. In the following examples, the specific case of the tugboat shows that when the centre of windage is particularly offset from th e vessel's centre of gravity, the neutral positions are significantly different from those of "standard" vessels.

The vessel will: _ luff under the effect of the moment MF/G.

_ drift (Fy) and fall back with (Fx). The speed of movement in drift, at a stabilised heading, depends on the wind force and the squat of the hull (trim and drafts) . In a high wind, a light vessel can travel at a transverse speed (drift) of around 5 knots .

Neutral position of stopped vessel; windage centre to the stern

The loaded crude carrier drifts from its neutral position by between 1 and 4 quarters of the crosswind, depending on the arrangement of its superstructures. on the other hand , a light VLCC behaves like a vessel with a uniform windage, and finds a neutral position practically beam on to the wind.

Neutral position laden VLCC

Neutral position light VLCC

Uniformly distributed superstructures: A vessel with a uniform windage area experiences a force F on its superstructures when it is stopped beam on to the wind. The reaction of water on the hull generates a force R that opposes this drift. The centre of windage C and the centre of drift are vertically aligned with respect to the centre of gravity G, so there is no remaining turning effect. The moments

M R/G

and

MF/G

are zero . The vessel will drift in the direction of the wind.

Neutral position of a vessel with uniform windage distribution.

Superstructures towards the bows: A vessel whose instant centre of windage is forward o!_.the centre of gravity, and which is stopped beam on to the wind, encounters a force F caused by the wind action on

~ Bearing

away

the superstructures which causes it to fall off MF/G , drift (Fy) and move forward (Fx) The reaction of the water on the hull generates a force R that opposes this falling off (MR/G), drift (Ry) and the forward way (Rx). As the vessel changes heading, C and Cd move, and the vessel finds a neutral position. Neutral position of a vessel with windage for ward.

The vessel will: -

fall off under the effect of the moment MF/G . drift ~ and make headway ~ . The speed of movement in drift, at a stabilised heading, depends on the wind force and the squat of the hull.

The tug exploits this stable neutra l position to stabilise itself in the lee of the assisted vessel. It can then secure its towline safely. If its drifting speed is greater than that of the towed vessel, the tugboat will compensate for this by reversing its engine, thus strengthening its neutral position (see section 9 .3.3) .

9.3.2 Vessel making headway (helm to midships) Starting from its stationary neutral position, the vessel starts its engines to move forward, with the helm midships, so the vessel is then subject to the following actions: - Hull resistance R increases with the square of the speed of the vessel, and its point of appl ication moves forward, tending to make the vessel luff, - The action F of the wind on the wind surface causes a falling off, the intensity of which varies with the force and angle of incidence of the apparent wind (the apparent wind approaches the axis of the vessel as the vessel's speed increases) . Its point of application also moves forward .

The vessel therefore turns more or less into the wind and finds a neutral position when the action of R is balanced by the action of F . The balance position therefore varies along with the wind speed and the vessel's speed . Since water is a more dense fluid than air, the hull resistances rapidly predominate as soon as the vessel has enough way on. In some cases, when the vessel is more resistant to drift, its inertia can help it to cross the path of the wind without using the helm . This also helps to explain why a vessel is in its neutral position when it is sailing into the wind . If the vessel deviates slightly from its initial heading, it therefore starts to drift, but resistance to oblique motion tends to bring it back upwind .

Some typical tendencies can be provided of representative vessels as examples of their wind load and for manoeuvring in high wind conditions . - Vessels whose windage surface cen t re of gravity is close to the mid perpend icular, with a significant wind area such as ferries, need to travel at fairly hig h speeds before feeling the luffing effect. On the other hand, they easily re ach the crosswind sailing trim . - Vessels whose wind surface is towa rds the rear, such as vessels with stern superstructures, luff more readily in order to come into the wind. The la de n crude carrier, for instance, tends to drift less and luff more readily than the li ght crude carrier. It therefore turns m ore easily into the wind. In high winds, under certain conditions, the li ght crude carrier has to be ballasted to remain manoeuvrable since it ha s a particularly adverse windage area t o drive power ratio. - Finally, vessels with windage towa rds the bows, such as supply vessels , luff with difficulty. This type of vessel is said to have a "lee helm". In some cas es, it can even be easier for it to cross t he wind astern by falling off.

Neutral position of a vessel making headway

110

g,3.3 Vessel making way in reverse When the vessel is making way astern , hull resistance R becomes more important than when moving forward , since the hull shape is less streamlined . However, since the stern shapes of the vessel are normally flatter or rounder, they have less resistance to drift. With this hull resistance, whose point of application is still behind the centre of gravity, the vessel 's stern turns into the wind , but normally less sharply than when making headway. Vessels can still sometimes find a

neutral position near to the tailwind, while only rarely crossing the wind . A vessel whose trajectory is caused by the combined effect of wind and its propeller pitch will follow a path that is the compo nent of these two actions . The heading depends on wind force, direction of swell an d the vessel's speed. The action of the helm, even when hard over, can only rarely counte r this effect. Neutral position ; MR and M; are co mbin ed

-

The ship moves backwards,

M;' C moves back further,

MR

the pairing of MR/M F causes it to fall off

wi t h the win d from the stern ; ~ and M; beg in t o counter each other back, the vessel

Neutra l p osition when moving in reverse

9.3.4 Vessel stopped in any position A vessel that is not in its neutral position at the moment it stops, will nonetheless reach it after several oscillations, while drifting downwind . This theoretical "floating leaf" trajectory is imperceptible in the following range . The drawing is exaggerated to assist in understanding how the vessel behaves .

With the ship stopped , only MF acts: the ship begins to make way astern and falls off.

rJ;

heoretica l alignment of the vesse l in its neutral p osition

111

9.4

Heeling effects

The action of the wind F on the superstructure and the hull resistance R on the underside cause the vessel to heel. This heeling effect changes the shape of the hull that is underwater downwind (increasing the anti-drift plane) and leads to an increase in R. The vessel now resists drift and luffing more readily. Heeling reduces the efficiency of the rudder, however, which depends on the horizontal co mponent of the transverse force (lift) that the water flow exerts on its profile. On vessels with two drive shafts, the downwind propeller, further underwater than that upwind, may also slightly co ntribute to the increase in luff. A vessel exhibits a weather helm when it heels .

This latter case requires full attention, since the sinkage of the vessel's bows and its typical vertical shapes lead to an increased lift force and displacement of t he hull resistance application point forwa rd (therefore an increase in the lever arm and the yaw moment) .

Phenomenon of lift in turning

Testing the stability of heading and the vessel's responses to the helm, and noting the shape of hull and its draft, an experienced sailor can feel this phenomenon and anticipate its effects . Turning the problem rou nd, it can also be considered that the turning drift (slippage) will depend on lift as we ll, and will be experienced whenever the vessel changes its heading. This concept is very important,

especially in a port zone, when the ves. . _ . sel must follow a precise trajectory after a .............. turn (arrivi ng at an entrance lock or na r-

Q-.....H--

....

M;

~-...---

~ Luffing Vessel under the effect of heeling

9.5

Drag, lift and safe speed

In practice, the concept of the lift of the hull, which characterises its capacity to resist drift, is all the more im portant, the harder it is to understand empirically. It is however possible to demonstrate it in reality by noting a vessel's capacity to luff and heel when it has headway, when it drifts. In fact, in water, and therefore

These two phenomena together can lead to an unstable heading and problems in steering: the rudder blade, whose surfa ce area only represents 2% to 5% of the area of the hull, may be shown to be no lon ger efficient enough. Observation and expe ri -

row channel for instance).

ence are therefore valuable in properly assessing the hull's reaction .

In port manoeuvres, it is important to take this force into account very carefully. The fact must be taken into account that lift on the hull does not occur immediately,

Manoeuvring a vessel within a port area implies complete contro l of the posi tion ing and therefore heading and speed. This latter point is vital since it is main ly

but only after an instant wh ich is around the same length of time (length/speed) needed for the vessel to travel its own length, and for the flow of water to be es-

the square of the speed that contributes t o quantifying the forces app lied to the vessel (inertia, drag or lift) .

tablished around the hull. In transitional periods (on completing a turn, for instance) and when lift is needed to resist drift, this deficit must therefore be anticipated . The vessel will actually always be "drawn" for a time by its "ves-

The principle of precaution, essential in the application of marine sense, therefore

sel stopped" neutral position, before beginning to experience weather helm , and thus behave like a vessel underway. Similarly, the chapter on turning will explain that the vessel's trajectory becomes

and the vessel's safety often depends on the vesse l handler's ability to evaluate it. This speed can vary considerably for one

most complex at the point when the turn begins when hydrodynamic lift is gradually established.

to use all the engine power. Full ahead (or full astern) sailing trim is often the m ost effective, and in a critical situation, it m ay even be absolutely necessary. It is only completely inappropriate wh en

The angle of drift, trim, squat and the shapes of the vessel are also very impor-

requires a safe speed to be adopted at all times during port manoeuvres, the mi nimum speed compatible with drift and good helm control. This is a very subtle concept,

vessel, depending on circumstances . This does not necessarily exclude the need

tant because they affect the hull's capacity to "p roduce" a lift force.

it allows enough time to achieve a speed that is incompatible with safety. All these comments clearly indicate how

plane, lift has a heeling effect.

As an example, a light vessel, with round-

difficult it is to assess the limits of fea si-

The greater the vessel's weather helm, the better it resists drift. Lift is determined by the vessel's trajectory (drift, speed, turning), as well as its trim, squat and shapes of hull (especially the bow and stern shapes, where the hydrodynamic forces are concentrated.

ed shapes and a very positive trim clearly finds it much harder to counter drift (by

bility of a manoeuvre in terms of safety speed . This decision is the captain's re-

its lift) than a loaded vessel with straight shapes and bow down.

sponsibility, his judgement being suppo rted by the advice of the pilot, whose local experience and knowledge of manoeuvring are valuable .

below the centre of gravity of the water

112

10 Summary To conclude, the action which exerts a force on a manoeuvring vessel can be summarised as two translations (the vessel moves forward or back, and drifts from one side to the other) and one rotation (yaw), similar to a change of heading . A vessel handler works with the vessel's inertia to follow a trajectory accurately, and using the helm and engine mainly,

superstructures that are especially difficult to evaluate since their intensity, direction and point of application vary with the circumstances which create them . In addition, depending on the principle that every action has a reaction, water which allows the vessel to float, also exerts a resistance force which counters its movements. This force is amplified by the confined space within the port basin . It must also be broken down by these

be countered by turnin g t he rudder, and creating another force . All these forces listed above are therefore closely linked t ogether. The vessel handle r the refore has to ana lyse this complex situation continually in order to co nt rol t he manoeuvre . Finally, th e bal an ce of forces exerted on a vessel wh en it is manoeuvring is difficu lt to assess in rea l time . The proble ms to be managed during a ma -

exerts thrust that has to be incorporated using these three degrees of liberty. During its manoeuvre, the vessel experiences the effects of the port environment, mainly wind and current, which create complex forces on its hull and

three degrees of liberty. It shows resistance to forward movement which can be compensated by the propeller's thrust, but also a capacity to counte r the drift, which forms a quality of the vessel itself, together with a turning effect which could

noeuvre appear very quickly one after another, leavin g very little time for reflection. Manoeuvres must therefore be prepared in ad vance or left t o trai ned and ex perienced seafarers.

The resistance the water exerts on a vessel moving obliquely can be broken down into a resistance to direct motion,

resistance to drift and a turning effect whose intensities vary with the shapes of

the hull, th e rad ius of turn, the drift angle and the squa re of the vessel's speed.

Complementary concept of ship hydrodynamics

.

.

. . =

•• • •

1.

Introduction to complementary concepts of ship hydrodynamics

The basic concepts covered so far highlight how important it is, when undertaking a manoeuvre, to correctly evaluate the forces exerted on th e ship. The magnitude of such forces as well as their direction must be evaluated, as well as their point of application, in order to anticipate the ship's movements . Nonetheless, analysis of the physi cal phenomena gi ving rise to these forces which occur when a ship turns, especially when it is manoeuvring in a port, is very complex. In water, the confinement of the hull in a narrow basin and the frequent changes in the engine speed and helm angles complicate analysis of fluid flows so that even the most significant phenomena are difficult to predict.

Research into hydro- and aerodynamics, however, provides ways of quantifying the individual types of data relating to manoeuvrability, (propeller thrust, hull resistance, appendage efficiency and wind surface area). Experiments in the hull test tank, together with the develop ment of digital calculation models have also increased the realism of computer simulations. Studying some of these hy-

Three hundred years ago, Leonard Eul er

drodynamic and aerodynamic concepts thus enables us to evaluate behaviour and manoeuvrability of ships and reduce the part played by intuition when guiding a manoeu vre.

-

2.

General remarks on flows

The air and water surrounding the ship are fluids, which are subject to the same physical laws, at least in the conditions we are considering . Newton was the first to determine the resistance encountered by a body moving in

The port structures and other obstacles to wind circulation above water reduce the ship's stability of speed and direction. There is therefore a need to approximate values, in order to study these fluid flows as a whole. It is extremely difficult, maybe impossible, for even the most powenful of computers to deliver real time calculations of all the forces exerted on a ship as it manoeuvres. The very few computer systems that have been developed with the aim of replacing the ship handler's function of bringing a ship alongside or taking it out of the port always need major manoeuvring capabilities (transverse and azimuth thrusters, high-spec rudders , etc.) and considerable power resources . The most effective means therefore are still the sailors with their manoeuvring experience.

-

equation of co nt in uity (or mass balance equation), equation of quantity of moti on ba lance, energy balance equation .

In hydrodynami cs for instance, solving Hydrodynamics and aerodynamics are very complex sciences, normally studied by means of mathematics. They are used by marine engineers when designing vessels, but rarely by sailors. Our simple approach avoids using esoteric language. The reason for mentioning this is only to point out that while sailors treat manoeuvring as an "art", it is also a science .

Ferry getting underway. (th e different colours show how the water masses interact with the environment)

applied these equations t o indivi dua l flu id particles . The cru cial con t ribu tio n of Navier, and t he n of Stokes, was then to add a term to th e Eul er equ ations for fri ction between the va ri ous laye rs of flui d, proportional to th e coefficient of v iscosity and variations in ve locity. Navier- Stokes equations combine the following:

a fluid, using the principles of mechanics . Other scientists (Bernoulli, Euler, Bouguer, d'Aiembert, etc .) subsequently expanded the understanding of physical phenomena associated with the movement of fluids around a ship, or any solid. These flows all create turbulence, such as that which forms in the wake of ships. Given the complexity of the subject, there is much research taking place about it even today, with applications in fields as varied as biology, astrophysics, meteorology or hydrodynamics. In all contexts, fluid in its liquid or gaseous state is treated as a continuous medium represented by density, pressure and velocity fields that can be described by the well-known Navier-Stokes equations. These are differential equations with nonlinear, partial derivatives that describe the movement of fluids . For instance , they govern movement of air in the atmosphere, flow of water in a pipe and many other fluid flow phenomena . They are essentially Newtonian equations linking force and acceleration.

these equation s all ows the quantity of motion equation to be so lved for f lu ids : I

FOrCeS

= Ma ss x Accele ration

It is thus possibl e to qua nt ify accele ra-

tion of fluid particl es f low ing aro und th e hull, as well as pressure and f rict ion forces that apply at each point of t he hull , w hile turning the vessel. So lving th ese equ ations also allows a th eoretica l study of f low along the hull of a sh ip . The ir comple xity, however, m eans t hat they often have to be simplified ( mod elling turbulence for in sta nce), and only a few experts using powe rful co mput ers can sol ve them . Specific deve lopments a re also needed to tak e account of t he effect of the formation of th e waves t hat acco m pany ship movementsll. This means that hull resistance can be cal culated , so that ship motion can be explained .

r .;; -. fl

d ~ ~ 6' V Q -/

/

Turbulent flow in the wake of a cylinder

1 l It is not yet possible to solve these equations exactly. Only some reduced

forms, representative of the most important phenomena, provide mathematically coherent solutions. The RANS (Reynolds averaged NavierStokes) method, together with a turbulence model provide a faster method of solving these equations, by defining each fluid variable by its average and its variable part.

'"'---

The progress made as a result of wind tunnel and test tank experiments now allow us to better understand the aerodynamics and hydrodynamics that are of most interest, through quantifying the physical interactions between a moving ship and the surrounding air and water. The dynamic behaviour of the ship can be anticipated by evaluating these forces of interaction and their points of action.

2.1

Definition of pressure

In a fluid environment, the force the liquid (1) exerts on the solid (2) across a surface element "ds" has any given direction . But this force Cif can always be broken down into : - a tangential component dfT - a normal component dfN The quantity ~/ ds represents the tangential stress and ~ /ds the normal stress. Pressure is by definition the normal stress

I

'--. dfN

2

dfN ds Unit : The Pascal (Pa) [p] = M L- 1 T- 2 •

1 Pressure and friction forces on a surfa ce element ds

Aerodynamics In fluid statics, only normal pressure forces dfN, apply to the element "ds" . Tangentia l forces dfT only appear in fluid dynamics: they correspond to the viscous friction of fl uid layers moving against each other and against the side of the ship. A study of hull resistance may therefore begin with the study of tangential stresses caused by friction from the water on the hull, then proceed with the study of pressu re forces: hydrostatic pressure caused by immersion and hydrodynamic pressure from the velocity of the water flow along the hull . This velocity varies with the flow of water pa rti -

Hydrodynamics

A visualisation of the path followed by fluid particles- surrounding the ship reveals deceleration, acceleration (different colours on figure), changes of direction and vortices, and thus variations in dynamic pressure. In the water, these vortices are found mainly at the bows and the end of the keel, when the ship is drifting or turning. The eddies or turbulence which develop in the wake of the ship, in air or water, make a study of these flows hugely complicated. Such turbulence, forming a series of vortices, increases with speed and angle of drift. They significantly affect the hull resistance and aerodynamic resistance, but their chaotic behaviour makes them unstable and difficult to predict.

cles following the hull's waterlines . The significance of the flow velocity and the concep t of hydrodynamic pressure also come to the fore during the ship's turning phases . When following a curved, drift trajectory, the ship actually presses on the water an d its hull is subject to pressure phenomena that can be compared to those allowing an aircraft to fly (aerofoil theory). This study is finally completed by considering the ene rgy expended in creating waves that form as the ship's velocity increases . This is easier to understand by taking a reference point linked to the ship's centre of gravity. Imagining this point as immobile, it can be seen that it is the surrounding wate r that is moving. The problem with considering hull resistance during manoeuvring pha se s comes mainly from the complexity of the pressure analysis, since this is linked to th e speed of flow, which is random during turbulence. Friction and wave phenomena are less significant, since turning speeds when manoeuvring are normally fairly low. The first concept to be understood when considering physical phenomena arising in a fluid flow is viscosity.

2.2

Viscosity

They can only really be approached by means of statistics. Along with cavitation phenomena, they always form a major problem when developing simulators. Moving on from concepts of flow to concepts of stress, it is necessary to talk about physics .

116

V

---.... Vs

ships to steer. "D'Aiembert's paradox" illustrates the fact that a body immersed in a non-viscous

---....

v4

----..7 v3

fluid is not subject to any force. Water and air surrounding a ship are viscous fluids. Viscosity therefore has to be considered when calculating the forces exerted on a ship by these fluids . A fluid's viscosity

Water flow around a drifting hull

---....

dS

The viscosity of air allows aircraft to fly. Similarly, the viscosity of water allows

characterises its resistance to distortion. A simple experiment demonstrates the concept of viscosity:

n

v;j ~/

~ Viscosity of the water Let us consider a tank filled with wate r t o level n. We Jay a plate , dS, on the surface of the water and give it a certain veloci t y V. The experiment shows that because of the viscosity, water resists the move ment of the plate, creating friction forces within the fluid.

--

Friction provides adherence between plate and water. In fact, the water particles touching the bottom of the tank remain immobile, and the water particles touching the plate are dragged along at the speed V of the plate . Information about the movement of the plate and the immobility at the bottom spreads through the fluid as the particles of liquid begin a gradual and linear motion. Each molecule of fluid flows at a different speed, through the effect of the interactive forces between the molecules themselves, and between them and the molecules in the sides of the tank. In this way, viscosity is manifested on a ship's hull by surface forces: tangential

Corrections can then be applied to take account of frictional resistance and wave resistance, using empirical formulae deduced from laboratory experiments. When the ship's trajectory incorporates significant drift, the physical phenomena caused by viscosity that occur in the boundary layer are much more complicated in terms of analysis . As a specific example, the importance awarded to this concept and the quality of flows can be seen from a rudder that stalls and loses its efficiency when the depth of the boundary layer increases to the point that it tends to detach from the ship's profile (see section 4 on pressure resistance).

2.4

frictional stresses proportional to the gradient (rate of variation) of the velocity within the flow created . Using the example of the moving plate, it can be seen that the effect of a ship begin -

Anyone can see the various types of flow that may occur, such as the stream of wa-

ning to move, is to start the liquid particles gradually moving, thus creating a flow whose dimension - "n" - is this time theo-

ter from a tap. A perfectly smooth flow (lam inar) over a length, L, then degrades and becomes uneven (turbulent) .

Quality of flows: Reynolds experiment

retically infinite . Within this flow, the variation in the gradient of velocity within the depth of moving fluid is simply the effect of viscosity, which causes shear stresses that counter the movement of the ship.

2.3

pressure . However, the complexity of flow inside this zone often means the analysis has to be simplified by imagining that it is not the ship that moves, but that sur-

The Reynolds num ber describes the ratio between the velocity forces and the friction forces . Thi s number is often used for hydrodynamics (or ae rod y namics) because the flow characteristi cs (lamina r or turbulent) have a m ajor effect on th e frictional resistance t he flui d exe rts on t he ship . The fluid/ side frict io n coefficie nt va ries significantly, depend ing on w hether th e flow is laminar or turbule nt. The value of this coefficie nt vari es in sign ificant propor-

are almost exclusi vel y tu rbul ent. The sig nificant angles of drift during the various movements of the ship, along with shallow The physicist, Reynolds, studied these different types, showing that a flow may change after a certain time, or rather at a certain distance from its source, allowing for the velocity of flow, its length and the temperature of the flu id. In other words, at a certain distance , the laminar flow undergoes a transition to turbulence (with vortices in which the direction of movement is constantly changing).

- - - - - - - - + L a mina r trajectory

rounded by its boundary layer like a shell, it remains immobile, immersed within a laminar flow moving at a speed equal to that of the ship . This process, which is theoretically limited to basic calculations of resistance to forward movement (with no drift or turning) does involve ignoring frictional resistances in favour of pressure resistances (potential theory).

= ---v

the length scales on ships show t hat th e Reynolds numbers (Re = V.L I v) associated with the flo ws t hat interest us are very large (Re ~ 109) an d thu s th ese flow s

Boundary layer

tres in air, for a streamlined ship . The boundary layer concept is very important for ships, particularly in terms of hydrodynamics. The speed of the flow must be theoretically evaluated within this zone to quantify the stresses linked to dynamic

Vx l

Re

tions from 1 to 4, in favour of lamin ar fl ow . From the point of v iew of th e sh ip , the flui d is more or less v iscou s and it is th erefore more or less easy t o turn in it. Low viscosity of water and air, as well as

The boundary layer is the interface zone between ship and fluid. It is defined as the zone of the fluid where viscosity effects are exerted, which in theory propagates to infinity. In fact, these effects (manifested by the drag of the fluid) very quickly diminish through a zone extending a few tens of centimetres in water, and a few millime-

By varying the parameters - V, velocity of flow; L, typica l length of flo w; and temperature (thus viscosity ) of th e fluid , Reynolds discovered t hat t he product (V x L)/v (where v re presents the kin ematic viscosity of th e f lu id ) was co nst ant. This constant was named th e Reynol ds number, "Re" . It is a dimension less quantity, which characterises the f low at every point on the hull , t hus identifyi ng its state (laminar or turbul ent) :

Laminar and turbulent trajectories

waters, use of engines for forward and reverse movement, and the direction of the helm in manoeuvring , all increase th e random nature of these flo ws, w hich tend t o be very turbulent and often cause stalling. By employing chaos theory, it can be seen that similar turbulent flows might develop very differently if even one of their initial conditions is varied very slightly. All of this shows the random nature of the various phenomena during a manoeuvre and the difficulty of modelling them if all magnitudes remain the same . At the moment, it is only possible to create a digital simu lation of the Navier-Stokes equation that has a large Reynolds number, by using vector supercomputers. It shows that turbulent flows are mainly three -dimensional and rotational. They are the cause of very intense variations in velocity and vorticity. Tests in the tank also show, especially in transition phases, that measurements are not 100% repeatable , meaning that there are random phenomena tha t ha ve t o be addressed from a statistical viewpoint.

117

3.

Frictional resistance

There are three things to notice in these diagrams:

The frictional resistance which a fluid flow exerts on a solid can be quantified by the formula:

1.

R = 1h.p.S.V 2 .Cf p: density of fluid (it varies slightly with the temperature of the fluid) S: surface wetted by the flow V: velocity of flow Cf: friction coefficient The friction coefficient and velocity are variables that are subject to change. Specifically, study of this resistance and the coefficient of friction "Cf" may be driven by an experimental approach. For instance, William Froude towed plates of an identical size, but with different levels of surface roughness to quantify different friction coefficients. In order to understand the boundary layer concept better, a flat plane may be immersed in a flow created by a wind tunnel. The flat plane is a useful study medium, since its slimness means that the stress exerted on it by the moving fluid can be assumed to be entirely due to friction. This flat plane is placed parallel to the flow . The only forces therefore exerted on the plate are frictional stresses. These stresses are studied by exploring the thickness of the boundary layer, measuring the speeds along a perpendicular to the plate (the capacity to slow down the air flow expresses the tangential stress of friction). A Pitot tube with a pressure probe is moved above the plate (1). The plate itself can also be moved forward or back so that measurements can be made from the leading edge to the trailing edge (2). Static pressure

2.

The turbulent boundary layer deepens much faster than the laminar boundary layer. Using the same image of the shell mentioned before, a flat plane seen from the point of view of the flow is much deeper when its boundary layer is turbulent, than when it is laminar. A hull experiencing this deepening effect will appear more bulky. It will therefore resist forward movement more strongly. The velocity gradients on the surfa ce of the plate are much steeper in tu rbulent flow than in laminar flow . Th is indicates that the tangential stress is much greater in turbulent than in laminar flow. This latter point explains why the turbulent friction coefficient is higher than the laminar friction coefficient.

3.

Pitot pressure

(

The velocity gradients inside the fluid, and thus frictional resistance, are greater to the rear of the plates bot h in laminar and in turbulent flow . This means friction is greater on the stern of the hull than on the bows.

1 Lom; 00 ,~ ~! 1 ~ ~.J---------------

owflow

Boundary layer

~(2)

Differential pressure = static pressure - Pitot pressure

The differential pressure measured is the dynamic pressure (1f2.p.V 2 ), which is representative of velocity. This assembly provides a way of exploring the whole of the boundary layer, through its depth and length, and measuring the velocity profiles for the various types of flow (the more the flow is slowed down leading to a greater variation in speed, the greater the frictional resistance). In order to make sure the boundary layer is really laminar, a flat plane with a bevelled and cutting leading edge is used (to minimise entry disturbance) and the air flow speed is kept low. The speed of the airflow is increased in order to obtain a turbulent boundary layer at the leading edge, and the leading edge of the plate is blunted so that the slightly rounded edge causes a disturbance locally. The result of these steps is as follows:

(1)

3.1

Shape effect

The extreme instability of the laminar

(1)

state in a flow must also be emphasised . Considering the water flow over a stream lined hull, for instance, the boundary layer is seen to behave as if it were expecting only one thing: the· transition to turbu -

(2)

118

lence, leading not only to a significant in crease in the friction coefficient, but also to an increase in the growing depth of th e boundary layer.

0

In fact, it is very difficult to measure the tangential frictional stress at every point on a hull. Many experiments have been carried out (towing plates with different surface roughness) to eva luate the friction coefficient. The ITTC 57 formula adopted by the test tanks is one solution used to quantify the average friction coefficient of a hull:

Actual and virtual hull of the ship

0.075 1.

z. 3.

Eddies Boundary layer Eddying

4. 5. 6.

Turbulent Laminar Transitory

In other words, when the limit layer changes, its frictional resistance while the flow itself perceives the hull around which it must pass as This thickening of the boundary layer, making the ship appear bulkier is called the shape effect . In a flow along a hull, boundary layers cohabit, passing rapidly from lent . The transition point is more or less close to the bows .

Cf

increases sharply, becoming thicker. than it actually is, laminar to turbu-

This formu la shows that the friction coefficient diminishes when the ship's speed or length increase . The frictional stress, however, increases with velocity and with the size of the ship, si nce it is linked to the square of this velocity and the friction surface area (wet surface). The friction coefficient is very low. Its value may be estimated at between 0.005 and 0 .001 for the ships we are looking at. The two images below show th at at 5 knots, the ship (300 metres long) seems to sit on the water, slipping along without forming waves or eddies . At 8 knots, the ship begins to pull the water along and form waves . The boundary layer merges into the wake from the bows.

Boundary layers cohabit In both theory and practice, variations in frictional stresses are often ignored in favour of other forces that come into effeet during manoeuvring . The shape effect

More specifically, the friction coefficient equally depends on the roughness of the solid. Especially in water, the ship handler should pay attention to the roughness of the hull- this varies according to the level

is included when calculating resistance to forward movement, as a coefficient k, which increases the value of the friction

of fouling and has a significant influence on the nature of the flow and the manoeuvring qualities of ships. It is estimated that

coefficient . The frictional resistance more generally becomes viscous resistance .

frictional resistance increases by 15% a year if the ship is not careened.

R,iscous

Transition flow along a plate Its position will depend on a number of

= (1 + k) ~riction

The ITTC 57 (International Towing Tank Conference) formulas, determ ined from experimental measurements carried out in towing tanks.

parameters (roughness, velocity, etc.). In the open sea, the velocity (Reynolds) means that the flow can be considered as almost completely turbulent. When manoeuvring, the problem is more complex and the boundary layers may cohabit. All the parameters are present to initiate transition, however: velocity, drift, roll, pitch, ram bow, uneven surfaces, roughness, undulation, vibration, etc. It is easy to understand why in port manoeuvring, every turn will encourage this transition, and the related increase in hull resistance. Frictional stresses increase, as well as the virtual width of the ship around which the flow must pass .

119

3.2

Fouling of the hull

Over time, the ship's hull becomes covered with seaweed and barnacles that increase its roughness . This fouling varies with conditions and navigation zones. A lengthy period spent in the harbour encourages its development. - A dirty hull increases turbulence and prevents the ship getting up speed normally. Empirically, an increase in roughness of 0. 1mm on the hull causes an increase of 11% in the hull res istance in the open sea . - Conversely, a clean hull slides through the water, is harder to stop and easier to accel erate. Constant improvements in antifouling paints reduce hull rough ness, w hich today, is normally less than 0.1mm . -

Similarly, fouling of the propeller and rudder reduces their effectiveness and clearly has a very significant impact on the turning qualities of the ships and safety of manoeuvres (six months' foul ing on a propeller reduces its efficiency by about 8%) .

3.3

Ageing of hull

Even after careening and sand-blasting of the hull, the performance of a new ship is never restored. The hull ages by gradual distortion and corrosion of its plates . It is difficult to determine a rule for quantifying the effects of this ageing. Tests carried out on different ships have nonetheless shown that after some years, an increased engine power of up to 30% may be necessary to achieve the original velocity in open sea . In conclusion , it is clear that the frictional stresses on the ship from air and water are complex . The viscosity and density characteristics of air lead to friction from air on the superstructures being ignored. On the other hand, water flow over the hull causes greater friction phenomena, and also increases the thickness of the boundary layer. In the forward shapes, flow is laminar, then quickly degrades towards the stern, until it becomes turbulent. Friction constraints increase with this change, and are therefore greater towards the stern than at the bo ws. These forces are always applied tangentially to the hull. The magnitude of frictional stress increases with the square of the velocity, the wetted surface area of the ship and variations in the friction coefficient. Fouling of the hull helps to increase this coefficient.

·•

Fouling of the hull

120

-

Comparison of the velocity in the open sea, or at manoeuvring speeds with t he same values measured in the bedding - in period, gives a simple way of assessi ng the effects of fouling. It is also possible to evaluate the qual ity of flow around the hull when the ship is underway, by observing the magnitude of the eddies in its wake . At manoeuvring speed, when the ship is not drifting, v iscous resistance - which includes fricti on and turbulence phenomena linked to t he boundary layer - form the main source of hull resistance. In large ships it represe nts about 80% of the hull resistance, R, at speeds under 8 knots. In practice, the frictional resistance m ay be considered to be equal on both sides of the ship, and so have no progressive effect on ship manoeuvring . It can be compared with a "braking" force. This force is very low compared to the ship's inertia, whi ch involves maintaining a low safe speed when circumstances allow, especially in good sailing conditions with reduced drift. On the other hand, the thickening and detachment of the limit layer which occu rs when the ship drifts, are more comp lex to understand. These phenom ena affect pressure resistance .

4.

4.1

Resistance to pressure

The stress caused by pressure exerted by a fluid flow on a solid of any shape may be studied by testing (in the wind tunnel or test tank) . For instance, the pressure resistance applied to the hull of a moving ship may be understood by imagining that the ship consists of an assembly of several hundred small, flat plates, all linked together like welded sheets, known as the meshing.

Continuity equations

Nature abho rs discontinuity. This is expressed by th e con tin uity and conservation equations . A law of conservatio n states t hat a particular, measurable pro perty of a physical system rema ins constant throughout the development of the system. The fundamental prin cip les of flu id dynam ics are the laws of conservation of mass, conservation of momentum and conservation of energ y. The conservation of energy equation takes a simple form whe n viscous frict ion can be ignored . It is expressed by Bernou ll i's law.

Mesh ing of a hull and an app endage A different pressure stress is applied to each of these plates, measurable with a sensor. The total resistance is calculated from the sum of all these stresses.

L.

.i . . .... •••••··· ......... .•••.1/

. ·.·at

Measuring pressure at different points of the hull by sensor A more theoretical study of flow over the ship's hull via distribution of pressure on all its plates is also possible by quantifying the static pressure for each of them linked to immersion, and dynamic pressure linked to the velocity of the fluid flow .

Correla tion bet ween velocity fields and pressure fields

This approach, to be accurate, must incorporate the complex phenomena of eddies linked to viscosity. Pressure resistance is the sum of all these stresses applied to

In water, for instance, hydrostatic pressure is given by the formula:

the surface of the hull. Using a simple approach it is therefore possible to try and study the physical phenomena that influence the speed of the flow over each of these plates.

Pat m: atmospheric pressure p: density of water g: gravity acceleration z: immersion depth

By definition, pressure is the force per unit of surface that a fluid exerts perpendicular to the surface of a solid . It is the sum of the static pressure caused by immersion and the dynamic pressure of the velocity of the fluid flow . For a moving fluid,

Dynamic pressure is added to static pressure. Its value is:

static pressure is the pressure that a sensor moving at the same speed as the fluid would measure .

CHAPTER 2 • PART 2.4 • RESISTANCE TO

Pstatic

= Patm + p.g.z

pdynamic

= 12 .p.V 1

2

V: velocity of fluid flow The physicist Bernoulli showed that in a flow, total pressure is constant:

Pstat

+ Pdyn

=Constant

121

4.2

Bernoulli law - Conservation of energy

Bernoulli's law states, that along a stream tube, total pressure Pt, flow rate Q, and total energy E, remain constant. Pt = Constant Q = Constant E = Constant This stream tube may be real , in the form of a pipe whose sections may vary. It may also be virtual as in the case of a hull . This gives a dummy section zone perpendicular to the flow, through which a constant flow passes . The stream tube consists of "streamlines " that the "waterlines" mark out . It is obvious that what enters the tube also leaves it. Neither more nor less.

In figure below, the phenomenon is amplified by the confined and shallow waters (sinkage), and it is clear to see that sta t ic pressure falls amidships . This is where the relative flow is moving fastest.

Pressure zones weakest amidships and stronger towards bow and stern.

A digital image shows the pressure fie lds along the hull distinguished by different colours. This fact is expressed as follows :

Q1=V1.A1 Q2 = v2 · A2

(Flow rate 1 = velocity multiplied by the area of the tube in 1) (Flow rate 2 = velocity multiplied by the area of the tube in 2)

The conservation of flow (Q =constant) implies: V 1.A 1 = V2'A 2 This means that if the cross-sectional area of the tube decreases, the velocity of the water streams must necessarily increase ; in other words, the ratio of velocities in 1 and in 2 is equal to the inverse ratio of the sections. Since total pressure Pt is the sum of static and dynamic pressures, and this value is

Digital image of pressure fields along the hull

constant along the length of the stream tube, the energy equation can therefore be written as follows :

4.3

To conclude, if the speed of flow increases locally inside the stream tube, dynamic pressure also increases and conversely, static pressure falls. Comparing the image of the stream tube with the shape of the ship, and imagining the ship as immobile and immersed in a frictionless flow, at velocity V (velocity of the ship) , we can deduce that the static pressures along the hull change with the waterlines and shapes of the hull itself. An impact pressure appears at the bows. As the hull broadens, the velocity of the flow increases and static pressure diminishes. Towards the stern, the flow slows and static pressure increases again.

Euler's theorem

Bernoulli's theorem is too limited in its scope . It does not provide a way of expressing the mechanical interactions that may appear between fluids and solids, for instance. Hence, a second theorem is needed. The following explanation uses a perfect fluid (non-compressible, non-viscou s), and a stationary flow . Euler's theorem is determined from th e fundamental relationship of the dynam ic (theorem of quantity of movement) :

P

is the quantity of movement in the

system and L FExt , the sum of the external forces applied to the system .

Proportion between speed and section

122

In a stream tube, a part of the fluid is rnarked out by a closed surface . The rep-

Wind tunnel experiments on solids of different shapes allow their coeffi cients of pressure resistance to be identified. They clearly show the impact on pressure resist ance of the

resentation of this surface is used to define the system. Calculating the variation in the quantity of movement between mornents t and t + dt shows that :

shape a solid presents to a flow.

-~

-)

qrn is the mass flow rate of the fluid (that is, the mass of the fluid that passed through section 51 and 52 during the period dt). ii;_ and

\i;_

c

Shape of the solid

are the average velociti es in -

Flat disk

~

1,1

Concave half-sphere

- C> 0

Sha pe of the so lid

~ Loog

1,4 -

Sphere

Cy lind rica l bar

d I

plote 1/d = 30

c 1a 0,35 0,8

a 0,66

0,3

~ Streamlined 0,05 aerod y nam ic bod y

I

l o og plote 1/d = 8

0,1

sections 5 1 and 5 2 . i: FExt represents th e volume forces (weight) and all the surface forces (pres-

Coefficient of penetration in the air C

sure). This theorem clearly ex presses that th e force exerted by a fluid on a solid is equal to the variation in the quantity of movernent per unit of time and that the beam of the ship, which determines the variation in the stream tube section, is a determining factor in calculating hull resistances , since

For instance, when the flat plate is perpendi cular to the fl ow, in t heory it ha s no surface subject to friction and is therefore free of frictional resistan ce. However, it obstructs the flow, which then has to divert in orde r to avo id th e obstacle. The variation in quantity of movement linked to the slo w ing of th e fl ow causes dyna mic overpressure on the front surface. In the wake of the plate, th e m ass detachm ent of th e fluid , which no longer adheres to the wall of the plate, causes ed di es t hat are syno nymous with low pressure . The overpressure to the front and low press ure to t he st ern

it has a direct influence on speed of flow (see Bernoulli) .

4.4

Demonstration of pressure resistance

Let us try to consider the impact of a flow on a flat plate, using a simple approach. The pressure stress a moving fluid exerts on a solid may be calculated by:

thus contribute to the creation of pressure resistance.

:j @ Viscous fluid, detachments with stable recirculation on a disk

Rp

= 1f2 p S V

2

Cp

Rp: pressure resistance 5: surface area the ship presents to the flow (LPP x draft of vessel Te) Cp: coefficient of pressure resistance p: density of flu id For a given fluid and ship, pressure resist-

In absolute values, this resistance is considerably higher th an the frictional resistance mentioned above . Experience shows that because of viscosity, the discontinu it y surfaces are unstable. The zone to the rear of the plate becomes a fa irly large recirculation zon e (figure above) or eddy zone (figure below). The counter-pressure loses its uniformity (b), leading to fairl y high values for t he pressure resistance coefficient (Cp) (the stern counter-pressure may in some circumstances

ance mainly varies with V 2 and Cp .

be weaker than the ambient pressure P) . Experience shows that it is the geometry of the plate and the velo city of the flow (Reynolds) which determine the nature of the downstream flow and thus the pressure

The hydrodynamic (or aerodynamic) re-

resistance coefficient. For instance, this pressure may vary from singl e to do ubl e ( 1. 1 to 2 .01) between a square and a lengthened rectangle with the equi valent surface area .

sistance of a solid does not depend only on its width, but more generally on its shape , which affects the coefficient of pressure resistance (Cp). In fact, the distribution of velocities and therefore of pressures along the solid and then in its wake, is

Unstable detachment

determined by the profile of the solid and its capacity to counter and interfere with the flow . For instance, when comparing a flat solid , perpendicular to a flow, with a sphere or water droplet shape, it can be seen that, for an initial given velocity of flow, the sphere offers 40% of the resistance of the flat solid, and t he water drop let only 5% .

CHAPTER 2 • PART 2.4 • RESISTANCE

Viscous fluids, detachments with unstable recirculation

Pressure stresses are by definition perpendicular to the surfaces exposed to the flow. They depend on the square of the velocity of flow, but also on the shape the solid presents to the flow, and its capacity to slow it down, creating fairly active eddies in its wake. The discontinuity (or detachment) surface will be fairly stable with a stern zone offering recirculation (for the square), or rather unstable with an eddy zone (for the lengthened rectangle) . The increase in speed , or more precisely the Reynolds number linked to the flow, also encourages instability in the downstream flow. Nonetheless, in manoeuvring, this variation is fairly low compared to the variations in shape the ship presents to the water or air flow (apparent wind) when it drifts or turns. So, in terms of the distribution of pressures towards the bows, or distribution of counterpressures to the stern, the shape of the ship, its squat, velocity, trajectory and thus direction of flow, are the main factors in the difference in pressure stress coefficients observed from traction experiments in drift and turn carried out in the test tank . It should be noted again that, for the flat plate perpendicular to the flow, detachment bounda ri es are stable since they match the plate's geometric boundaries, unlike rounded bodies such as hulls which, because there are no edges, have moving detachment boundaries that are difficult to define . Laminar flow

Positive Z

sure

\ Negative pressure

Deta chments with eddies When the ship drifts, for instance, wa-

These comments already reveal the difficulty of representing the evaluation of the coefficient of hydrodynamic pressure resistance for a drifting ship whose hu ll is confined within port waters, or that of t he coefficient of aerodynamic pressure for a ship exposed to wind effect . While it seems possible to calculate t he pressure exerted on surfaces the ship presents to the flow, it is much harder on t he other hand to quantify the effects of detachment and eddies which develop in its wake. Wind tunnel experiments show, for instance, that the aerodynamic coeffici ents on a ship are affected by the variation s in surface area that the ship presents t o a flow. When the ship is light, the aerodynamic coefficients are actually often slightly smaller than when it is loaded (the lig ht ship is similar to the square, and the laden ship similar to the elongated rectangle ). Other laboratory measurements show t he effect of the underkeel clearance on t he hull resistance . When the clearance reduces, the velocity of the flow increa ses, along with hull resistance. It must also be emphasised that pre ssure stress coefficients are much greater (around 30 to 150 times greater, depe nding on the angle of incidence of the fl ow)

ter flows along the hull, but also passes beneath the keel, forming turbulence. Nonetheless, the detachment zones where the eddies form, are concentrated mainly near the bows, to the rear of the keel and the appendages 1 l . The figure right shows a drifting hull, with the overpressure ap-

than friction coefficients (Cf = 0.003; Cp = 1.1 approx for a squ are plate and Cf = 0.0048; Cp = 0.16 for a streamlined hull underway forward); th is is why friction coefficients are often neglected (perfect fluid) in calculations .

plied to on the port bow (red) and a low pressure area on the starboard quarter

In manoeuvring, under the effect of apparent wind in the air, or the effects of drift, variations in trim, and draft for wa-

(green) .

Water flows around a drifting ship

ter, the ship's profile that is presented to the air and water can vary considerab ly. Coefficients and surface areas to be considered when calculating pressure res istances therefore also vary hugely. It is very difficult, and often expensive, to simulate all loading configurations for different types of ship in the test tank or wind tunnel. The available coefficients are thus often only representative of standard configurations (light and laden), and

1 lThe deepening of the boundary layer, leading to increased friction, sometimes improves the profile of a moving object and limits the eddies in its wake while generally reducing its pressure and forward resistance. The cells on the surface of a golf ball give an example of this phenomenon.

124

in order to evaluate the stresses that water and wind actually exert on the ship, it is necessary to know how to interpol ate these (for programming simulators, for example).

5.

6.

Wave resistance

Hull resistance

AS a ship passes through the water, it puts liquid particles into motion over a wide area Hull resistances are created by all wave, pressure and frictional stresses exerted on around the hull. The ship transfers energy to the water from friction forces and especially from pressure the ship's hull by the water, in countering forces developing in contact with the hull. The ship pushes the water aside with its bows. the ship's movement . The water then passes around the hull , accelerating along the gunwale. In terms of tests, these va rious forces are Near the stern, the velocity falls and the water pushes the ship. isolated as far as possible to understand This driving phenomenon becomes particularly noticeable on the surface of the sea, the phenomena better, but it is clear that where waves similar in appearance to those created by the wind form. they are actually intimately linked . The energy that has built to form these waves increases with speed, and in the open sea Resultan t of friction forms the main component of hull resistance. The ship therefore forms a wave system, and pressure forces which when it is moving uniformly, seems fixed with respect to the ship, hence its name of accompanying waves . Eddies The size of the wave system depends on the velocity and type of ship . It operates in the same way for all ships, and has been the subject of several scientific studies, including those by Lord Kelvin. Hull resis tances

The particular case of a ship following a straight trajectory at a constant speed

Modelling a wave system

It consists of 2 different systems: - A first field of divergent waves forms, mainly at the bow, but also astern . It is at an angle of 19° 28' either side of the ship's heading. - A second field of transverse waves appears within the first, forming an angle of 54° 44' either side of the ship's heading. At the edge of the wave fields, the two systems meet and propagate in the same direction . -

Ripples form along the hull. They begin ahead of the ship, with a fairly prominent crest which breaks easily: the bow wave . The distance between 2 ripples along the hull increases with the Froude number : F

=

V

Wave system on a ship underway

v(g.L)

.•~,-

..'.. :r... ;;·

~.-

•• ~ ..~

.. 1. - ,._-

--

t04rv r;o

.c:.-r---...

1,0

/

0,8

:~~fJ( rr~

fn,-, I~

0,05

lL

~~

' VV I

0,6

I

0,4

6o 80 100 120 140 160 15o•

Longitudinal coefficient Cx

0,1

1,2

0,2 20 40

0,15

1,4

ec '

V

-0,05

r\

'-~

car go

" 0-

J "~

-0,15

e

\

~~

V\ ith ~ec ea g6- ~

-0,1

1\

V

-

+ u•

lf

t

r·

,?/

-0,20

.~

Overall transverse coefficient Cy

Coefficien t of turning moments Cn

Coefficients of aerodynamic drag of the container ship

2.2.3 Ferry or liner curves It can be seen that between 30° and 60°

the Cx is positive, which means that the ship behaves like a sailing vessel. The superstructure becomes propulsive . The

curve of the coefficient Cy also shows that the transverse force exerted by the wind, which causes drift, is at its maximum from 50° and not across the wind, as might be expected. Observing the yaw moment Cn,

Cn

Cy

1,6

"

0,20

1,4

0,15

1,2

0,1

1,0

I

0,6 0,4

"0 •8 f--2--'-0-4-'-0----'6-0-8-'-0-10._0_1_..2_0_1.._40----'16'--0--'180° -1,0

Longitudina l coefficient Cx

0,2

V

1\

-0,05

\

I

11

-0 ,1

L

\

8 0

Overall transverse coefficient Cy

Coefficients of aerodynamic drag of the ferry or liner

144

0,05

V"- v r--- r-- h

0,8

-0,6f--l--+--f-+--+---+-+--+---!

it can be seen that the effect of the sh ip's sway is strongest when the ship makes an angle of 45° to the wind. When it is full across, the ship only drifts.

~

-0,15

'" t'""\

1/ _j_

V

+

~

t

"' '\ 1\..Y

/

-0,20

Coefficient of turning moments Cn

I~

-

3

Influence of superstructures

observation of the surface as well as the shape of the superstructures that the ship presents to the wind is a determining factor in evaluating the coefficient to be used when calculating the force exerted on the superstructure . The graphs of aerodynamic coefficients have been plotted using studies of typical ships in the

-

-

wind tunnel. The following comments may be considered in practice to further refine the evaluation of the coefficients of a given ship : _ The ship superstructures often have many sharp edges. The consequent aerodynamic flows are therefore separated and excessively turbulent. Some particularly well streamlined ships (liners, ferries) are however locally subject to particular effects, to flow acceleration effects creating a force similar to the lift exerted on a streamlined sail (figure A) . The effect of wind on the ship is increased . For these ships, the coefficient Cy often reaches its maximum value at an angle of less than 90° (see curve right below). _ The cells that form "windtraps" must also be considered .

-

The position of these cells on the longitu dinal axis is also important, because they may affect th e yaw moment of the aerodynamic stress. The further these cell s are from the middle of the ship, the greater is their influ ence on the yaw moment. The representative aerodynami c coeffi cients of the forces exerted on the ship are linked to the reference surfaces used in wind tunnel tests (figure C) . For a light ship, these coefficie nt s are generall y slightl y weaker than for the same, loaded shi p. Conversely, the light ship clearly presents a greater surface area t o the wind (figure D) . For container ships in particular, the pos ition of the ship's su perstructure and the su rfaces which loading exposes to the wind with respect to th e centre of grav ity sign ifican t ly affects the yaw moment of the aerod y na mic force (e .g . : aft superstructure and deck cargo partl y loaded , shifted t owards the bows or stern) .

They increase the aerodynamic coefficient . They are created for instance between containers when the ship is only partially loaded, or by cabin walkways on liners (figure B) . The walls of large modern liners are generally covered with balconies, which form cells . They have a lateral aerodynamic coefficient Cy which can reach 1.4 .

0

-

0

. . . . .....

... ............. .... ·-lilllll!lll- !i!. ············· ······· -·· . -==-"""'

·-··

~-

11

mmmmll l Ro -ro ship with strea mlin ed shape

Cells housing walkways and lifeboats on a liner

Windtunn el test

Light ship

145

3.1

Example of aerodynamic coefficient Cy weighting

The lateral surface of the ship is divided into several basic zones, for which the aerodynamic coefficient Cy can be estimated using laboratory experiments on the reference shapes:

Coefficient Cy for each basic surface Distribution of zones into surfaces:

Zone

Percentage of total lateral surface

Shape

Coefficient Cy 0 .5

Zone 1

10%

cylinder

Zone 2

40%

cells (walkway)

1.4

Zone 3

10%

long plates (lifeboats)

0.8

Zone 4

40%

flat (free-board)

1.1

Total surface area

100%

I

1.13

3.2

Simplified formula

The transverse aerodynamic coefficient of ships lies between (0.8 :5 Cy :5 1.4) Using

1,5

+-----,•""

~

lllf l J 11 11

~

101 l J 11 11

)>

)>

~

llll 11 11 11

)>

__., Max -

------7-----------l_~~ ~ Min ----~----------~----------~

-. o

---------------;---~~ Max------~----------~~~

--M in

Helm hard over and engine running ahead ~ Min

~ Min -----~~----------~-~~ Max ----~----------~--~~

--------i--.--Min

When the overpressure zones meet, the lighter ship moves away from the heavier one, and when the low pressure zones merge, the ships are drawn towards each other. The velocity of the overtaking ship drops

F:

sharply ( max) when the overtaken ship lurches ( Mz) towards the overtaking

2.4.3 Example from real life Ship B, after agreement with ship A, begins to overtake the latter. The river is not excessively large . The block factor is over 3.

t

1111 I J I I

----~----------r-~~~ 'Max ----~----------~~ Vs --------1----i--~ Min -- Vs

Fx

Mz

~

~O

-----~----------~--4~~Max

ship, in what may be a violent and difficult to control event (4th column) . It should also be noted that the block factor of ships that are crossing or overtaking each other increases (for the same volume of water, the sum of the volume of the two ships is much greater than the volume of each one of them).

This means that the squat of th e two ships will be greater than if they were alone . It is only possible, therefore , to ove rtake in a river at slo w speed , in t he wi dest part of the channel, in order t o li mit t he effect of the interactions .

The low under-keel clearance of the ships causes a significant squat effect. The overtaking manoeuvre begins comfortably, with ship B travelling faster than ship A. The ships are balanced,

Ship A loses control when ship B passes the bow of ship A. The latter can no longer steer once its bow is in the low pressure zone left by ship B. The hydrodynamic effects mean that ship A is sucked towards the stern of ship B.

despite the hydrodynamic forces present, and they can still control their course.

Given their respective tonnage, the low under-keel clearance and the configuration of the river, a collision is inevitable . It is interesting to note that the recording of the speeds of the two ships complies with the theoretical concepts ex plained

0,1 mille

above . Just before the collision the speed of ship B dropped significantly, while that of ship A increased, without a command being given to increase engine speed. Initial positions of A and 8 Vessel A •

Ship Ship

Vessel B •

B-

18:56

Kinemati cs of A and 8

CHAPTER

18 :58

)(

A-

......__

19:00

Record of speeds of A and 8

-

10

.___

6

Knots

19:0219:02:30

__ o

3

Manoeuvring in the current

The basic manoeuvring principles described in this section are only intended to highlight the effects of the current on the hull. Th e simple comments made must not conceal the problems involved, especially the scale of the forces that the current exerts on the vessel. For obvious safety reasons, actions from the bridge and within the manoeuvring ranges must be perfectly coordinated. Only the most basic manoeuvres are described (without a tugboat or bow thruster). A shiphandler may, in certa in circumstances, use the cu rrent to assist a manoeuvre if prepared . This may be of help since the force of the current is regular and known at a specific time at a particular place. For instance, the ship handler may: -

-

Angle the ship into the current, and create a transverse current that is very helpful when coming alongside or moving off When turning, the pivoting moments generated by the difference in speed of the current between slack water zones (zones with no current) and the current flow itself can be beneficial.

This simple approach shows that this force, F , mainly varies with the square of the speed W of current, with its angle of incidence i and a coefficient K representing the density of the fluid, above all the shape of the hull and its capacity to oppose the flow of water. The length of the ship, its trim and draft, obviously have to be included in the calculations. Intuitively, it also appears that the hull confinement, linked to the crosssection of the ship and the cross-section of the current flow, affects the speed of water (Bernoulli) and therefore also the force F . The point of application C of the force

3.1

- Effects of current on hull

current on the hull. Breaking down these forces along th e ship's axes shows that the ship moves forward or back, drifts and turns. Tank experiments on models representa tive of typical ships (crude carrier, con tainer ship, etc.) provide more accurate figures using dimensionless coefficients, longitudinal, transverse and yaw moment forces, for a given draft and depth . Studying these experiments improves understanding of the phenomenon a shiphandler must anticipate .

Cx

Cn 20

0,4

40

60

80 100 120 140 160 180

0,4

~

20

40

60

80 100 120 140 160 180

0,2

0,2

/

0

V

"

V

/

"""'

ao

0

"""~'---

r-- -....._,

/

/

~

...

-0,2

-0,2 -0,4

-0,4

Cn of yaw moment

Cx longitudinal

ao

0 -0,2

As a first approximation, and reasoning within a reference base linked to the ship's centre of gravity, the force the current exerts on the hull may be understood with reference to thin plane theory.

F

on the hull is also important,

since it significantly affects its yaw mo ment with respect to the centre of gravity. There are therefore many parameters t o consider when assessing the effects of

"""

-0,6

/

/

-.......

"'-...

/

'

-1, 0

_..-/

Crude carrier type hull, with a depth-to-draft ratio of 3 (depth-to-draft= 3).

-1,4 -1,8

Cy total transverse

Cy Cn Cx

-

20 40

0,4

0,2 ~I

fluid W ---..

-------- /

B

0

Thin plane

This simple approach shows that this force, F , mainly varies with the square of the speed W of current, with its angle of incidence i and a coefficient K representing the density of the fluid, above all the shape of the hull and its capacity to oppose the flow of water.

v ,...\

-0,2

The hull forces the flow of water to change, experiencing a force from the water whose inte nsity and point of application vary with the current's velocity and angle of incidence. F = K X S X W2 X sin i

\

1// \

/

\

(1 0

I

-0,2

~

-0,4

-2,8

\

\

/

/

\ "'-...

/

/

-3,6

Cy

40

80 100 120 140 160 180

60

V / /

1\ \

/ V Cn of yaw moment

ao

0

-2,0

20

0

I

ex longitudinal

-1,2

~

0,2

v-,

-0,4

-0,4

0,4

80 100 120140 160 180

60

Cy total transverse

..___/

/ Crude carrier type hull with depth-to-draft = 1. 05.

L b.. \

\

....

The diagrams (figures left) may be used as reference for ships with the same shape but of different sizes. They highlight a number of points : 1. Confinement with the depth-to-draft ratio has a major influence on the forces the current exerts on the hull : when this ratio is divided by three, the longitudinal and transverse forces are multiplied by approx imately three . 2. The transverse forces, assessed using the coefficient Cy, are the most significant, following an almost sinusoidal curve (in accordance with the simplified thin plane theory) . 3. Longitudinal forces are much less significant (about ten times), but harder to predict. Apart from confinement, the shape of the hull, especially the bow, influences these forces greatly. It is not always as easy to control longitudinal speed as might be thought. 4. The yaw moment varies depending on whether the ship is heading into or moving with the current . The turning

5.

effect appears greater when the current is flowing from behind . In the example given in figure 34, the positions of equilibrium are reached when the coefficient Cn = 0, that is at an angle of flow of the current at 0°, 80° and 180° . The ship then drifts in the direction of the current .

Quantifying these forces, using the equations given in the chapter on complementary aspects on ship hydrodynamics (Fx = V2.p.V 2.Lpp .d .Cx, etc.), shows their

3.2

Coming alongside against the current

This is often described as the easiest and safest manoeuvre . The current slows the ship down . It is thus easier to control th e steering and t he inerti a. The ship-handler keeps the speed very low, mainly using the engi ne in fo r ward motio n. He/she may also create a transverse movement, by increa sing or decreasin g t he angl e of incidence to the current, and thus increasing or decreasing th e speed of approach to the wharf. This is known as a "docking" angle. The turning effect , however, must always be borne in mind, as this tends to make the ship move across th e current. This effect is controlled by gradually reducing the angle of in cid ence close to the dockside . Generally speaking, the first mooring lines passed t o t he quay are the bow spring and the stern spring.

z

....

z

Coming alongside against the current

3.3

Moving off against the current

The ship may move off and away from the quayside by allowing the current to flow between the quay and the hull. In general , the stern spring is used to "open up" . The stern mooring line may be hauled in to start the movement . The force the current exerts on the ship combines drift and a turning outwards .

The stern spring compen sates fo r t he lon gitudinal component of t his force and also causes a turning moment th at assist s th e manoeu v re . Th e engin e is run in fo rward motion, to redu ce the fo rce appli ed t o th e stern spring. The ship m oves off at an an gle of incidence t o the current that allows transverse movement seaward . This is known as an "underway " angle .

significance (several tens of tonnes) . The effect of the current also quickly exceeds that of the effect of the wind, especially for loaded ships .

100 m -

emit regulatory audible signals, in the event of poor visibility (Morse letter D )

Fog signals Anchor a-cockbi/1 Anchor stowed The following operations have to be car-

-

the front panel displays on the bridge the heading for anchoring and the number of shack les paid out (normally

take a bearing at the time the anchor is dropped (bearings with optical sights, radar, MOB on GPS).

ried out for each item of the anchoring gear: - prepare the fire pump; draining via hawse pipe - check that the brake is applied

-

-

-

-

turn on and test the windlass (sprockets and winch drum engaged) open the hawse pipe and chain locker

-

covers engage sprocket and release brake

-

-

-

let go the anchors by releasing stopper, so that the anchor tension is taken up by the sprocket release brake and let out the anchor

-

th e warp required apply the brake, swing the stopper into a link of the cha in, release the mooring line until the stopper takes up the tension again, then release the windlass . The stopper locks the chain and secures the mooring . The ship could hold on the brake alone while at anchor in good weather

one metre to check it is not jammed

-

two commanded , then each shackle in succession passing through the wind lass and into th e water) show the moment the ship turns into wind (the ship comes to a st andstill, then pivots to tu rn into wind or current) pay out any further shackle lengths for

in the hawse pipe shaft (check must be carried out especially after a long crossing in bad weather) lower the anchor to water level (only if the sea state allows) tighten the brake and engage the sprocket : the ship is ready to anchor

-

MOB = WP 500

inform the en gine of the wait period before moving off (e ngines warmi ng up or longer movi ng off time). If bad weather is forecast, the engine needs a short start- up ti me so the anchor can be lifted if it drifts notify local port au t horities of your position

199

4.8

Monitoring anchors

It is essential to monitor the ship at its anchorage . According to regulations, although the engine is not ready, the bridge watch is

organised as at sea. The watch officer's job is to: - make sure th e ship remains well within its t urning circle - complete the anchoring log with regular fixes - observe any deterioration in the weather - ensure continuous visual and auditory watch (monitoring water surface, traffic, safety, communications, weather, etc.) -

make sure that the foredeck helmsman makes regular inspections

4 .8. 1 Monitoring methods The most effective means for checking whether the ship is drifting are : -

mark out the turning circle with distances from radar guards and guard bearings

Distances and guard bearings -

Optical bearings

set paramet ers f or GPS alarm defining th e radius of the turning circle within w hich the ship must stay. Th e radiu s includes the length of the warp plus the distance from hawse pipe to GPS antenna (one ship's length if antenna is on aft superst ruct ure)

Anchor a la rm: Anchor alarm alerts th e operator when the anchor has broken or is dragging . If the vessel drifts more the distance defined by the Radius field , th e alarm will sou nd . Radius: Select the radius large enough t o account for : 1. normal drift about the ancho r 2. 3.

the lenght between the bow/ antenna position the nor mal fluctuation in GPS position.

GPS alarm on turning circle

Rad ius: Calculation of turning circle

-

put the anchor point and turning

-

circle on the electronic chart, add ing the alarms to the safety circle the limits of the shallows over which the ship must not cross , (alarms on shallows only operate with ENC digital charts) set sounding alarm to a depth the ship must not enter

Alarms on Ecdis

200

Alarm on depth sounder

4.8.2 Risks of dragging the anchor When the wind freshens and becomes gusty, the ship swings at the mooring and exerts force on its anchor that is no longer constant.

@et@

,J fJ

/

,"I",I""II

I I I

I

~--------------,1'/

I

/:

f"\~~:)

c~ ::.__ ../ _,/ ~\

0

\, \.® \.)

CD A ship dragging its anchor

The chain tightens (1) and relaxes and the ship begins to oscillate (2) around its anchor point. The changes in dynamic forces exerted on the mooring line (3) may unhook the anchor. When the latter is no longer buried, the ship turns across the wind and sea . The anchor line is dragged and its weight can no longer hold the ship . The ship drags (4). This phenomenon can be anticipated by monitoring the ship's behaviour.

Visual elements for observing whether the ship is dragging: - the chain tightens and relaxes abruptly and jerkily (oscillations at the hawse pipe outlet, which causes vibrations that can be felt on the ship) - the ship comes to its neutral position -

When the ship begins to swing wide around its mooring point w ith the effect of a rising wind, it must be stabilised w ith its head to the wind, before it begins to drift. The responses should be : - increased monitoring - lengthening the warp (the weight of the chain on the bottom provides better horizontal traction on the anchor, to keep it buried; friction caused by the chain on the bottom only slightly assists in holding the mooring)

(across the wind) the ship leaves its turning zone (anchor alarm triggered) the ship may also drift with the current or swell as it increases . -

dropping second anchor straight down keeping the engine ready to go in case the ship has to get underway or in extreme cases start the engine moving forward very slowly to relieve the anchorage if no decision has been made to move off.

201

4.9

Specific issue of VLCCs

Given the mass and therefore the inertia of these ships, traction forces on the mooring line and its gear are very high. When approaching and anchoring, the inertia of a supertanker is such that the following points must be respected: - the anchor is released when almost stopped (making sternway or drift at less than 0.4 knots)

-

-

-

if circumstances allow (fine weather, slight current, etc.) the chain is released by the windlass then laid on the bottom while the engine is running slowly in reverse otherwise, place anchor a-cockbill to reduce strain on the brake and gradually tighten the chain the chain is attached to a stopper not the brake, regardless of the wind con-

-

-

special care is required for monitorin g at the anchorage, in most cases because the ship is in shallow water the windlass power is not always enough to set the ship moving an d raise the anchor. It may be necessary to start the engine in forward motion t o bring the anchor almost upright

ditions

VLCC at anchor

4.10

Moving off using one anchor

There are no particular problems to deal with when moving off, if the exit route is downwind. The sequence reverses that for anchoring, namely: - pull down the mooring mark and switch - arrange the chain washing system - weigh the anchor on command, with - engage the sprocket to take up the tenship's heading aligned with the moorto navigation lights if necessary when the anchor is tripped, sion on the mooring line and release ing line - apply brake, release sprocket and prethe brake or lift the stopper - inform the bridge of the direction on pare anchors for mooring again while - provide enough time to haul in the which the anchor is being brought up, passing through the channel mooring line, according to the de- using the engine running forward very slowly (windlass braked) if necessary to relieve traction or pull the anchor from

parture time (about 3 minutes per shackle length) (Before the pilot arrives to assist the ship, he/she normally asks for a number of shackles to be hauled in already)

-

its bed haul in anchor ashort, apeak, tripped until it is sighted and clear stow the anchor

-

-

engage stoppers, attach anchors and isolate the windlass once the ship ha s left the confined waters open the hawse pipe and chain locker covers notify local port authorities of your intentions

If the exit heading is not that on which the anchor is drawn in, it is hauled until it is ashort in the line of wind or current. Using engine and helm, move close to the desired heading, taking care not to allow the chain to pass over the bulb, then haul in until the anchor trips. In exceptional cases, the anchor may be held to the bottom by one "hook" - an anchor held in an uneven rocky surface or a hard silt bed, after a long period at the mooring, etc ... After stopping on several links, the ship may force around the anchor to try to release it. This can be a very tricky operation. 1.

Command to haul in until

2.

anchor trips Anchor ashort

202

3. 4. 5.

Anchor apeak Anchor slipped Anchor sighted and clear

If it fails, the ship may be forced to separate from its anchor by cutting a link with a welding torch.

5

Glossary

Anchoring Shackle length Wind rode Let out Warp Radius of turn Draw in Lifting anchor short A peak Tripped Sighted and clear Drag

: : : : : : : : : :

: : : Lash Cause to drag : : Drift Chain round the bow : Caught : Anchor a-cockbill : : Release hook Anchor stowed :

CHAPTER 3 · PART

dropping the anchor and letting out the necessary warp length length of chain 27.50m (15 fathoms) Valu e round ed to 30m the anchor has hooked and the ship is head to wind lower anchor using windlass length of total warp paid out for anchoring total length of warp + length of forward deck I bridge (radar antenna and/o r GPS) lifting anchor with windlass heaving to keep just enough chain to hold the ship before tripping the anchor to move off the chain is vertical, with only the anchor still being hooked the anchor has been pulled off the bottom the anchor is at the surface of the water and not in use the anchor is not fouled on the bottom and is lifting along its shear plane drop anchor dow nwind to hold the ship when subject t o w ind drift deliberate action to draw the anchor along the bottom the anchor moves on the bottom without hooking the chain passes above the bulb anchor jammed and held by an obstruction which prevents it being raise to anchor in depths between 20m and SOm, first releasing the anchor at the windl ass hook that can be opened at a distance to release the end of the mooring line in an emergency anchor in place in the fitting in the hawse pipe

203

Man overboard manoeuvres

1

Recovery conditions

Although the man overboard manoeuvre is not specifically a port manoeuvre, it must be performed without delay by the watch officer. Aside from the manoeuvring aspect itself, a man overboard situation is a major event requiring emergency actions with which the whole crew must be familiar. Regular training must take place in specific procedures to prepare a capable response team and ensure that the equipment is working correctly, particularly in order to launch the casualty recovery lifeboat, called the rescue vessel. This chapter covers the appropriate ma noeuvres which, in terms of emergency procedures, refer to the ship's capac ity to come to a standstill (See chapter, "Manoeuvres for stopping the ship").

2

Man overboard observed at once

If the watch officer observes someone go overboard or if th e in cident is re ported t o t he bridge, he/she must respond at once, implementing a pre-set proced ure acco rding to the bridge file. This stressful situation must not lead to uncoordinat ed acti ons that coul d prolong the time taken to recover the victim (particularly t he choice of t he direction of turn according to wind direction, and the side on which the rescue vessel is stored) .

2.1

Planned actions

At the time the fall is observed, the watch officer must ap ply the foll owing proce du re , in sequence: - Put the rudder hard over to the side on which the fall occurred, in ord er to com e around the casualty from the rear (even if the manoeuvre is delayed, w ith the ship moving at speed so that it is in fact a long time since the person in t he water slipped past the side of the ship, thi s move is still necessary in order to si gnal th at th e fall has been observed) . - Launch the man overboard marker devices downwind , - including the "man overboard " buoy, with a self-righting light and smoke generator. The SART* * is also launched to monitor the position on rada r - switch the radar adjusted to relative mode with the Real Move m ent (Trails) "RM(T)" function .

Survival conditions for a person in the sea, if still conscious after falling from the side of a ship, a drop of over ten metres, depend mainly on the temperature of the water* , and the physical and mental condition of the casualty. With few exceptions , the risk of drowning for a person overboard is real. This is related to the victim's exhaustion and to hypothermia . Time is of the essence and this is the reason for the urgent response of the crew to recover the casualty as quickly

Smoke generator and lifebuoy

-

as possible. Whatever the conditions of their fall and the temperature, someone who has fallen in the water is in a state of shock . There are two possible man overboard situations: - the fall is observed, and action is taken at once to rescue the victim -

the time the person fell is unknown, and actions are reflective (searches on board, coordinated actions)

-

-

Remote lifebuoy launcher

fix the position on the GPS or Ecdis by pressing the MOB key *** . The GPS and Ecdis displays switch to "auto- course " mode, showing the heading and distance between the ship and the man overboard activate the general "Man Overboard" throughout the ship choose the best manoeuvre to bring the ship upwind of the victim, with the rescue vessel downwind reduce speed and make the engine "ready to manoeuvre"

-

organise t he crew mem bers reporting to th e bridg e so as not lose sight of the person in t he water send a radio di stress call on VHF 16 (if other ships are in the immediate v icin ity of the victim and can help with th e

-

rescue) hang th e rescue vessel ove r t he si de, above the water stop upwind of the v ictim , pu t t he rescue vessel in th e water put the crew on board and send t he raft to recover the v ictim enter the incident in the ship 's log

-

*IMO IAMSAR manual. In theory, for a water temperature below l0°C, survival could only be a few hours.

When the symbol is located in the middle of the center lane, the vessel is on course , and XTE is near zero ; R:Right ; L ; Left.

**SART : Search and rescue transpond-

When the arrow points toward the next waypoint, the vessel is headed in the correct direction (COG = CTW)

er which emits a signal as concentric circles, detectable by radar when the radar receiver is close to the beacon . The echo in the centre of twelve circles is the primary echo of the beacon.

***Pressing the Man Overboard, MOB, key records the present position at WP 500.

SART beacon

---+-----

WPT 500 : The next waypoint in the active route is identified by number and name. The relative velocity of the vessel is represented by the rate of advance of the horizontal lines located outside of the center lane. The arrow represents the current heading relative to the desti

COG : Course Over Ground in the graphic, is based on active route and current leg

GPS screen in '"'auto-course " mode

2.2

Launch of lifeboat

Ships above a certain size must have a lifeboat specifically for man overboard recovery. Depending on the ship's layout, the rescue vessel is stored either on a clear deck, which allows it to be launched either side by a derrick, or on port or starboard. The lifeboat must always be launched downwind and sheltered from swell. After its half-turn manoeuvre, the ship's final heading must bring it upwind of the person in the water to protect them, but the rescue vessel must be launched downwind to make it easier. These two requirements combined determine the side upon which the ship must begin its half-turn manoeuvre.

The turn is the fastest option, whatever type of vessel is involved. Nonetheless, it does require the person in the water t o be kept in sight (the lifebuoy drifts faster than the person in the water whose bod y underwater acts as a sea anchor* ). Turning hard to one side helps to slo w the ship down (See the effect the ship 's speed has when turning, in the chapter "Turning") . This is why it is important not to try to slow down too soon, in order t o remain manoeuvrable and be able to adjust the final heading, and in order to put the rescue vessel in the water. When turning, the launch of th e lifeboat takes up most of the time . Priority shou ld therefore be given to placing the ship correctly with respect to wind and swel l, and not to keep trying to get as close as

Life- raft at sea station

Rescue vessel on port side of ferry

Launching the lifeboat is a tricky manoeuvre, for which the first officer is responsible, coordinated with the bridge . It is performed at very low speed. Before the crew is embarked, the engineer makes sure the engine is working correctly. At least two people are needed to pull the victim on board the inflatable. The rescue vessel is recovered under the same conditions as for its launch. The victim is harnessed and pulled on board using a jib arm if unable to climb on board unassisted . Special care must be taken to secure the victim as he or she is being brought on board, especially if conscious and the pilot's ladder is used . There are some ways of hoisting the lifeboat that can bring the crew on board at the same time as the hoist manoeuvre .

2 .3

Rescue manoeuvres

The methods for recovering someone fallen overboard depend on how visible he/she is, on which side the rescue vessel is stored and on the wind direction. The ship can then choose to turn , or else to use the Boutakov manoeuvre and its derivatives .

2.3.1 Turning. Anderson manoeuvre As seen in the previous section, the lifeboat must be launched downwind . At th is point, the ship should also go to a position upwind of the casualty. Depending on these parameters, the ship then begins a full turn, or else a racetrack pattern.

Turning to port with rescue vessel on port side

206

Racetrack to port with rescue vessel on starboard side

possible to the casualty. Once the resc ue vessel is in the water, it will get to the person in the water much faster than t he ship could . * A sea anchor is a tapered , fabric device supplied on survival rafts, a kind of water pocket t hat is let ou t into the sea wh ich helps to slow th e drift of the lifeboat.

2.3.2 Boutakov or Williamson manoeuvre The Boutakov manoeuvre involves carrying out a calibrated turn to bring the ship back precisely into the wake of its initial route . This movement takes place without reducing speed. It is a slower manoeuvre than a full turn on the same side, but has the advantage of being fairly accurate if there is no way point . This is the method the ship should prefer to use at night, in fog, or else to turn back on its course if the time at which the casualty disappeared is unknown . The ship puts the helm over by 15° to the port (the side does not really matter) and continues the turn until it has moved 70° off course. It then puts 15° of helm on in the opposite direction and adjusts its final course to come round to a heading opposite to its initial course .

Helm 15° to starboard

Boutako v man oeuvre

'

Scharnov manoeuvre

For ships with very high tonnage , the amplitude with respect to initial heading is reduced to 60° because of their high inertia when turning . This is also known as the Williamson manoeuvre . As a final option , the ship could use the Boutakov manoeuvre but in the reverse direction, starting out with an angle of 240° with respect to the initial heading. This is called the Scharnov manoeuvre.

2.3.3 Manoeuvre with engine in reverse In favourable wind and sea conditions, a ship with two variable-pitch propeller shaft lines can try to stop quickly by putting the engine into reverse . If its sea speed is reasonable, it can come to a standstill at a distance of just a few ship lengths, and can continue to move in reverse until it is level with the person in the water. The rescue vessel is put into the water downwind when the ship is stabilised . The manoeuvre is used only for ships of medium-tonnage.

Moving in reverse to recover a man overboard

207

3

Methods for recovering casualties without rescue vessel

It is extremely risky to plan recovery of a

casualty directly from the ship because of the way the propellers stir the water, the rolling and pitch movements of the ship, and the high freeboard. It is of course preferable to launch the lifeboat if there is one .

SAFETY

Otherwise, if by chance the man overboard is conscious, he will be exhausted and may find it difficult to get on board by his own efforts . The following procedure may then be planned : - recover the man overboard using the appropriate resources (Markusnet)

-

send a member of the crew down th e pilot ladder to secure the traumatised casualty and haul him on board using a harness with a jib arm , or by the pil ot ladder, or even the gangway.

FIRS.,..

' Markusnet MS, Recovery basket

Pilot ladder

208

4

4 .1

Man overboard at time unknown

Planned actions*

It has been observed that someone has

When the watch changes or on resuming work a member of the crew does not appear. A quick search, then other more thorough sea rches are ca rried out to try to find him. If they are unsuccessful, the master of the ship decides to co nsider this person as probably having fallen in the water and starts the appropriate searches. This procedure is part of planned actions.

gone missing. The actions to be undertaken are as follows: - carry out a search throughout the sh ip (last place seen; patrol, wardroom; last witness, when the watch was changed ; -

ss• 30 'N

49°

S7'N

44°

Boat 51 °27.4565 'E 000 °20.0529'E

To WPT 207 0053.90 Nm 064.1 °T

COG 077.0°T SOG OOO .OKn -

Man overb oard manoeuvres

at a meal, etc .) send a distress message corresponding to the navigation zone (GMDSS, Global maritime distress and safety system) bring together the navigation reso urces needed for the search: • the course fol lowed at the time at which the accident probably occurred • meteorologi cal factors (drift caused by wind, current, direction of swell) make a half-turn (Boutakov manoeuvre) and fol low the opposite course organize and increase the watch, ensure CSS** functions to organize a search in the area with reinforcements if possible (ships in the area, search and rescue aircraft, etc.), enter the events in the ship's log .

After marking out the likely man over-

J;- :-: 111 F

r;'

.\ p .- ·• .\R ( -,.-.

~

Man overboa rd search pattern parameters

* The bridge sheet for man overboard at a time unknown (guide for managing emergency situations) ** CSS: Coordinator surface search : skipper of the vessel coordinating the search resources in the area . The ship concerned transfers its role to the SAR facility

[ ·;:

board zone , the search and rescue operations begin. If t he man overboard incident took place several hours previously, and the precise location is unknown, the ship may : - look for the casualty following an ap-

-

-

propriate pattern from the IAMSAR* ** manual , or a dedicated module on the DGPS or Ecdis . These modules provide optimized courses according to wea ther co nditions, the number of ships involved in the search, etc. ask for help from external SAR resources (hel icopters, marine patrol aircraft if the incident took place within range of the coast, etc.) coordinate search on site with the resources available there (as CSS, Coordinator Surfa ce Search)

when it arrives in the area, as it is better equipped for the job. The commander of the specialist rescue resources in the zone, then takes on the role of OSC (On -Scene Commander). ***The IMO IAMSAR manual gives tables for the speed of drift of a person in the water, as a function of wind and swell.

209

0

Stopping manoeuvres

1

Ship's stopping capability

Depending on the urgency of the event requiring the ship to stop, there are several ways of bringing it to a halt: - by inertia, slowing down and stopping -

without power, with the engine running in reverse, by turning alone, by a zigzag movement, combining turning and engine in reverse , dropping anchors to act as a drag, using external means (tugboats, mooring lines) ,

The need to make the ship lose way and stop may be an emergency manoeuvre, or a normal stopping manoeuvre . In both cases, for most ships the stopping manoeuvre is a significant source of stress . The IMO defines manoeuvrability criteria for emergency stops*. A stop test, the Crash Stop ** is performed when the ship is brought into service to confirm that the engine and the propeller

Type of ship

Coefficient A

Cargo

5 -8

Passenger ship/ferry

8 - 9

Gas carrier

10 - 11

Tanker

12- 13

VLCC

14 - 16

Type of propulsion

%power in reverse

Coefficient B

Log ( 1+B)

Diesel

85%

0.6 - 1.0

0.5- 0.7

Steam turbines

40%

1.0 - 1. 5

0.7- 0.9

Length of ship (metres)

Time to reach reverse power

Speed of the sh ip (knots )

Coefficient C 2.3

100

60

15

200

60

15

1.1

300

60

15

0.8

The formula ( 1) shows that, for all ships, the coeffi cie nt A is of prime importance . It is linked to the shape of the hull and to displacement (inerti a). Coefficient B can only be reduced at the cost of an increase in the engin e po we r. Coefficient C is influenced particularly by the rate at which the ship reaches its fu ll reverse velocity.

~knots

are adequate to make the ship lose way and stop within the regulation distance . Under test conditions (ship under maximum load , at full sea speed), the stopping distance must be less than 15 ship lengths . This limit may be increased to 20 lengths for ships with large deadweight

14,8 L

13 L

2000 m

(VLCC). In practice, the stopping distance is often nearer to 10 ship's lengths . A ship's stopping distance may be assessed according to the formula:

11 L ' , ,

12,2~',,~'-........ \

S = A .loge ((l+B)+C)

S: stopping distance in ship's lengths, A : coefficient varying with the ship's inertia (displacement and resistance to forward movement), B: coefficient varying with the ship's resistance to forward movement and the propeller's reverse thrust when the ship is stopped. This coefficient depends on the type of engine, C: coefficient varying with the ship's length and speed and with the time needed to establish the propeller's reverse thrust .

10 knots

\

5

kn~s I I

I

15 kn ots: ----------------~L--------------------

__.

Influence of wind on stopping distance and trajectory of an oil tanker of 200,000 tonnes (Stopping distances are marked in lengths L of the ship).

The initial conditions of velocity and the marine environment (sea st ate, depth , weather conditions) also affect the ship 's stopping distance: - during the deceleration phase, the wave stream created by t he ship exe rts a propulsive force linked to the previous movement and th e add ed ma ss of water. This force -

increases the ship's inertia by about 10%, confined or shallow waters (depth less than 4 times draft) increase hull resistan ces but reduce efficiency of the propeller in reverse. In the final phase of th e stopp in g manoeuvre, the prop walk effect is greater. The stoppi ng distance is general ly sl ightly

* MSC circular/ Circ. 1053 16 December 2002; see chapter on "Regulations". **Engines full astern when the ship is at sea speed m ov ing forward ; see section 3.1.

-

increased, finally, wind and waves modify trajectory, help to increase hull resistances and generally reduce stopping distance (except by following wind or sea) . Each situa t ion is affected differently with wind direction, swell and th e drafts of th e ship, with the co mbination thus create d being difficult to predi ct w ithout great experience. The use of crash stop is therefore limited to emergency manoeuvres.

211

1.1

Types of propulsion

1.1.1 Ship with single-shaft, fixedpitch propeller

The following diagram describes the propeller's operation:

Coefficient of propeller thrust

Most ships are fitted with a single, fixedblade propeller driven by a diesel engine. Diesel propulsion gives maximum power in reverse, which is not the case for a steampowered engine. As well as the power developed, the other vital point is the time needed to put the engine in reverse . For diesel engines this is around one minute, whether reversible or not, it is half that for electric or hydrojet propulsion systems . The most important factor is however linked to the speed of the ship when the engine is put into reverse. At manoeuvring speed (less than 7 knots), reversing of the engine is reliable and the engine gives its full power very quickly in reverse. This is not the case when the ship is at a fair speed or at sea speed. Reversal of the direction of movement may be haphazard. Two co nstraints emerge. The first diffi cu lty is to start the propeller moving in reverse, while it is still moving forward by inertia and by wake, the engine being set to Stop. The greater the ship's velocity, the greater the propeller's drive power. To overcome this force, some ships are fittep with a brake which stops the shaft while the ship is reversing . The available pneumatic power to start t he motor may also be enough to reverse the propeller's direction of rotation . It is therefore often desirable, to ensure the direction is correctly reversed , to reduce speed when circumstances allow by stopping the engine and waiting until the propeller and ship's speed fall to a reasonable level so that the engine can be reversed (around 7 knots). This period obviously increases the distance over which the ship has to travel before it stops. The second problem is linked to the efficiency of the propeller when moving in reverse. So when the ship has significant headway on, the propeller moving in reverse operates in poor conditions. It has difficulty in

producing negative thrust when accelerating a mass of water forward whose initial speed is towards the stern. It also ofterr experiences cavitation thus losing efficiency.

212

A

n.D

V G With n being the num-

/

/

Propeller's operating curve The propeller's operating point in the open sea is point A. Reducing speed quickly, thrust falls and disappears, although the machine is still moving forward (n>O and V>O). If the propeller is decoupled , it is driven by wake, and operates at point B. The motor applies braking torque to the

shaft, so the propeller's operating point is at C when the engine is stopped with the propeller in gear. If the engine braking is enough, the operating point goes to D. If a shaft brake is used, it moves to E. When the engi ne starts to run in reverse, the propeller operation goes to F, then slides gradually to G as the speed increases to Full Astern . The difference in the positions of the thrust coefficient in sequence from Full ahead (A) to Full astern (G) shows the propeller's inefficiency when in reverse.

ber of rotations, D being displacement and V, velocity.

1.1.2 Ship with twin-shaft, variab le pitch propellers Ships with variable-pitch propellers can move quickly from a full ahead speed to full astern , reversing the angle of th e blades while the engine rotation contin ues in the same direction. This flexibil ity gives greater security, but if the directio n of movement is applied too abruptly, th e load on the engine becomes significan t , the propellers develop cavitation and th e whole structure of the ship vibrates. Und er these conditions, th e propellers ' thrust is limited. When circumstances allow, it is therefore better to reduce speed before going into reverse, then gradually increa se the pitch of the propellers. The flexibili ty

engine in reverse.

achieved by quickly reversing the angle of the blades is at the cost of less efficiency in reverse compared to a fixed-pitch propeller (often around 50% of the availabl e

A significant change of heading (around 90° or more) caused by the propeller's

power in forward motion). Ships with twin shafts and variable-pitch propellers, ferries for instance, can keep a

The transition phase between points C and D show the unpredictability of starting the

thrust effect also occurs in the final phase of the stopping manoeuvre. The rudder no longer controls the ship's trajectory, as it becomes inefficient because of the eddies generated by cavitation on the propeller.

straight heading when the engine is run ning full astern .

2 2.1

Planned stopping manoeuvres Inertia for stopping

The distance and stop time depend on sea conditions, the initial speed, the displace-

The reduction in speed follows a logarithmic curve. It is very fast at the outset, then

When a ship with its helm amidships stops

ment and shape of the vessel (block coef-

slows down graduall y towards zero after a

its engine, only the hull resista nce exerts

ficient). An effective exa mple is given by a

theoretica ll y infi nite t ime. Under the same

a braking force and absorbs the k inetic en-

VLCC of 250,000 tonnes in a ca lm sea, un-

conditions, a containe r vesse l under load

erg y of the movement until it stops.

der load with an initial speed of 18 knots,

with an initia l speed of 18 knots stops in

stopping its engine and moving under its own momentum for about an hour, travel-

30 minutes over a distance of 4 miles .

ling a distance of 15 miles . Initial speed : 18 knots

15 sea miles in 1 hour ..--- - - - - - - - - - - - - - - -•

Final speed: stopped

In ertia stopping distance of a VLCC

2 .2

Stopping normally with engine in reverse

This is the normal use of the engine, in a scenario whe re the vessel enters confined waters and needs to com e to a halt. The reduction in speed, often controlled by an automatic pilot, is usually planned to fall from "sea speed" to "manoeuvring speed". The shiphandler complies with these va lues for speed reduction associated with the engine's efficient operation (tem perature , etc.) by ha ving an accurate grasp of the distances travelled between each cha ng e of speed. Regulation tables (Resolution A 601 / 15; see chapters "Regulations " and "Appendices") displayed in the wheelhouse give details of the times and passage distances for each one. Approa ching the port, for instance, at manoeuvring speed, the ship's engin e is ready to go into reverse . The available power is normally 80% of the power at running speed. After a long cross ing , it is better to check that the engine controls are ready to go into reverse. Normally this check can be carried out when the pilot is taken on board and speed is reduced . Some procedures, inspired by the US Coastguard regulation s, may be required before the pilot is taken on board .

2.3

Zigzag manoeuv re The time taken for the ship to reac h t he

Starting at running speed, the zigzag ma-

speed at which it can begin to move in

It is also possible to compensate or increase the pitch effect of the propeller in reverse by initiating a swing to port or starboard just before going into reverse.

noeuvre involves combining helm move-

reverse is reduced.

ments with th e reduction in speed.

The stopping distan ce can be cut by 25%.

The ship is slowed by changes of heading

The inertia which the ship gains on

and the drag generated by the rudd er

changing heading using the helm main -

This manoeu vre is advisable w hen navigating confined wate rs, or in heavy

angled first one way then the other.

ta ins control over the co urse for most of

traffic. Under some conditions the stopping

the manoeuvre.

distance may be reduced to around 7. 5 times the ship's length especially if a swing has been in itiated in t he direction of the propeller's pitch . Change of heading

Full astern

Zigzag manoeuvre with reduction in speed At a constant r unning speed, this method is also

altering the angle of the helm first to port then to

used for the regulation testing of the ship 's

starboard.

manoeuvrability and directional stability, by

213

2.4

Calibrating stopping distance

A shiphandler has to know the distance the ship will take to stop, whatever its initial speed and the engine setting (running or manoeuvring speed) . The question for the shiphandler faced with the need to stop at an unplanned point is to know the distance over which the ship has to manoeuvre in order to stop at the desired point, with an approximate final heading . The ship's trajectory during the regulation crash stop is displayed on the bridge , in order to assist the skipper's decision-making process in an emergency. When manoeuvring in a port, every case is different, hard to predict, requiring the minimum speed compatible with drift give n the co nfined space, and good helm control.

Emergency procedure

3

The emergency stop procedure is a manoeuvre only to be used in exceptional circum stances such as when avoiding an imminent danger, such as a collision or grounding . It involves making maximum use of the ship's potential, in order to stop it over th e shortest possible distance, by putting the engine into reverse at full power. This is the crash stop procedure . There is also the option of starting a turn, in order to avoid a hazard, by putting the helm hard to starboard or port. Finally, drag anchors to slow the vessel down are used only in low-speed port manoeuvres, for instan ce if th e engine does not go into reverse at the intended moment (approaching th e wharf) .

3.1

Crash stop procedure

The emergency Crash stop or "Exceptional full astern" procedure involves putting th e engine into reverse, going to Stop without touching the helm. This abrupt reduction in speed is carried out without considering the eng ine's co ntrol parameters, and so could trigger alarms . Special provisions may be taken to activate this manoeu vre, distinguishing the emergency procedure clearly from the normal procedure. This may include by-passing particular engine safety or automatic control procedures, by activating a special command.

Stopped 0 BUI

i~:::oo

~~

!ft~

ltJ'204:00

Ill

~-~·.Engine reverse \' I ):03:00

\~ ,7 knots .

~·.\

Telegraph with pushbutton for the Crash Stop procedure

\~\ ~~

Helm 35° starboard

This is a regulation emergency procedure on all vessels. It involves stopping the engine, then wait-

\:\

'] .;0100

{~: ~~~~ 8.

\1

·~ ·

· ~1::01 • 00

Engine stop

Helm 35 ° starboard

~--

----

;$

ing for as short a time as possible to put the engine into reverse, and quickly in-

motor with a lot of choke in reverse, overcoming the torque of the engine induced by the wake . It is difficult to control th e ship's heading when starting the engine in reverse. With transverse thrust, especiall y

creasing speed to Full Astern. On ships with the engine running and a fixed-pitch propeller, it is difficult to reverse when moving above a certain speed. Starting the engine in reverse of-

noticeable in the final stage, the change of heading may be more than 90°. Emergency stopping distances on ships with twin-shaft, variable-pitch propellers are often much shorter, only a few shi p

ten means waiting until the ship's speed has dropped enough, then starting the

lengths, perhaps 5 or 6 lengths only for a ferry running at 20 knots for instance.

~Q :OO:OO

._

Lgx3

16 knots

$t

Emergency stop distance on container ships Keeping velocity low is above all the safest way to reduce uncertainty associated with the stopping distance. There are multiple criteria to consider when evaluating the stopping distance . Generally speaking, the main factors are the initial speed, and the ship's tonnage over drive power ratio.

214

0: --t---

Ferry 11.000T 2 variable-pitch propellers

.-----r--A

Starting up in reverse for a ship with two variable-pitch propellers

'Jt 'It

3.2

Stopping by turning alone

At running speed, coming to a standstill by putting the helm hard over is often the most effective emergency manoeuvre to avoid a hazard and allow the vessel to lose way. Every 90° change of heading reduces the vessel's speed by about 40% once the engine is stopped. Although forward progress (longitudinal distance covered) is reduced wi t h this manoeuvre, the sideways shift (lateral distance) is greater. For this reason, the manoeuvre is only possible if there is available water to move in . At manoeuvring speeds, however, a loaded vesse l finds it much more diffi cult to turn without the help of the engine driving forward.

......---+----2000 m I

I

Crude carrier 220,000T 1 fixed-pitch propeller

-------~-------------c:»Q

_)

Turning hard to one side

It is therefore impossible to stop within the reduced space of the port without the engine running in reverse . The stopping distance of ships can often be reduced nonetheless by combining a turn initiated by a kick ahead * in the direction of the propeller's thrust effect. This action of the engine running in reverse is then combined with an increased hull resistance caused by the turn . *Kick ahead: short, powerful burst on the engine driving forward.

3.3

Emergency stop with anchors

This procedure is used to stop the ship quickly, as long as it is moving forward at slow speed. Dropping anchors above a certain speed actually risks causing damage to the windlass and the hull, and Iosing the mooring line .

The method involves paying out a short length of chain (between 1 and 2 shackle lengths, depending on depth) so that the anchor can drag along the bottom . As speed is reduced, the length of chai n may be increased to add braking force and

This manoeuvre is normally kept for confined spaces, if there is a fault on the helm control or engine, or to avoid imminent danger. Small ships often use it however, to help them come alongside .

immobilise the ship. This manoeuvre often causes a swing in the direction of the lowered anchor. The two anchors may be dropped one after the other, to control the heading and improve the effectiveness of the manoeuvre, then paid out alternately until the ship is stopped.

215

4

Illustrating emergency stop distances

Figures on this and next page illustrate the various procedures for two different types of ship, the container vessel and the ferry. Simulation conditions are identical for both ships, namely : initial speed of 12 knots, then emergency stop manoeuvre

4.1

using crash stop, by turn combined with crash stop, and finally by turn only. For container vessels, the engine is stopped then started in reverse once speed falls to 7 knots .

For both ships, it can be seen that only shallow depths have a significant effect on the radius of turn and on the change of heading caused by the thrust effect, but have little influence on emergency stop distances .

Emergency stop distance on container ships

4 .1 . 1 In deep water In open water, container vessels will preferably use a turn with helm hard over if there is enough sea room .

Turning + Crash stop

..j..

p. ~I

J

~? 0000

r

In deep water- initial speed 12 knots -~---- · '·

4. 1. 2 In shallow water In confin ed waters, however, crash stop is preferred. Sideways movement (lateral displacement) is greater if combined with a turn .

Crash stop

In shallow water- initial speed 12 k nots

216

4.2

Emergency stop distance on ferry stop

Turning + Crash stop

4.2.1 In deep water Whether in open water or confined waters, crash stop is preferred at this speed.

In deep water -

4.2.2 In shallow water Turning is penalising in shallow depths. The flexibility of variable-pitch propellers means that the distances travelled are small (3 to 4 ship's lengths). As the ship has twin shafts, the ship stops in a straight line.

Crash stop Turning

+ Crash stop

In shallow water initial speed 12 knots

5

Summary

As we have seen, the emergency manoeuvre to be used depends on the type of ship and its speed, as well as on the environment in which it is turning. There is therefore a difference between ships on passage at their running speed normally controlled by the automatic pilot, sailing in deep water, and those at manoeuvring speed in channel or manoeuvring in shallow water in a port. In the former case, turning is preferred, with forward movement always less than the distance covered in crash stop . Manoeuvrability is also maintained, by preserving a minimum speed on leaving the turn. In the second case, in a restricted space, crash stop is preferred to turn, the distance travelled being less, and the engine being available to respond quickly in reverse. Although the final heading is difficult to control with the engine always running in reverse (although it can be anticipated with careful, early use of the helm), this method stops the ship over a shorter distance.

CHAPTER 3 • PART 5.4 ILLUSTRATING

Moving forward -

Speed greater than 12 knots: turning. Speed between 12 and 7 knots: zig zag

-

manoeuvre (crash stop with turn) . Speed less than 7 knots : crash stop .

Comparing emergency stop methods

2 17

Towing

1

Description and general poi nts

Towing in port

2

Turn ing a ship 100 metres long within a zone 400 metres in diameter does not

The most important part of the towing process takes place within the port envi-

seem impossib le ; if the ship measures 300 m etres long , then a different ap-

Sea-going ships are rarely manoeuvrable

ronment. All ports around the world use

proach is needed. Only loca l best prac-

enough to turn safely under all conditions

tugs - whether with specialist or more

ti ce can provide the right answer.

within confined port zones .

basic f acilities - appropriate to their ow n

Even when they have specific resources

layout, local conditions and rules. The

such as transverse thrusters or sophis-

starting point for this is the limitations in

ticated rudders to assist manoeuvring,

manoeuvrability of the sh ips using th ese

these have their limitations, and so tugs are also needed in these circumstances.

ports . Port towing operations are governed by

Port towing services thus make the work

the commander of the ship being t owed .

of the ports more reliable.

The towed ship is responsible for an y ki nd

These co ntinually adap t to t he changes

of da ma ge t hat may occur during the

in ship technology, to cope with their size and improve security of manoeuvring .

t owing operations, unless t he t ug ca n be shown to be liable . (A r ticle 2 6 of Act 69-8

Many states also take special measures in

on ship fitting-out and sales.)

-

line with current environmental policies t o improve safety measures for certain types

2.1

of ship, not just within the port, but also

The first questions which come to the cap-

when passing through territorial waters,

tain 's mind w hen he/she has t o manoeu-

channels or inland waters. These escort tugs are a completely sep-

v re in confined waters and in the port are: "Do I need tugs, and if so, how many 7"

arate category, with their ow n particu-

The ans we r is far f rom obv ious, since

lar t echnical characteristics and working

there is a conflict between safet y and cost

rul es.

tha t unf ort un at ely, cannot always g uara n-

It is also worth covering the two main phases of to w ing assistance: the em er-

Rules -

T he pla nned ma noeuvre : here too, a manoe uvre may be achievable unaided w ithin a particular situation, depending on the resources of the ship. The need to pass through a lock, its direction an d size; the need to make a t urn; th e traffic encountered ; the need t o limit speed to a level below that which the ship's technical capabilities can maintain ; the likely need to have to wait in a pool ; the presence of parti cula r cu rre nts or shearing effects; any para met ers affecting the decision . In addi t ion, some of these are only kno w n at the last minute. The ship: the ship has powerful ma noeuv rin g ca pabilities . A bow or stern thruster, f ixed- or variable-pitch propeller, a nozzle, special type of rudder syst em - Seeker or Schilling perhaps - and active Azipod-type rudder systems are

tee a positive outcome .

all fa ctors to be considered. The power and limitations for use of

difficulties has to be secured, following

It is often the ship ow ners or charterers who must approve the use of tugs , whether because of th e met eo rologi cal condi -

damage , for example ; bringing the ship

tions or because of parti cular local circum -

t hruster, for instance , it must also be

into a safe place, into a harbou r or to an

stances or re gulation s; in m ost cases , t he

in good condition and of a size suitable

area where it can be repaired and made

ship's master does not hav e all thi s info r-

for th e shi p itself. An anchor can also

seaworthy again .

mation available .

It is impossible to describe all the likel y scenarios here, or all the manoeuvres that can be carried out with a tug. These will depend partly on the technical properties of the ships or tugs and

Regul ar practi ce, along w ith con sid erab le

be used effectively on small ships and may "replace" a thruster. The type of

gency phase, when a ship that is in

each of t hese devices are known fact ors: it is not enough to have a bow

experience and specific training are often

ship, it s shape, displacement, length

needed to assess the need for tugs, since

and beam , draft, the position of its up-

there are many parameters involved :

perworks, t he weig ht/power ratio of its

-

engin e and t hrusters must therefore

the marine and meteorological condition s

Topog raphy : obviously the more space avail able for manoeuv ring, as re-

in which they are moving, and partly on

gards the size of the ship and its own

cu r rents and wind experienced when

the level of expertise of those carrying out

technical resources, the less there will

making a decision .

these manoeuvres - pilots and skippers of

be a need for tugs .

al so be co rrelated to the depth of water,

tugs who through their daily work have, over the years, become expert in the particular techniques appropriate to their own local circumstances. The corollary to the improved safety contributed by the tug, is that it must never impede the ship's own manoeuvres . This requires the ship and its tugs to be perfectly matched to each other. The safety of the ship, the tug, their crew and sailors working on the manoeu vring decks, all depends on communication and coordinated action between the ship and the tug.

Towing manoeuvres

Tug in action

-

~

Particular conditions at the time: weather conditions, wind, tide and current are obviously taken into account . Thi s data relates directly to the charac-

20T

20T

il

~DDDDDDD[J9)

teristics of the ship, its und erside and its superstructure. When a ship has to turn, it necessa rily moves through a

t 15

stage when it is beam on to the wind or current. Although this stage is not necessarily the trickiest, it is the peri-

knots= 40 T

Force applied by the wind to the ship's windage surface

od during w hich the maximum su rfa ce

Although the result is naturally approximate, a rough ca lcu lation can be quickly

area of the ship is exposed to external elements. All of the resources imple-

made, thus avoiding a manoeuvre w hen the necessary safety cond iti ons have not

mented must be sufficient to ensure it

been met .

will be able to pass through this stage. The forces , which pilots call "control

The following table gives the results of the formu la ( 1) ca lculated for various wind -

forces ", must at all times and in all cir-

speeds in knots, with the necessary contro l force expressed in tonnes .

cumstances be greater than the forces exerted by the elements.

Wind

In this field , humility in the face of the

speed

greater force of nature is necessary,

0

1000

2000

4000

5000

6000

8000

9000

10000

12000

13000

14000

16000

4

0.3

0.6

1.3

1.6

1.9

2.6

2.9

3.2

3.8

4.2

4 .5

5. 1

10

2.0

4.0

8.0

10 .0

12 .0

16.0

18 .0

20 .0

24.0

26 .0

28 .0

32. 0

16

5.1

10.2

20.5

25.6

30.7

41.0

46.1

51.2

61.4

66.6

71.7

81. 0

20

8.0

16.0

32.0

40 .0

48.0

64 .0

72 .0

80.0

96 .0

104

112

128

26

13 .5

27 .0

54.1

67 .6

81.1

108

122

135

162

176

189

21 6

30

18 .0

36.0

72 .0

90.0

108

144

162

180

216

234

252

288

36

25 .9

51.8

104

130

156

207

233

259

311

337

363

41 5

40

32.0

64.0

128

160

192

256

288

320

384

416

448

51 2

and

adaptation

is

required.

Surface exposed to the wind, in m 2

Simple

"rules" which everyone can understand can be applied here : •

The forces and moments used to quantify towing

requirements are

determined by assessing the forces that the wi nd , current and perhaps waves exert on the ship . These forces and moments vary according to the angle of incidence of win~

•

and current.

Control forces to counter the force of the wind across the beam are expressed in an empirical fo rmula already stated: 2

X

s X V2

Fe=----100

This process is used to quantify the force needed to keep the ship beam on t o the

Fe in to nn es-force .

wind and to also find out the moment - with a tug attached in line, applying a

S is the surface area of superstruc-

traction T - to produce it to bring it face on to the wind:

ture in thousands of m 2 .

T x Lo Mt= - - - - 2

V is the windspeed in knots in cold air (2°C).

with T = 2 X Fe X C

For insta nce, for a container vessel

T

= traction in tonn es-force,

300 metres long where the "wind-

Fe

= control force in t onnes-force,

age" surface is estima ted at 9000

Lo

= overa ll length.

m 2 , the control force of a 15-knot

C

w ind w ill be about 40 tonnes.

= coefficient: C = 0 .17 for an oil tanker

A minimum force of 20 tonnes pe r-

C = 0.12 fo r a container ship

pendicular to the longitudinal axis of

C = 0.11 for a ro-ro ferry

the ship ahead and astern is need-

C = 0.14 for a liner

ed to keep the ship immob il e w hen beam on to the wind.

•

The co ntrol force for managing the effect of curren t quickly becomes very significant . It depends on the square of the speed of the curren t, the longitudin al surface area of the ship's undersid e, as well as the under-keel clearance (R: ratio of depth-to-draft), which leads to a local increase in the speed of flow . Fo r instance, for a ship 300 metres long , and with a draft of 10 metres or an immersed longitudinal surfa ce area of 3000 m 2 , in shall ow · water (R = 1.2) , the thrust necessary to counter a current of 0 .5 knots is 50 tonnes (see figure next page) .

220

Current force in knots

Wetted longitudinal surface areas 1000 m' 2000 m2 3000 m2 4000 m' 5000 m'

-

Tugboats: the number of tugboats is obviously and "mathematically" the first point to resolve. It is also esse ntial to consi der the type of tugs to use . Power is the first aspect, as a g ross, quantifiable value. The type of propul sion and t he particu lar working facilitie s are also infl uential: sometimes it is better to have two "small" tugs rather than one powerful ship, espe cially with lighter ships such as ro - ro ferri es; it may be useful to have a more "flexible " tug, which can move quickly and is "ag il e" ,

>3

2

or which can work in "push-pull" mode , or

R=PIT

with a close- co uplin g in very tight spaces .

1,6

Each of these resources has its ow n char1,4

acteristics, benefits and dra w backs, so that what wo rks in one particular port or at a

1,3

given moment does not wo rk elsewhere . 1,2

Force in T

-

Loca l rul es : around the world , more and more ports are imposing specific rules for some types of so-called "at-risk" ships .

Force at the hook needed to overcome the thrust of a cross current

This is often independent of the ships ' actual manoeuvrabil ity and is generally added

The moment needed to bring the ship into the direction of the current with a tug

to the considerations listed above.

attached in line is:

These sh ips of course include, among oth-

T x Lo Mt= - - - - -

ers, crude carriers, gas-tankers, chem ical with T

=2

X

Fe

X

C

2

tankers or som e ships carrying hazardous or "sensiti ve" goods. Rules are also being put in

C = 0 .11 when R (D/d) = 1.05

place for some large container-ships whose size may cause problems in terms of safety

The control force needed to overcome the effect of a crossing sea is given by:

harmonized wi th the obligation to have an

112 X LPP X H2 Fe=

for port infrastru ctures . These rules will be escort tug, but use port tugboats that assist

1000

the manoeuvre until it is com pl eted .

Lpp : length between perpendiculars (Lpp) .

In such cases, the only question the captain

H: height of waves in metres.

needs to ask is w heth er further t ugs need

In an emergency, the coherent control force to stop a ship may be assessed

to be ordered . This invol ves considering the

using the formula:

various elements described above.

D Fe=----3000 •

Where D = displacement of ship in tonnes .

These various considerations show that it is difficult, perhaps impossible, to set absolute

The force exerted on the towline is greater as a function of the angle it makes

rules that will allow a ship's master (or own-

with the horizontal, than the tug's "effective" traction force.

er) to know for certain, in advance and in all

The towline tension T' is equal to the tugboat's traction T divided by the co-

situations, how many tugs he/she will use .

sine of the angle a of the towline with respect to the horizontal.

The various aspects covered here will help the

Thus, when the tug exerts a traction T of 20 tonnes, at an angle a of 45°, the

captain in the discussions to be had with the

towline tension T' is 40 tonnes.

port's regular users , the pilots . Guided mainly

T

T'=

cos a

by safety aspects rather than economics, the latter will be able to direct decision-making. Mutual trust and understanding of problem s that arise will help improve the situation .

It may sometimes happen that adding further resources will no longer be eno ugh to produ ce an acceptable solution. It is also essentia l to know when to give up, especially wh en the safety of the tug s is at stake. The difficulty of controlling a tug when it is towTowline tension as a function of th e angle o.

ing should therefore be emphasised, along with the insecure position of its crew. Manoeuvring

The stretch of the towline allows the tension it experiences to be limited, but

with tugs thus requires accurate comm un ica-

reduces the tug's speed of manoeuvre . A tu g 's traction capability is measured

tions and trust between the crew of the ship

as static force (bollard pull) . Normally, this is expressed as tonnes-force.

and that of the tug .

221

2.2

Types of tugboat

This is no place for a complete description of the various types of tugs as it would require another book. Some technical points of interest and some safety rules, however, are necessary. The work of a "conventional" tugboat is very different from that of modern tugs, which have useful capabilities in terms of both power and of versatility. The following is a brief description,

T'

in chronological order, from the oldest (conventional) to the most recent (trac-

Transverse force applied to conventional tugboat

tor tugs, ASDs, Rotors). Their benefits and drawbacks are summarized in a table.

2.2. 1 Conventional tugboat Th is is the old-style, conventional tugboat . It has one propeller, normally in a nozzle, placed at the stern.

T: towing point T': tension of towline G: centre of gravity V: transverse speed R: resistance to drift

In order to limit this risk as much as possible, most conventional tugs have a "stoppe r". This accessory (a holding system applying a return force to the stern of the tugboa t, close to the propeller) shifts the point at which the traction force is applied towards th e stern, limiting the torque created and thus reducing the negative effects.

Towline

Conventional tugboat with "stopper" It takes longer to attach or release the tug, but the safety of the convoy is greatly im proved. However, great care must be taken . This accessory is often fitted to a winch , called a "stopper winch", so that its length can be adjusted. The The tug. The

Conventional tugboat It is rigged with a single stopper winch placed between the aftermost third and the middle of the ship. Its superstructure is placed forward. It can only tow an attachment from the stern, which is why great care must be taken when working with these tugs. The point at which the towing force is applied is at the tow winch itself; the tug's traction force is applied at the propeller,

first rule is therefore to keep the speed as low as possible . design of the conventional tug makes it much less versatile than all other types of This can be seen in the following sections where they are described . risks inherent in speed with the consequent limitations, have already been covered .

Moreover, the propulsion unit is located to the stern of the ship, along with a conventional rudder system, or a nozzle at best. Its design is similar to that of an ordinary sea-going vessel. When close-hauled, the towing or thrust force can therefore only be along the tug's longitudinal axis, forward or astern. It cannot be moved across . It is therefore less versatile and the greatest of care must be taken when manoeuvring . This lack of flexibility also causes disturbances of which the shiphandler must be aware and must predict as far as is possible: since the tug cannot move across, no change of direction can take place without stress on the towline, which could have a negative effect . Let us consider a tug attached to the stern axis of a ship . The pilot asks it to come to starboard, in order to make the ship pivot to port. The tug then exerts traction on its towline and uses the helm to move laterally.

which creates a pair of forces that tend to place the tug "across" the general trajectory of the convoy. This means that if the longitudinal speed of the latter is high, the tug begins to list; if no action is taken, it may capsize. If this situation arises, the only possible action open to the tug is to let go its towline using a remote release system. The ship must stop as quickly as possible.

222

Tug comes to starboard

Traction on starboard side

After a while, if the tug is then asked to come to the other side, to stop a lurch for instance, it can only perform this manoeuvre by exerting a force on the towline which, along with the action of the helm, will allow it to move laterally to the other side .

The action that consists in exerting this force is applied first on the starboard side, and therefore is the reverse of the desired effect . Until the tug has passed the ship's axis , this force will be exerted on the wrong side. These effects, familiar to the shiphandl er, are obviously unavoidable and amplified by speed. It is very important to be aware of them, so as to be able to predict them if possible and thus anticipate them .

Force exerted on the wrong side

The force moves to the correct side

The reduced manoeuvrability of such tugs is also felt when taking up or releasing towline s. These transient situations are especially sens itive, all the more so for conve ntional tugs. When the tug is ready

Difficult passage of towline in the high

to take the towline across the bow, it must approach from alongside the bow with a

p ressure zone ahead of the towed shtp

parallel course and significantly higher speed than that of the ship. As it does so, it is in a turbulent zone of higher hydrau lic pressure created by the ship's way; the greater the ship's displacement, the higher its speed and the more turbulent this zone. Close to the bow, the tug takes action to pre ve nt being pushed away. As it passes the bow, its stern then tends to be pushed away, bringing the tug's bow in line with the ship's axis . If it is unable to bring its bow in line with the convoy 's axi s, the tug ay find it is ahead of the ship, beam on, w ith no way out . Conversely, when it comes up from the rear, it may be sucked into the stern by the depression created by the propeller.

These two manoeuvres , equally valid for other types of tug , are especially difficult for conventional tugs. The shiphandler must therefore be very aware of all these points. The conventional tug, more than any other type, can only work properly and above all safely at the lowest possible speed. This can cause

Suction of tug by the low pressure created by the propeller

problems for the current generation of large ships, especially container vessels, some of which have fairly high minimum speeds. This lack of flexibility in terms of speed, and especially manoeuvrability, requires personnel who are particularly careful, with an experienced and vigilant pilot and tug master. These tugs are very efficient, about 1.2 tonnes per 100 hp, with a moderate draft.

223

2.2.2 Tractor tugboat The best-known "tractor" tug (still known as VWT (Voith Water Tractor)) is the VoithSchneider type. This type of tug is different in that it has two separate, multi-directional propulsion systems. These are placed on the forward third of the tug boat, approximately beneath th e bridge . The first positive effect of this arrangement, is that whatever happens, the application point of the towing force is to the rear of that of the traction force, thus avoiding the risk

There are some drawbacks, nonetheless: - the transverse action, if any, is less tha n the power developed longitudinally - stability of the forward heading is re-

-

of capsizing for tugs with propulsion at the stern. Voith-Schneider tugboat

duced since the thrusters are locate d forward they experience problems keepin g themselves in the wake of the ships: the longitudinal drift beneath the ste rn of the tug's hull helps to stabilize its helm, but may also be affected by th e ship's water streams beneath the towline (attached at the stern) and caus e sculling effects mainly at high speed

The tow winches are placed at the stern. Depending on its power, the tug has a single winch, as is the case with the 40-tonne port Voith , or two winches as with the 70-tonne seagoing Voith. With the latter, the second winch, fitted with a heavier cab le, is located ahead of the port wi nch .

significant draft: often over 5 meter, the draft of these tug s involves some precautions - all the more so as th e thrusters are placed ben eath the hu ll. Even if they are streamlined, they are directly affected if the ship grounds the general efficiency is around 20 % less than all other types of tug at abo ut 1 tonne per 100 hp .

Despite this, they were the first alte rnative to conventional tugs and have generally replaced them because of th ei r sa fety, flexibility and available power. The investment and maintenance cost s have often limited their distribution to pa rticular ports . Cu rrent tugs, ASD or Rotor, are mo re "economical" and are now taking over.

Port winch of a 40-tonne Voith

Port and sea-going winch on a 70-tonne Voith

These tugs have many advantages: - almost no risk of capsizing caused by traction on the towline - versatility because of its two propulsion systems: greater power available; -

independent steering control of thrusters; 360° operation; transverse action identical power ahead or astern much better security; it can easily detach itself if there are problems option of transverse action: it can move aside quickly, with little interference with the towline a drift plane located beneath the rear section to stabilise the forward heading gives it significant braking power, much higher than the tug's theoretical power : even a small angle of incidence increases the drag of the attachment. This is closer to the thin plane theory, where F = k x S x V2 x sin i; the drift plane greatly widens the wetted surface, thus tending to increase the force generated.

224

IN PORT

2.2.3 ASD tug

Thorough training and good experience

which is unsuitable and especially difficult

From one point of view, the ASD (Azimuth

are needed in order to perfectly control

to control (see section 2.4 .2) .

stern drive) tug may be considered the mid-point between the conventional tug

this type of tug.

Thrusters are obviously less exposed than

and the tractor. As its name indicates

These tugs have a winch facing across the

those of a simple tractor, and the draft of these tugs is normally less for the same

(stern drive), it is a tug whose propulsion

stern placed towards the middle, and thus

size vessel.

system is placed at the stern (as on a con-

ahead of the propellers. However, they

They have a better performance , compa-

ventional tug), and can be turned through

also have one or two winches forward of the bridge . These can therefore be used

around 1.2 tonnes per 100 hp.

does combine the benefits of both the pre-

in the "tractor" configuration with no risk

The fact that the propulsion system is lo-

vious types .

of capsizing . When attached, these tugs normally work head on to the ship: mov-

cated to the rear and the hoists normally used for towing are at the bows means

ing forward, on the stern and moving in

that these tugs can be used in another

360° (azimuth) . Despite its drawbacks, it

It has two streamlined propellers on the stern (and sometimes a low-powered bow

rable to those of a conventional tug, at

be turned through

reverse, on the bow. The attachment point

way, thus increasing their versatility: the

360°. It therefore has almost exactly the same power no matter which direction it

is therefore always between the ship and

push - pull manoeuvre . This configuration

the propulsion system , improving safety.

allows the vessel to change very quickly

is working in. Having two propulsion sys-

In some cases, there are two winches fit-

from tug to pusher, a very practical , safe

tems provides better control of the direc-

ted to the front, allowing close-coupled

and flexible capacity when coming along-

tion and transverse action to move aside .

working with greater precision , without

side (see section 2.3) .

thruster), that can

having to use the ship's hawser,

ASD tug

Stern sea win ch on ASD

Forward sea winch on ASD

2.2.4 Rotor tug The Rotor tug is the latest development for port tugs, having three identical thrusters: one to the rear and two at the bows. It may be compared to a tractor tug with the drift re placed by a thruster. Obviously, this increases flexibility in use, giving more options. If any one unit is faulty, there is no change to the safety of the tug or the convoy. It just means that the available power is reduced by one third . Its three omni-directional thrusters give it great turning capabilit.

CHAPTER 3 - 6.2

Rotor tug

Despite its lateral hull surface area, th e Rotor Tug still has significant sideways movement speed. The available power is almost identical in any direction (360° ). Finally, the installed power is higher fo r the same size, displacement or draft. Turning in its own length

The most positive point for this type of tug is of course the flexibility given by th e

60%

~ 100%~ ~

95%

i

/

-• ) - 100%

/

l

~ i 100% ~ ~ ~

-

/

~

side , turning on itself, transverse move-

~

Conventional

and much experience. Section 2.3 w ill

Distribution of thrust

Tractor

Complete rotor

ASD

3 independent thrust-

each type of tug will highlight the vari-

2 independent

1 rudder centre

thrusters on the

2 independent thrusters on the

ers (2 on the bow,

ous benefits and drawbacks of each mo re

tow winch,

bow, drift plane,

stern, 1 winch

1 on the stern),

clearly. Technological progress has ob -

stopper towing

towing winch in

on the stern, 1

1 winch on the bow,

viously improved the capabilities of t he

hook

centre

or 2 winches on

1 winch on the stern

latest tugs to a significant extent . In th e first place, these developments are main ly focussed on resolving safety issues con -

Shallow draft,

High-power, safe

Flexibility

Great flexibility in use,

low cost, good

manoeuvring

in use, high power, close-

safe manoeuvring,

cerning conventional tugs . There are al so

even with one thruster

gains in terms of performance, power de-

Significant risk

advantages

2.2.5 Summary A summary table of the main features of

1 propeller,

performance

Dis-

handling the three separate and independ ent controls requires thorough trainin g cover its use in more detail.

the bow

Benefits

ment with no interference effect. Howeve r,

95%

Distribution of velocities

Brief description

/

) - 100%

l

60%

third thruster: working indirectly, along -

at high speed

coupled option

broken , higher power

livered and versatility of use in different

(if 2 winches

for the same size,

situations.

on bow), good

good performance,

This table does not attempt to provid e a

performance

power almost equal in

thorough description of every type of tu g.

every direction

It covers the typical main features th at

Deep draft, low

Risks if at-

Three controls, thus

performance,

tached by stern

more demanding

expensive, poor

winch, and

training

directional

when towing in

stability

line astern

have an important impact on manoeu vring, and those that the pilot and th e ship's master must understand. Each tu g has its own limitations and cannot be expected to provide any and every service.

For the purposes of comparison, a diagram shows the traction performance at zero speed for each type of tug with an identical propulsion power. We ca n see for instance the capacity of the Voith Schneider tractor tug to exert a significan t traction force beam-on, as well as that of the ASD tug to push or pull in line with th e same efficiency forward or astern.

90%

45%

Of traction in moving forward

~. ~ ~ Asd

226

Voith-Schneider

Conventional

Diagram of traction at zero speed for each type of tug

3 · PART 6.2 TOWING IN PORT

2.2.6 Tugs and thrusters It may seem strange to cover the subject

In terms of safety, towing requires specific precautions, which have been generally

of thrusters here, in a chapter covering

covered.

towing. Nonetheless the shiphandler must

There are some common points, however: - the developed power (as well as per-

know the limits of each type, their drawbacks and benefits, independently of the

2.3.1 Speed This is the first essential point to cover, since it is the most vital. Although it is not the only cause of potential accidents, it is still the most important aggravating factor. A towline may break even at low speed,

economic aspects, of course.

formance) for each one is similar; large container vessels these days have

A tug is not a thruster and vice versa. How

thrusters with 4000 to 5000 hp; the

separating itself from the lines of the ship,

often does one hear, "I don't need a tug, I have a thruster I" Doesn't a bow thruster

same power levels are available from

even at low speed ; if the convoy's speed

port tugs. Some liners have thrust-

is also higher, this fault may cause both

have its limits? Can a stern thruster re-

ers reaching 9000 hp' and some port

human and physical disaster.

lieve the captain of the need to use a tug?

tugs can develop traction forces of 80

There is no question , of co urse, of de-

a tug may "get stuck", or have difficulty

We will look at the general subject of

tonnes . This may also cause problems

monizing this operation nor manoeuvri ng

transverse thrusters, only operating lat-

in terms of the resistance capabilities of

erally. Some of them, normally those on

the deck gear onto which the towlines

in general, but speed is the first factor to control.

may be warped

This does not mean that a minimal speed

smaller vessels, or river/sea-going ships, may be turned through 360° giving them a longitudinal action, but these are specific

-

in both cases, the ship's speed signifi-

must always be maintained , but only that

cantly reduces efficiency. Some of the

it should always be appropriate for the sit-

types. The first restriction to consider for a

latest tugs, however, can still work at

uation, under control , and agreed among

thruster is its transverse effect.

higher speeds of up to 7-8 knots, but

the various parties.

It therefore cannot slow a ship down by its

thrusters are often ineffective at more

There are commo n, often imp li cit rules

action alone (turning has a fairly significant

than 4 knots.

that are found in port after port with all

effect, but is not the purpose intended by a thruster) . A tug towing in line exerts a

pilots and tugboat skippers, depending

It is important to be aware of these vari -

on the type used. These rules should be

ous points so that resources can be fully matched to needs. Few owners have cho-

It should of course be added that this ap-

thruster does not replace a tug.

sen to fit stern thrusters to their ships, in

plies to a manoe uvre proceeding as nor-

A captain that does not use the help of

fact . It is rare to forgo the use of a tug for

mal.

a tug because his/her ship has a thruster

these large ships, especially because the

require extreme measures, which means

must be aware of this, and must be certain that the ship's own resources are enough

ship has to be slowed down rapidly to give

the limits that were set have to be exceed-

it greater manoeuvrability, even at low

ed. In such cases, everyone does what is

longitudinal action which the thruster does not have . It is thus apparent that a stern

shared.

Some

emergency situations

may

without this assistance.

speed . Installation of a stern thruster can

needed to stabilise or minimise the conse-

In particular, the captain must not only be

only be justified if the ship regularly uses

quences of a deterio rating situation, while

sure of the power of the engine relative

ports where the presence of a tug cannot

maintaining an acceptable level of safety.

to the ship's displacement, especially for

be guaranteed, so that nonetheless, ma-

This is not our concern here .

variable-pitch propellers (stern power re -

noeuvrin g is safer.

In section 2.2.1, we emphasised the risks

2.3

The convoy speed should be limited to 4

duced by almost 50%) and its reliability (risk of failure to start in some cases, or

caused by speed for conventional tugs.

Usage

maximum speed in forward motion pos-

Having covered the benefits and draw-

knots for this type, 5 knots maximum . Fo r

sible before going into reverse) , but also

backs of each type, we can proceed to de-

other types of tug , a speed of less t han

that the thruster can counter the prop

scribe how tugs are used: there are many

8 knots must be maintained. This applies

walk effect, as well as the effects of wind

common factors and some differences, especially as regards safety and manoeu-

phases of the towline - very tricky situ-

As regards the use of tugs, it must be re-

vrability. Once again, it is not possible to

ations - as well as during passages un-

membered that this needs enough space

describe all the options in detail. We will

der tow. It is also important to be aware,

to allow the whole convoy to turn; some-

only consider the main rules and the most

as regards safety, that the position of a

times in deeper water.

commonly found situations.

tug when ahead of the ship is often more

and current.

both during the attachment or release

The manoeuvrability of conventional tugs

dangerous and in this case, speed should

is quite sensitive, and on any type of tug is

sometimes be kept below these values.

less than that of a thruster. It does not take

During manoeuvring phases, speed will

long to reverse the direction of a thruster

naturally be lower, and thus less than the

with respect to the tug's course, in order

tug finds comfortable . It will be adjusted

to move from one side to the other.

in terms of value and direction to achieve

This flexibility is also available in terms

the desired effect, and to suit the com-

of facilities on the tug. A thruster can be

bined effect of the various resources (see

used at once if the ship has the power.

figures next page).

A tug is an extra tool that has to be planned; it needs specific facilities appropriate to the tug's power (deck gear, winches, fairleads) and human resources (crew) for implementation on the ship.

227

2.3.3 Pick-up and jettison of towline • f4

Once again, we will devote a specific section to an operation which appears fairly

,.

~~L""· ••~-~!-.,.• I

simple, but which also generates prob lems. The procedures for this are comm on

'

to all tugs and all ships. Compliance with a few simp le rules allows this operation t o be performed quickly and safely.

on the w inch followed by a nylon spring, with a breaking strain of 150 tonnes, which acts

Taking up the towline: • The tug must approach the shi p (normally from the stern or dow nwind), which maintains as straig ht a course as possible; turning may cause hydrodynamic effects th at could cause problems for the tu g. Especially when approaching fro m ahead, however, a certain distan ce must be maintained so as not to be pushed away by the bow wave an d risk coming beam-on (figure * ) . A suitable hauling line must therefore be provided (not just a simp le ballast that could injure the tug' s crew), attached to a tripping line (figure ** ). The effects of wind, and the roll and pitch movements caus ed

as a shock-absorber and a "tail" , a cable attached to the towed vessel which serves as

by the sea are all factors that can

a fuse, and at the far end as the tripping line for passing the towline. On ASD tugs, the

make this manoeuvre more difficult,

Presentation from th e stern at 7 knots Presentation from ahead

2.3.2 Composition of the attachment Composition of the port towline attachment is different from that used for assistance or at sea, whic h is limited to a single cable. When towing in the port, the towline used is very short, and its elasticity is provided by the textile content of the mixed towline. Generally speaking, on Voith-Schneider tugs, the towline consists of a steel cable stored

towline is made entirely of Dyneema , synthetic high-strength fibre.

increasing the risks taken by the t ug and its crew. The ship's captain mu st be informed when the operation begins and told of any incident th at may occur. •

The ship's heaving line is attach ed to the towline then hauled on boa rd the ship, normally with a winch, but sometimes

by

hand,

since som e

modern materials are very light an d flexible (figure ***) •

The towline eye must be passed through a fairlead , then attached t o a bitt strong enough to match th e tug's power. If this requirement can not be met, and it is not possible t o

Towline from a Voith-Schneider

do other than hook on the towline, it

Towline from an ASD

is essential to notify the ship's cap tain, the pilot and the captain of th e tug so that both the static and th e dynamic effects are limited •

When the towline is attached to th e ship, care must be taken to ensure its eye or its shackle is not carried on the fairlead, which could create a breaking point (figure ****) Having done this, the tug is notified so that it can let go if necessary an d lengthen its towline. A simple gesture, signa lling with crossed arms, confirms this operation. The ship' s captain must also be informed.

•

*Presentation of tug

* * Tripping line attached to towline

The crew must keep away from th e towline once it is in place, since it could heave tight at any moment

CHAPTER 3 · PART 6.2 TOWING IN PORT

2 .3 .4 Tugs in line This section covers th e work of port towing as such. We will consider here the various methods for using tugs. Carried out by th e pilot and the captain, th e planned manoeuvre take s many factors into acco unt. The ship 's own resources or external resources are used and adapted as required. Tugs form part of this scenario . The manoeuvre as a w hol e w ill include forces and moments that the ship's resources alone cannot cou nter or appl y. Tugs overcome this deficiency. Usin g their "manoeuvring" plan , the shiphandler, pilot and/or cap tain can use the tugs effectively. The tug is sai d to be "in line" when its towlin e is attached to t he towed ship in the longitu dinal axis, or very close to it, either over the bows or over the stern .

***Tauten ing tripping line

The tug's wo rk is carried out in this way along an arc of a circle about 180° centred on the point at which the towline crosses: - In line astern: the tug works in a lin e ahead or 90° eith er side of the axis . It can haul the sh ip along this axis and slow it if it has sternway ; it ca n help it to turn one way or the other, increasing or reducin g a turn; comb ined with other resources, it ca n he lp to draw the ship sideways .

~

. ·~:":~.

Tug in line astern

**** Towing line attached

-

Releasing towline • The ca ptain must give th e order

At the rear: t he t ug is wo rking in line aste rn or 90° either sid e of t he axis. It can slow the ship or reduce its headway; it can help it to turn one way or t he other,

increasing or re du cing a turn ; combined with other resources, it can help to draw the ship sideways.

to let go the line , norma ll y w hen t he ship has some way on. A tow line must never be released without the captain's order, even if it seems slack. In particular, when a tug is

•

placed astern of the ship, a towline released prematurely can slip beneath the hull and become fouled in the th rusters When the tug is ready and the order given (the tug has approach ed, the crew is on the manoeuvring deck, the towline is no longer taut), the towline can be detached and the tripping line slowly slacked off until the towline has been hauled in completely by the tug

•

The tug will then be able to release itself; notify the ship's captain that the opera tion is complete

Tug in line ahead

Th ere are m any uses for this method . In some specific cases, for instance, it is useful to use a tug in lin e ahead, whi le keeping the ship's eng in e moving forward : reducing speed by stepping up the tug's power kee ps co ntrol of the ship's cours e with the help of the helm and engine . Eve n on large ships the power "ahead very slow" rarely exceeds 2500 to 3000 hp. A tug ca n the refore maintain a very low manoeuvring speed for t he convoy, without having to put too mu ch strain on the towline. Another advantage of this situation is that tugs are hooked onto either end of the ship: this means the moment of the forces exerted when turning is at its maximum . Given the ship's hull shapes, on the other hand, it is very difficult for tugs to work precisely at 90 ° to the axis. This always causes slight drifting, which has to be controlled so as not to be taken unawares in confined pla ces .

Similarly, when a tug has to work alongside, when speed is not zero (tug ahead with headway, and astern with sternway) it tends to increase speed, simply because it has to keep a minimum angle with the ship to ensure it is not overtaken. Analysing the forces identifies a longitudinal component.

Water wedge caused by the tug's water streams

Tug making headway The towline take-up and release phases must be carried out correctly, with tugs in the zones where the hydrodynamic effects are greatest (bow lines, propeller suction, steering system). The tug's position in line astern natural -

Finally, we must address an effect that is mainly linked to the action of tugs in lin e astern, which sometimes significantly reduces the efficiency of the traction force applied : a tug hooked on, whether in line or alongside, normally works by exerting tracti on on its towline . The water streams generated in this co nfigu ration by its propell ers or thrusters may be directed towards the ship's hull, depending on their orientation . The action-reaction principle means that they therefore cause a pressure force on th e ship's hull which directly opposes the tug 's traction. The higher the po wer developed by the tug and the shorter its towline, the greater this effect is (up to 50% of the traction ) . Similarly, for rounded hull shapes, these water streams can be diverted and can flow along the ship's hull, reducing the tug's efficiency by modifying the hydrodynamic pressure zone (Coanda effect). A longer towline limits these negative effects .

ly increases the space the ship needs to turn: whether in transit, for manoeuvring, or passing through locks, tugs "sweep" an extra zone which has to be considered as the manoeuvre takes place . The pilot has to increase the margins available for passage or positioning, in both surface area and depth of water : modern push tugs have drafts of about 5 metres . Some precautions are needed with a tug's towline ahead, which can be used to reduce or even stop the ship's forward movement. A dynamic component is added to the tug's intrinsic force, produced by the speed which can be very high, depending on hull surface area and especially on the convoy speed.

2.3.5 Push-tow tug

The ship's crew must always keep well away from the towline, even if it is slack,

Second mode of use : the tug is used just to push. There is no need to take up a tow and it stays mobile, able to move along the length of the ship to vary the effects.

since it can tauten suddenly and even break, causing a whiplash effect. The effects of a tug in line astern may be reduced or even cancelled out by the "water wedge" effect : close to wharves or

In general, the tug takes up position at marks painted at specially strengthened points on the hull (TUG), or opposite reinforcements or bulkheads.

lock walls , the tug which exerts traction on its towline creates water streams which slide between the ship and the dockside. This causes a "water wedge" that can even prevent the ship coming alongside (figure*). This situation occurs when the ship is approaching the dockside while the towline's angle of incidence is necessarily small and the tug has to force in order to achieve a greater lateral movement: the water wedge (and thus the ship's headway!) dominates.

&ooooorn~~ In this case, the tug is of course no use in slowing down the ship; nor ca n it hold on to it. Its action is in one direction only, perpendicular to its point of pressure. It is naturally more limited in its action.

Push-tow tugs

230

CHAPTER 3- PART 6.2 TOWING IN PORT

Nonetheless, depending on its position

In these situations, the effect required is

relative to that of the ship's pivot point, the effect varies: with headway on, a tug

to apply a variable force to counter anoth-

Their flexibility allows t hem to turn mo re freely, limiting "parasi tic" effects. This flexibility has naturally led to changes

at the midpoint of the ship on the star-

er coming from a known direction . In particular, the manoeuvre involves con -

board side tends to make the ship come

trolling the balance between the push-tug

in working methods . In some cases, tugs can work in "push -pull" mode: the tug is therefore attached alongside , to a latera l

to starboard as it pushes and to port, with

and the force concerned.

sternway on (and with the opposite effect

It is important to keep this balance, since

bollard pla ced in the straight sections of

if the tug is on the port side). It is also important to know accurately

the ship-handler must be aware that the

the ship's sid e t o pull it (figure * );

push-tug acts in only one direction and its effect cannot be reversed: stopping

it can also push on the side of t he ship

where a push-tug is placed when the ship has way on, so that it does not have the reverse effect to that intended.

the action is not enough to stop the ship's movement . Reverse forces must do this

In any case, it allows the manoeuvre to be

job: thruster, holding tug, engine and rud -

action: it is a push -tug that can also hold;

closely controlled by combining the vari-

der control, wind , sea defences, etc.

it can help w ith a turn, then stop the turn.

ous resources, engine ( headway or stern -

(figure ** ) . The tug thu s has at least two directions of

The ad van tag es are obviou s -

smaller

is still useful . Less space is used than for

2.3 .6 Push-pull tu g The previous sections have shown that each situation has its limitations : significant space requi red, single direction of ac-

tugs in line astern (at best, minus the

tion, etc . Until now, conventional tugs or

ship' s deck gear and attach itself as close-

length of the towline). On the other hand, there are many situations where unilat-

tractor tugs were not sufficiently versatile to resolve all the problems that arise at

It is enough simply to reverse its own di-

era I action from a tug is required: hold-

once . The number and position of towing

rection of move ment to reverse its action .

ing a ship at the dockside, or bringing it

winches have restriction options for a long

It neve r moves away.

alongside slowly ; countering the effects of

time.

way) , rudder control, bow thruster, tug, etc. Despite its limited action, the push-tug

crosswind; overcoming lack of a thruster

Modern tugs, ASD or Rotor, have been fit-

or supplementing it when the wind is too

ted with winches at the bow and stern,

strong .

with improved safety for their operation.

-

*Push-p ull tug pulling

wo rkin g area, safe push ing , fast, conv enient t urns . This operation takes place withou t delay, since the t ug can pass its towline to the ly as possible to th e side of t he ship.

-

**Push-pull tug pushing

231

This is where the theory stops. It is clear, of course, that carrying out this type of operation takes a particular combination of conditions: - the ship must have bitts and fairleads on deck that are strong enough for the job, 50 to 60 tonnes or more - the ship's deck height must be within a range of values that allow the tug to work without its towline being at too steep an angle with respect t o the horizontal - the ship must have an effective way of coming to a standstill on its ow n : some longitudinal action is possible, but very limited; it also causes a turn which may be undesirable The typical type of ship for which this method may be very useful is a loaded crude carrier (VLCC and ULCC) : -

these ships have a clear deck, with specially designed and tested gear for the purpose when they are loaded, their deck is at the correct height. This is not always the case when they are light and when they have significant freeboard these ships, with very high displacement, have relatively low power engines for their mass: they therefore use two,

-

-

three or four tugs, or more One or two push-pull - tugs can therefore be used with another in lin e ahead to slow the ship in an emerge ncy On the other hand, this method is not suitable for many other types of ship: No solid mooring points on th e deck : most ships may have bitts and fa irleads on the deck or the side, but these are intended for barges and small ferries. They are not normally strong enough for

-

the work of t ugs; except for the "S unken Bollards" designed for the purpose, which tolerate a tension of 80 tonnes

Sunken Bollards

232

-

Unsuitable freeboard: chemical tankers, too small ; ro-ro ferries, container ships, too high

Nonetheless, the method can be slightly adjusted to improve manoeuvring of these many other ships in a variety of circumstances: - the tug may be attached in line astern or ahead; it can fulfil its ro le of tug completely; it is possible, however, that the ship's shape does not allow it to be pushed. In this case, it moves a few metres along the side to push on the straight sections - the tug can be attached to the quarter or the loaf, or even, if the deck gear allows, to the side, and a few seconds are required to move away slightly thus reducing the horizontal angle of the towline.

2.4

Special cases

Special cases here means uses derive d fro m conventional situations that are less ofte n used, or which relate to particular needs .

~ @

Working indirectly

\

at htghec speed

~

2.4.1 Working alongside

This working method is not specifically an exceptional case . Some tug captai ns do it naturally, depending on circumstances and with all type s of tugs . It requires considerable kno wledge, allowing for the hydrodynamic forces involved which increase tension on the towlin e. In general, the traction force of a tug in balance is exerted in the line of its towline .

The action is not reversed at once, of course, but it does not take long: time for the tug to let out or haul in its towline and move aside . This solution is often used . It is only possible with ASD or Rotor tugs, whose manoeuvrability is adequate, and whose winches are easily controlled from the command station . This greatly improves the safety and convenience of the manoeuvre. Although the push-pull solution is very practical, it cannot be used in every case. The pilot, ship' s captain and tug captain have to discuss it in advance in order to determine its feasibility and limitations : strength of bitts and fairleads, speeds, respo nse times, possible fouling of towline in the propeller, etc.

Working directly at low speed

A tug working ind irectly, alongside, in princi ple exerts traction so that its longitudinal axi s is not in line with its towline. The traction force of the tug is suppleme nted by a part of the forces applied to its hull. These forces are of course a function of th e square of the speed and its exposed surface. (F = k x 5 x V2 x sin i; (see chapter "Vessel in motion, basic concepts", thin plane theory)). The speed is a necessary condition in setting up the angle and finding a neutral position so th at the tug can work alongside.

It cannot find an angle without speed , and

This is relatively low compared to its

cannot work indirectly (except for Rotor

theoretical traction force, but may be enough . Working alongside in this way

tugs). The resulting forces may be very high . The effects gathered for slowing or turn -

-

in order to operate on one side, the tug sits at a sl ight angl e to the side concerned , which appl ies traction to the

also offers another benefit over that

element of t he other side : the effect is greater than if it were hooked at the

ing are increased ten - fold as tension is ap -

of another kind of tug working in line astern : if the rotor tug shortens its

plied , since the speed is high. The more

towline, the

component

hand, the force is applied on one of the

the speed falls, or the speed of the turn

is practically zero and the ship has no

two side s and not on the centre line, and o n th e oth er, the tug's angle of inciden ce m ay be greater for the same lat-

longitudinal

increases, the more the effect is reduced.

stern way or headway on; the work

When speed fall s too low the tug must re -

of the tug's thrusters is also essen-

central line of the ship, since on the one

sume a normal position to work line ahead

tially transverse and there are no wa-

as usual: it has to "turn around".

ter streams between the ship and the quayside, which does not conflict with

pose tra ction almost instantly on the

Special precautions are needed ; always fit

the berthing force . This means there is

opposite elem ent then shift laterally

t he to wlin e t o deck gear w hi ch is strong -

no wat er w edge or Coanda effect.

asi de.

-

eral moveme nt (half-width of the ship), to chang e side and action, it can im-

er than the tug 's theoretical force ; keep some distance from the towline, because

2.4.2 T ug close-coup led

It is thu s a working method that provides

of the risk of serious whiplash if it breaks.

This technique is especially suitable when

great f lex ibil it y of manoeuvring .

All tugs can work in this way as long as the

space is limited, when a tug may have t o

There are ho wever several drawbacks that

convoy's speed is not zero.

turn and move frequently from one side

limit its use:

For rotor type tugs, however, they can

to the other. For instance , entering a lock,

-

also work in indirect mode at no speed :

following

a precise heading, a narrow

used, it mu st be of good quality, solid

the tug may in fact, with its third (trans -

and well-maintained; it could be a weak

verse) thruster, create a force perpendicu -

channel , etc . On the one hand , the tug has very little room either side to turn and

lar to the axis of its towline and thus shift

appl y lateral actions to the ship , on t he

mate rials, and thus their elasticity, are

from its axis. This situation is only possible

other hand, it may be required to supply

all different . Hence the problems in bal-

at a low or even zero speed , unlike th e

these actions either side as qui ckl y as pos-

above-mentioned mode of working along-

sible. It may also be useful for a ship with

side (otherwise, its thruster is ineffective

no central fairlead over which the to w line

and inadequate for movement sideways).

is passed (RoRo ship with axial door) .

Thus, the two options are available for this

Attaching it on board would cause lateral

if one of the hawse rs on board has to be

poin t , with all the risks that entails. The

anci ng -

the use of an on-board hawser consequently redu ces t he ship's mooring capacity, and at lea st man oeuvring time

-

with to w li nes passed from both sides,

type of tug boat, depending on the speed

effects whenever the tug applies traction.

the ove rall wi dth of the manoeuvring

of the con v oy:

Th e tug can be close -coupled to m eet

deck is occup ied,

these conditions.

moori ng complicated

Force exerted

Indirect method

Generally, this solution is used over th e

-

which

can

make

the t im e needed to take up or release a

ship 's stern . The tug is atta ched lin e

t ow line is significantly longer: passage

ahead , with its towline over one side of

of a tow line , passag e of a hawser, ad-

the ship . The ship then passes one of its

justing lengths; releas ing both lines.

hawsers from the other side to the tug . The latter attaches it, then the ship ad-

Direct method

4

This solution mu st be used j udiciously

justs the length of this hawser to match

after con sultatio n wi th t he tug's capta in,

the two lengths.

the agreement of the ship's ca ptain and a c------- r - r---:>'=---c-r crew b ri efi n g .

7 Speed in knots

2.4.3 Tug in hip configuration This working m ethod invo lves attaching a .___,___..___,__,._""'"""'__. tug to th e sid e of th e ship in paralle l, by

-

speed is fairly high (2-3 to 5 knots),

-

alongside speed is low (below 2-3 knots) or zero, it works in indirect mode. Clearly in

This forms an isosceles triangle centred

allowing longitudinal slow ing or acce lera-

this case, the aim is not to bring the

on the axis of the ship. Actions take place

tion action . Towline attachment fitti ngs

it can work in the conventional way,

,---,......,..--.c....,

Tug close-coupled

the bow and the stern . It gives very accurate lateral con t rol, w hil e

ship to a halt, but to apply a transverse

quickly, with minimum movement of the

must be suitable for th e powe r levels in-

component to it. This solution is found

tug. It means that little space is used .

volved. This solution is used more oft en to tow a ship or barg e wi t hout propu lsio n .

very useful when the tug has no room

The lateral actions are also greater than if

to move on one side or the other, for

the tug were attached line ahead on the

instance along a quayside or in a lock

axis of the ship:

and it has to approach the ship.

to slow down in line, the tug applies a

Since the tug cannot get enough of

central traction on its towline and on

an angle to come alongside, it works

the hawser,

indirectl y, al ong th e quayside or dock wall, and imposes a transverse force .

CHAPTER 3 - 6.2

Tug in hip configuration

Escort towing

3

3 .1

Rules

Escort towing differs from port towing in several ways: - the intended purpose: it is not a matter of resolving manoeuvring problems, but protecting from the effects of breakdowns or accidents that are judged to be potentially serious for

-

-

-

the environment or safety of goods and people . The environmental aspect has promoted this procedure, following several acciden t s that caused serious pollution. This solution is used in many situations nowadays, for ships carrying "at risk" cargoes (hydrocarbons, gas, hazardous goods). location: this type of towing, whether in confined waters or coastal areas, always takes place outside port zones the tug: given the speeds, displacement of the ships concerned and the operating mode, escort tugs are specifica lly designed for this use speed: as they are not usually in-

3.2

Types of tugboat

No particular type is required unless the escort is outside the port zone, otherwise, any kind of tug can be used. Before the manoeuvre actually begins, or after it is completed and the towlines are let go, the tug or tugs assigned to assist the ship may accompany it in case of any incident. This takes place during passages when a tug is not needed, or at least not needed at once. The speed is often not compatible with their safety, but their presence as an escort provides for any eventuality. Escort towing specifically excludes the manoeuvring phase . Let us consider the case of a tug intended to escort ships in confined or coastal waters, where they will nonetheless still be exposed to the elements: wind, waves, swell. Conditions of use therefore determine the type of tug suitable . Authorities and states that require this may thus adapt needs to suit their requirements. - The general aim is to stop a ship un der way that may lose its propulsion or its helm control, and then make it safe. First of all, the ship has to be brought to a standstill. The tug (normally there is only one) is therefore attached (if necessary) line ahead to the stern of the

volved in manoeuvring, the ship may be travelling at speeds much higher than those acceptable for port towing and sometimes even at sea speed . This can generate significant forces since

ship. It should be mentioned that the speed of tra ve l makes it dangerous and ineffective to have the tug ahead .

they are linked to the dynam ic effects directly associated with the square of the speed . Special precautions and proced ures are therefore needed. There are two possible situations that may arise requiring escort towing : - the ship does not have all its own re-

-

234

Escort Tug

-

Speed: this is an important issue, since escort towing is often carried out for sometimes very lengthy passages, during which the ship maintains a high speed, often more than 10 knots. As we have seen, such speeds are incompatible with particular types of tugs, depending on their attachment

to always be accompanied by one or more tugs . These are generally so-

point. A tug whose attachment point is placed forward of its propulsion unit

called "sensitive" ships, whose cargo poses a particular risk (hydrocarbons, hazardous goods) or the size of which

should therefore be used. This tug must also be able to travel and work at speeds of 12 knots or more; this also implies the dynamic limita-

requires specific, special facilities to be provided . Particular rules and procedures are applied by the authority and the local services .

The weather conditions: the escort is not within sheltered zones . The tu g must therefore be able to travel, secure or release and work in unfavourable sea and swell conditions. It must have th e necessary power and towing gear, bu t its freeboard and directional stability must also be much better than that of a port tug . It is more like a sea-going tug. These three conditions define the properties of tugs that can be used for this typ e of situation: tractor or ASD tug, increased freeboard, significant power capacity, extra-strong towing gear.

3.3

Procedures and precautions

As for tugs working in line ahead, the tu g approaches from the stern, while the shi p maintains a constant speed. The towline is secured to a bollard via a suitable fairlead. The tug then follows th e convoy, without working, and is ready t o respond immediately. When letting go, th e same procedure is used as for tugs in lin e astern . If the tug has to slow or stop the ship, it may apply its power for braking. The most effective way, however, is t o work indirectly: the tug takes an angle of incidence with respect to the direction of the convoy. This gives it a greater wetted surface (especially if it has a drift plan e, like tractor tugs) . The force may be con siderable in this case as it is proportional to this surface area and to the square of the speed. The first precaution is thus for the ship t o secure the tug to very generously sized deck gear, bollards and fairleads; second ly, it is essential to keep away if necessary

sources available, or there are good reasons to fear malfunction of its propulsion unit or steering gear. In this case, to be on the safe side, it may request an escort tug . For this, it may use an actual escort tug or a port tug that would then escort the ship . In the latter case the procedure and precautions to be taken are appropriate for this type of tug, the port or local authority, often a government agency, requires some ships

-

tions linked to speed.

from the towline, which can tauten very suddenly, without warning . If it breaks, the forces on the towlin e can cause it to sweep rapidly and sharply across a very wide and unpredictable area.

4

support towing

Towing operations in the open sea are un-

Strictly speaking, the need for assistance

-

may arise in any maritime situation . There are three different situations, manage-

Port zone : port tugs will be used in this case . The ship may have to ca ll on these tugs

der the authority of the tug's master. The

ment of which depends on the ship's posi -

tug is responsible for any kind of damage that may occur during the towing opera-

tion: in a port zone; during escort; at sea .

ately in such a situation. Th e escort tug

In the first two situations, the ship is close

can respo nd at once and make t he ship

tions, unless the towed ship can be shown to be liable . (Article 28 of Act 69-8 on ship

to means of assistance, which although

safe before taking it to shelter. Escort tugs will be used in t his case

fitting-out and sales.)

can provide help and make the ship safe. Dangers and risks are also closer at hand.

4 .1

The emergency is often more obvious,

are conventional or tractor tugs . Fewer

cident, propulsion or steering malfunction,

within a few minutes. In the third case, special means are implemented . The level

conventional tugs since a m alfunctio n-

or a major breakdown. The ship has to call

of urgency may appear less, and depends

ing ship does not risk pu llin g the tug,

in external resources to make it safe until

both on the type of f ault as we ll as the

overtaking it, or moving too fast .

it is repaired, or even to take it to a safe

proximity of hazards at sea .

The power of these units may reach

Types of tugboat

Assistance may be needed during an ac-

-

not specifically intended for the purpose,

port or to its destination.

-

Escort towing: this is j ustified immedi-

At sea: specific , sea-going tugs must be used for this. In technical terms these problems may arise in po rt towing wi t h

25,000 hp , with traction of 250 tonnes .

Sea-going tug

4 .2

Equipment: ships and tugboats

Some types of ships are however required

This dev ice is completed at the stern with a towline pendant, with a fixed point on a

The previous sections have already cov-

to carry specific gear. The Solas convention requires crude car-

ered the equipment on escort or port tugs.

riers and gas tanker ships, whose dead-

device has a traction resistance of about

We will now look at that fitted on ships in

weight is more than 20,000 tonnes, to

200 tonnes, the equivalent of the traction

general and on assist tugs .

have an emergency towing arrangement

at the sea-going assist tug's hook .

4.2.1 Ships

This system must be capable of immedi-

In most cases, ships do not have specific

ate deployment, without having to use the

UMS are not required to have a special

equipment fitted for assist towing. They

ship's main energy source.

towing device. Nonetheless, Solas regula-

drum fitted w ith a brake (figure **).The

fitted to the bows and stern . "Ships whose tonnage is greater than 500

use the tug's own gear, carefully stowed

At the bows, it consists of a mooring chain

tions require these ships to have an emer-

on board, on specific fittings such as wind-

about ten metres long and a stopper

gency procedure, specifying the apparatu s

lasses or winches. The traction force ap-

aligned on the axial fairlead (figure * ) or

plied, especially in a heavy sea, may be

a Smit Bracket, a fixed point to which the

(bollards, guides, etc.) needed to take up a tow effectively and safely."

much greater than the normal fairleads

chain is shackled .

and bollards can handle.

4.2.2 Tugboats These have two complete sets of towing equipment (one as back- up ). It includes the winch, the recovery devices (posts, stopper winches, etc.), a steel

towline (1500 to 1800 metres long,

64-71mm diameter), a joining shackle, a steel pendant suitable for the ship to be * Stopper at the bows

**Stern towing device

towed .

235

4.3

Procedure

4.3.1 Passing the towline from the tugboat When the equipment is ready, the tug approaches the ship. Depending on the circumstances, it may pass a towline, either manually or with a line thrower. A tripping line is attached to this, and then to the end of the towline. If the ship has power, it draws the whole rig on board and secures the towline to a suitable fitting. Otherwise, it passes the hauling line and tripping line to the tug in the same way in order to set up a pass rope, so that it can secure the towline on board . Winch

Tug return hook rope

Hawse pipe

Pass rope system for bringing the towline linkage on to the ship without power

4.3.2 Passing the towline from the ship It is tricky to pass a towline over the bows of the assisted ship because the equipment (chain pendant, towline, tripping line) is heavy to handle, especially if the

vessel is without power. In this situation, the towed ship will supply the attachment. Using a line thrower, the tug recovers its hauling line then the tripping line, and hauls in the towline, which it secures to its rear deck.

Correct length of towline

236

When the towed ship has its own as-

4.3.3 Towing

sist towing device on the stern, the ship throws the end of the towing system into the water, attached to a light buoy. The tug can pick up this end and heave the pendant on board . It then shackles the end of its towline to the pendant . Towing from behind is only a temporary

Once secured, the towline is set up . The tug pays out a sufficient length of lin e. It should never become taut and should always remain underwater. This ensures the convoy is stable and ha s sufficient flexibility. The length of the tow -

phase , especially to prevent the towed ship from drifting towards the shallows. A long-term line astern tow is then required.

line paid out depends on the amplitude of the swell . The weight of the towline itsel f acts as a shock-absorber between the two vessels.

If the length paid out is not in phase with the amplitude of the swell , the traction forces exerted on the to wlin e co uld be too great and too viole nt.

Incorrect length of towline The ship tends to luff with the effect of wind and the hydrodynamic forces . It may then be much easier to find a stable neu tral position before getting underway at

The towing speed is rarely more than 4 knots . The towing equipment must be regularly checked on board, so that the gear is not damaged and the operation

When approaching con f in ed zones, t he tu g may shorten th e towline , before handing over to tugs suitabl e for worki ng in port . The ship ma y th en rel ease th e towing de-

sea or towards a safe port .

proceeds to a successful conclusion .

vice .

237

G

Cooperation with pilots .

.

1

Involvement of the pi lot on the bridge - Bridge reso urce m anag ement

Their local knowledge and experience also

2

Information to be given to the pilot - Master pilot exchange

mean that they are recognised as experts by naval headquarters, and form part of the t eam s eva luating and contributing t o the assistance of ships in difficulties, as part of the State's maritime policies.

Piloting is defined as assistance to t he cap-

vising him/her about manoeuvres to be carried out during difficult passages and

The regulatory framework for th e work of

tai n by a perso n appo inted by the State

the pilots is that of a public service mis-

to guid e ships en tering and leaving po rt,

within the port environment . The pilot's

sion, with general economic va lu e, to

wit hin ports, nat ural harbours and mari-

presence on board is obligatory, according

ensure safety at sea . They contribute to

t ime waters of rivers and cana ls .

to particular criteria set by the State .

protection of th e environment, and en sure the port remains competitive by improv -

dures for maritime and deep-sea pilots are

The pilot assists the ship's captain, ad -

His/her daily work forms an essential link

Recom m endation s on operationa l proce-

in coastal protection and marine safety in

ing its safety and reliability. All ships are

speci fied in Appe nd ix 2, IMO Resolution

the widest sense.

therefore treated equally, and most of

A960 (Act of 1928, Article 1, fixing the pi-

Marin e pilots a re merchant navy offi cers holding a master's certificate.

t hem em bark a pilot w ho works close ly w ith the commander during port t ran-

lot system, copied to the port code) sum m arised he re.

According to Dean Ripert (French lawyer

sits and manoeuv r es. The pilot mo nopol y

(1880-1958) specialist in marine law) , their role is one of the oldest spoken of by marine law. Some historical perspective is needed therefore in order to describe their role. This shows that the first maritime expeditions could only be carried out w ith the assistance of experts in astronom y and marine sciences . Th e deep -

guarantees the pilot's independence and

2.1

honesty, particularly for the ship 's captain .

" Effi cient pilotage depends, among other

General points

"The pilot advises but never displaces the

thing s, upon the effectiveness of the corn-

captain". This terse statement does not ig-

munications and inf ormation exchanges

nore th e fact that th e pilot has a lega l and

betwee n th e pil ot, the master and the

disciplinary responsibility, as well as civil

bri dge pers on nel, and upon the mutua l

liability, fo r every piloting o pera tio n un -

un derstandi ng each has for the functions

dert aken . Deta ils of regula t io ns may va ry

and duties of t he other. Establishment of

sea pilot was responsible for navigation

from country to country, but in all ca ses,

effective co-ordinati on between the pil ot,

alon e and was al rea dy assisting th e sh ip's

t he ca pta in of t he ship and t he harbo ur

the master and t he bridge personnel, tak-

captain, normally a trader or a war leader.

pilot carry out their manoeuv res jointly,

ing due accoun t of th e shi p's syst ems an d

As marine activities increased, the need

each wi th their ow n responsibilit ies.

equipme nt avail abl e to the pi lot will aid a

also arose for an expert in coastal and

This means they are a close team , bound

safe and exp edit iou s passage."

river navigation : the berthing pilot, "pilot

by th eir commitment to th eir t ask, w hich

collected on the spot, knowing the straits (na rrows, channels) and hazards of which the ship 's pilot is unaware " . Masters may, when visiting the same port frequ ently, take an examination to achiev e a Pilot exemption certificate.

goes well beyond tradition, and even the legendary f r iend ships am ong marin ers.

2 .2

Duties of ma ster, bridge officers and pilot

"Despite th e duties a nd obligations of a pilot, his/her presence on board do es not relieve the master or officer

The rights and duties of berthing pilots were then duly codified and finally led to a state-controlled, monopoly organisation, which set rules, checked the skills, organ-

in charge of the navigational wa tch

ised recruitment and decided on rates to

of th eir duties and obligations for the safety of the ship . I t is importa nt that, upon the pilot boar ding the ship and before pilotage commences, the pilot, th e m aster and the bridge personnel

be charged for pilot services . Despite technological progress, especially in the field of navigation, pilot services are still needed in every port in the world . Safety remains its main purpose, but it is often al so concerned w ith safety of port traffic and manoeuvres . Pilots therefore embark on the ships, with the captain 's agreement, and direct manoeuvres on entering and leaving ports. The maritime authority imposes on them the duty of reporting on the state of the seabed and the buoyage, as well as any malfunction on board ship that could put at risk the personnel, cargo or other ships,

Pilot on the bridge with the commander

are aware of their resp ective ro les in the safe passage of th e ship. The master, bridge officers and pilot share re sponsibilit y for good comm unications and understanding of each other's role in the safe con duct of the ship in pilotage waters. Masters and bridge officers have a duty to suppor t th e pilot and ensure that his/her actions are monitored at all times".

or the port installations and the environment. They must also report any incidents that occur during piloting, or accidents of whi ch t hey beco m e aware t hat cou ld have safety implications .

2 39

2.3

Procedures for requesting a pilot

"The appropriate, competent pilotage authority issues procedures for requesting a pilot for an inbound or outbound ship, or for moving a ship. As human resources and technical means have to be planned well in advance, the operation of an efficient pilotage service requires information on the estimated time of arrival (ETA) or departure (ETD) to be furnished by the ship as early as possible, with frequent updates where possible. Communication must be established as quick ly as possible, so that the captain can confirm the ship's ETA and the Pilot Station can furnish relevant information rega r ding pilot embarkation. The initial ETA message to the Pilot Station must include all the information required by local regulations including: 1. Ship's name, call sign, ship's agent. 2 . Ship 's characteristics: length, beam, draft, air draft if relevant, speed, thruster(s) . 3.

Date and time expected at the pilot boar ding point. Destination, berth (if required, side alongside).

4.

5.

Any other relevant requirements and information . "

2 .4

Master - pilot information exchange

"The master and the pilot should exchange information regarding navigational procedures, local conditions and rules and the ship's characteristics. This information exchange should be a continuous process that generally continues for the duration of the pilotage. Each pilotage assignment should begin with an information exchange between pilot and master. The amount and content of the information to be exchanged should be determined by the specific navigation demands of the pilotage operation. Additional information can be exchanged as the operation proceeds. The method for exchanging information may be formalised as a check-list or other memory aid to ensure that essential information is covered. 1 lthe

"Appendices" chapter gives an

example of the Pilot Card and the Wheelhouse Poster. 2 lRefer to SOLAS regulation V/34 and resolution A.893(21) on Guidelines for voyage planning, and STCW Code section A-VIII/2, part 2.

240

This information exchange should include at least the following: 1. Presentation of a completed standard Pilot Card 1J. In addition, information should be provided on rate of turn at different speeds, turning circles, stopping distances and if available, other appropriate data. 2. General agreement on plans and procedures, including contingency plans 3.

4.

5.

6. 7.

for the anticipated passage. Discussion of any special conditions, such as weather, depth of water, tidal currents and maritime traffic that may be expected during the passage. Discussion of any unusual ship-handling characteristics, machinery difficulties, navigational equipment problems or crew limitations that could affect the operation, handling or safe manoeuvring of the ship. Information on berthing arrangements; use, characteristics and number of tugs, mooring boats and other external facilities . Information on mooring

arrange-

ments. Confirmation of the language to be used on the bridge and with external parties.

2.5

Communications language

"Pilots should be familiar with the !MO Standard Marine Communication Phrases and use them in appropriate situations during radio communications as well as during verbal exchanges on the bridge. This will allow the master and the officer in charge of the navigational watch to understand better the communications and their intent. Communications on board between the pilot and the bridge watch keeping personnel should be conducted in the English lan guage or in a language other than English that is common to all those involved in th e operation. When a pilot is communicating to parties external to the ship, such as vessel traffie services, tugs or linesmen and the pilot is unable to communicate in the English language or a language that can be un derstood on the bridge, he/she should, as soon as practicable, explain what was said to enable the bridge personnel to monitor and subsequent actions taken by those external parties. "

2.6

Reporting of incidents or accidents

"When performing pilotage duties, the piIt should be clearly understood that any passage plan is a basic indication of preferred intention, and both the pilot and the master should be prepared to depart from it when circumstances dictate. Pilots and competent pilotage authorities should be aware of the voyage planning responsibilities of masters under applicable !MO instruments 2 J. "

lot should report or cause to be reported to the appropriate authority, anything ob served that may affect safety of navigation or prevention of pollution . In particular, the pilot should report, as soon as practicable, any accident that may have occurred to the piloted ship, and any irregularities with navigational lights, shapes and signals." (Article 13, directive 95/21/CE).

3

Embarkation and disembarkation of pilot

The pilot's ladder is an essential element in ensuring safe embarkation and disembarkation. If freeboard is less than 9

The pilotage authority or the pilot deter-

metres, the pilot ladder only is installed between the stem quarter and stern quar-

mines the location of the boarding (or

ter of the ship. When free board is over 9

disembarkation) point for the pilot, suffi-

metres, the pilot's ladder is used together

ciently removed from the point at which

with an accommodation ladder.

pilotage begins so that the embarkation manoeuvre can be safely performed . The master must assist the pilot's boarding on arrival, and provide all necessary means for him/her to come alongside and embark safely. Once pilotage is complete, the pilot should be disembarked under the same conditions. (Article 2, decree 14/12/1929 covering the general pilotage rules.) It may be tricky for the pilot to embark or

Boarding pilot

disembark. The operation is therefore organized jointly by the pilot and the master. The normal manoeuvre involves using the ship as a barrier to shelter the pilot boat from wind and sea, in order as far as possible to limit roll, pitch and heave which are hazard during transshipment. The position, speed

(normally 6 to 8

knots) and heading are the main criteria to be defined.

Freeboard less than 9 metres

The design of the pilot hoisting facilities and a description of its installation are described by SOLAS chapter 5, rule 23 and IMO resolution A889/21. Departure of p ilot boat Excessive speed alone can make approach, boarding and disembarkation of the pilot very difficult, since it causes changes in dynamic pressure and waves alongside the ship which affect steering and stability of the pilot boat.

I

Pilot boat in eddies caused by the ship's speed.

REQUIRED BOARDING ARRANGEMENTS FOR PILOT ln~mr.ce.wichlM.O.req...iremen:csandi.H.P.A.r~n5

INTERNATIONAL MARITIME PILOTS' ASSOCIATION

3.1

Transfer of the pilot by helicopter

Several pilotage stations sometimes use a helicopter to board and disembark the pilot. This type of operation is governed by the IMO and procedures were developed, initially by crude carrier owners (OCIMF), the first users of this method. The International Chamber of Shipping published a Guide to Helicopter I Ship Operation , which is now kept on nearly every ship's bridge. Using a helicopter for pilotage operations assists: - safet y at sea: it allows one or more ships to be attended within a very short space of time, sometimes at fairly long distances apart, before entering very co nfined waters, clear of areas with heavy traffic or in extensive pilotage zones. Most of the time, transfer of the pilot can also take place without changing heading or speed . - safety of transfer: even in very heavy

-

condition of the deck and other factors involved: the deck of coal-carriers for instance is often covered with a fine layer of dust after unloading. The turbulence caused by the helicopter's rotor will make this dust fly up, making the operation very uncomfortable or even dangerous. In this case, winching onto the wing of the bridge or another clear space is preferred.

The deck or place where the transfer take s place must always be clear. In particula r, there must be no objects left loose and untethered nearby that could fly away : cloths, helmets, fire hose, etc . If the transfer has to be made by helicopter, and the ship has been duly notified in advance, the procedures include a mini mum advance warning of arrival of about 15 minutes . This period gives the safety and fire-fighting teams time to prepare.

3.1.1 Deck landing Bulk carriers (on large colliers, the area is prepared on a cargo hold) and passenger ships (ferries, liners) which have specific, marked zones . In some cases the stanchion rails near the landing zone can be folded back . Some ships (North Sea crude carriers) have raised, lighted platforms, with a net . All personnel must keep well away from the zone, especially to the rear of the aircraft (blind spot for the pilot and tail rotor area) . This is especially important during take-off and landing : the "wind" generated by the rotor is very strong (over 70 km/hr) and the pilot of the aircraft does not have an all-round view of the position. Only the maritime pilot who has received speci fic training in this procedure may enter or leave the zone. The aircraft normally faces into the apparent wind (there must be windsock placed nearby) after reconnoitring the area .

seas, it means ships of all sizes can be attended, even the smallest vessels, for which the launch or pilot boat might not be suitable given the weather con-

-

ditions, flow of traffic: it means a large number of ships can be attended within a short period of time. The time taken to move between each vessel is shorter and the ship can continue to travel at speed. Traffic flow remains smooth .

The pilot may be transferred in one of two ways: by landing on the deck, or by winching down . The decision to land on the deck depends mainly on:

-

the type of aircraft used: its size (rotor diameter, length and width with rotors turning); its weight (the bridge can tolerate a particular mass per m2 and specific areas should be marked out to show the appropriate place) ; landing method: skids, floats or conventional wheeled landing gear;

-

Zone to be avoided when embarking and disembarking personnel

space available on the ship's deck: the clear deck of a crude carrier is usually suitable for this kind of operation; it is clear that small vessels or ships with a loaded deck (container ships, for instance) cannot transfer their pilot by

-

landing the helicopter; weather conditions: no spray across the deck and little movement of the ship; this may be minimised if there is enoug h space by manoeuvring the ship in order to bring it sea astern, for instance; Helicopter landing platform

242

3.1.2 Winching On some ships, specific winching zones are provided : Winch Only. These are smaller vessels unable to allow an aircraft to land on deck. Some ships have even more basic zones, normally on th e wings of the bridge : a " P" marked w it hin a circle 1 t o 2 metres in diameter. Winching may be carried out in very restricted spaces, depending on circumstances .

It is safer to transfer a pilot onto a zone 1m 2 , on a catwalk, the wings or upper bridge than onto a contain er, which may be slippery and with no handrail.

Starting to winch a pilot from a chemical carrier

Pilot disembarked

The personnel must keep away here as well (risk of falling objects ) and not hinder the descent or ascent of the pil ot , who is responsible for attaching and releasing his/her own hoisting system (belt, chest strap, harness ) . When the hoisting operation takes place near the bridge, the radar must be shut dow n.

~-r ~~~~~~---- · - 1 -- '

Winching onto wing

Winching onto upper bridge

It is also v ital t hat the commander is du ly informed of the availabil ity of th e pilot by helicopter in good tim e, and direct co ntact

If possible, the aircraft faces into the wind,

In the same way as for th e pilot boat, the

away from smoke from the funnel and

ship's owner remains liable when trans-

from antennas, and especially to give the

ferring the pilot by helicopter. The com-

pilot a permanent visual sight onto a fixed

mander may of course refuse to allow this

should be established betwee n command

part of the ship (positioning) .

boarding method, for various reasons , al-

or bridge team and t he he li co pter pil ot or

though it is still extremely safe.

the maritime pilot, so t hat everyone involved is clear abou t th e va riou s stag es of the operation .

2 43

Dockside manoeuvring practice

Preamble

1

The final case involves cutting speed too quickly. The wake effects reduce the speed

It is difficult to cover in a few generic examples all the possibilities for ship manoeuvres

of the water flows from the propeller

within a port, as the data intrinsic to the ship and external to it vary so much for each manoeuvre, so the ship-handler has to make strategic choices appropriate for each of

which supplies the rudder. The latter is therefore not efficient enough to

these elements . The manoeuvre should be treated as a physics problem to be resolved, analysing its data, the theoretical knowledge for understanding the forces concerned,

ship may therefore lose he lm control.

overcome the limitations of inertia . Th e

and rationale for identifying possible solutions, from which only the most suitable ones shall be retained . If the limits of the ship's own resources are exceeded during one of

2.3

the phases of the manoeuvre, the problems involved should be evaluated and any additional needs determined .

-

Loss of helm control situations

Abrupt transition from fu ll ahead to very

The analysis should therefore be made preliminary to every manoeuvre .

slow ahead : the loss of helm control is ac-

This work is then continued as the ship 's situation changes. It leads to fast, transparent

companied by the ship turning in the same

decision-making. The ship-handler keeps control of the ship; in other words, maintaining rational solutions whose gradual loss cannot reasonably be overcome by immed iate

direction as that of the prop walk effect -

When the ship is underway with a stern

implementation of improvised rescue efforts.

swell running at the same or slower speed,

The success of a manoeuvre often mainly depends on the relevance of this forward-

the rudder becomes ineffective and helm

planning. The rest comes from the ship-handler's knowledge and experience, his skills

control is on ly restored by giving short

as an observer and sharp eye, as well as from the professionalism of the ship's crew and

kicks ahead, so as not to increase the

other workers (tugs, linesmen). Manoeuvring needs team work, true synergy that dem-

speed already boosted by the swell

onstrates the competence of a crew. Completion of a manoeuvre requires great concen-

The rudder

tration and self-discipline, since the ship-handler has to come to terms with events, plan

2.4

ahead to maintain his/her chosen strategy, observe how the ship behaves and make the

It controls the ship's heading.

necessary corrections, constantly adjusting speed and helm control.

Its effectiveness is linked to the ship's way, to the speed of the water streams that sup-

This section offers a method for putting this into practice: understanding and design -

ply it, to its technology (conventional, Becker,

ing a manoeuvre around the forces present. We will also assum e that the ship has only

Schilling, etc.) and to the speed of its re-

its own manoeuvring resources , since manoeuvring with tugs is covered in the chapter on

sponse to a given incident .

"Towing". We are first going to review the manoeuvring gear, which forms the ship-han-

Observing the turning data displayed on the

dler's tools. We will see how to use these tools when manoeuvring, as well as their limita-

bridge indicates its performance. When ma-

tions . The system of forces can be used by physically designing their points of application.

noeuvring, the quality of a rudder is judged

2 Resources available to the ship-hand ler on the bridge

effect when it receives water streams from

by its ability to give a significant transverse

2.1

The engine

the propeller, while avoiding an increase in longitudinal speed. When the propeller is giv-

I thruster unit

The various types of engine/propeller combination are described in the "Ship's charac-

en a kick ahead with the helm hard over, this

teristics" chapter. The benefits and drawbacks associated with the ship's movement and

often begins by giving the engine a few sec-

the engine power determine the different types of behaviour when manoeuvring .

onds on half-ahead , to initiate the transverse

This will be covered in the second part. Once the mooring lines are cast off we will start

effect, then cutting it to ahead very slow, to

the ship moving and create forces which will generate dynamic effects that we will

maintain the effect without increasing the

identify. A manoeuvre requires strict and appropriate control of these effects, especially

ship's speed . The type of engine/thruster

speed and inertia, for its completion. We will therefore see how to manage them.

combination determines the extent of the

We will then add further forces from active external elements such as wind, to resolve a

" kick ahead" effect required .

manoeuvring problem by determining the best solution.

Generally speaking, use of the rudder when running in reverse to steer, in order to coun-

2 .2 The

The engine speed I ship speed pairing combined control of these two "engine speed 1 ship's speed"

ter the prop walk or luffing effect caused by concepts is vital when

manoeuvring. There are three possible situations:

the crosswind, is ineffective, except on some small ships that have a large rudder propor-

1.

The ship's speed matches the engine speed .

2.

The ship's speed is less than that matching the engine speed,

tionate to their size. This is combined w ith the " mask" effect that may create a rudder

3.

The ship's speed is greater than that matching the engine speed.

angled to one side when the engine is running in reverse . Water streams sucked in by

-

-

In the first case, the water streams generated by the propeller pass over the rudder

the propeller are disrupted and the propeller

at a fair speed; the helm is efficient and complies with the results of the regulatory

experiences cavitation .

turning and directional stability tests. Thus a reduction in speed should be applied

This phenomenon can be minimised on some

soon enough for the ship to lose inertia and its speed to remain matching that of the

ships by disconnecting the rudder, which is

propeller's rotation

then in asynchronous mode. It is therefore

The second case is used to improve helm control or speed up a turn; this is the

better to put the helm amidships before go-

kick ahead. The faster the water flows over the rudder, the more efficient the helm

ing into reverse . Propulsion efficiency is in-

control. To achieve this, after putting the helm over, the engine is thrown powerfully

creased, and the forces on the steering gear

into forward gear to speed up the water flow in the direction of the rudder, then it is

reduced .I n port manoeuvres, both helm en-

reduced before the ship gains too much headway. The rate of turn increases without

gines are started up to make the helm's ac-

increasing the ship's speed too much.

tion more responsive .

245

2.5

The bow thruster

This auxiliary device for manoeuvring, fitted to an increasing number of ships, may be used to begin transverse movements, to turn, or for control in sternway. It becomes quickly less efficient, however, as the ship's speed increases (over 4 knots, the helm action combined with the engine is normally more effective in starting a turn than is th e bow thruster) . With a crosswind, its effectiveness depends on its power and the windage of the ship's superstructure. To put figures on this, and to give an example, the simultaneous action of the thruster or t hruste rs, with helm and engine, counters a crosswind of about 35 knots for a very powe rful and manoeuvrable ship (ferry, liner) , 25 knots for a standard vessel (loaded coastal crude carri er) or 15 knots for a ship with significant w ind age (deck- loaded container vessel) .

2.5.1 Ship m ovement in reverse The thruster is used t o overco m e pitch effect and steer a ship at low speed, in reve rse in good sea conditions . Drift caused by the thruste r is still added to drift ca used by prop w alk . In poor weathe r cond iti ons, drift cau sed by wi nd shou ld also be included.

It is fairly easy to make a ship with two shafts (variable or fixed-pitch propellers) move (depending on propellers and their direction of rotation) combining the difference between engines, helm and thruster power.

Thruster to starboard

~ \ll

~~

vl~F;

~ Using differentiation of

Shifting a ship with one shaft, fixed-blade propeller, right-hand pitch

Shifting a ship with two shafts and a

When leavi ng a wharf, the en gine and helm must be carefull y controlled at th e sta rt to en sure th eir effect does not pre-

shafts, turning stern to sta r boa rd

thruster If equal forces and moments are applied at bo w and stern, the ship moves sideways to port or starboard. Adju sting t he po wer of th e t hru ster and engines allows th e direction of movement to be changed , thus causing the ship to m ove forward or ba ck . Using engines, helm and thruster all together in this ways makes for good manoeuvrability, so for instance checking the effect of wind drift. The same manoeuvre is more difficult, however, on a ship with only one shaft. The propeller's turning direction (prop walk) determines the side to which the ship moves. The example given (see fig ure above right) is a ship with a fixed , right-hand pitch propeller running in re verse, where the stern of the ship is mov-

Ship with stern way a nd thruster only

2.5.2 Tra nsverse movement It is obviously easier to make a ship move forward or back, than to make it drift. This approach is generally preferred when manoeuvring in a port . However, if the intention is to apply transverse force to counter drift, or to manoeuv re parallel with a wharf, the thruster may be used jointly with the engine and helm set in the oppos ite direction to that of the thruster's action .

24 6

Thru ster astern

ing to port. The action of the bow thruster driving to port combined with the turning effect of the propeller in reverse, alternating with the action of the engine driving forward, and the helm over to starboard moves the ship sideways to port. Because of the turning effect of the propeller running in reverse, it is harder to move this ship (right- hand pitch propeller) to starboard without external assistance (wind, tug, etc.).

dominate over that of the thruster. When the ship picks up speed as it leaves its berth, th e thru ste r' s action becom es less effective and th e angl e of th e helm must be reduced to ma intain the balance an d the course of the drift out towards th e open sea .

2.5.3 Countering drift When approaching a lock, for instance, the ship must maintain a minimum speed compatible with drift and good helm con trol in order to maintain the heading and alignment with the lock entry. This speed is a compromise between thruster effect and the action of the propeller on the rudder. In practice, it should generally be cut to 2 or 3 knots so that the thruster re ma ins effective . If there is significant wind drift, the stern of the ship cannot be held with a low engine speed, so short bursts of power are used so that the thruster is not made ineffective by increasing the speed .

2.5.4 Turning

-

The thruster allows a ship to turn within a small area . To turn to starboard, the

this force , transporte d to the ship's centre of gravity, gives it longitudinal and drift-

engine is put into reverse and the thruster's action assisted by the starboard prop

ing movements co mbined with a turning

walk effect. In order to stop the ship 's sternway, the engine is run forward with helm to starboard. When the ship is stopped, the engine is stopped and the turn

moment : the ship moves forward or back, drifts and turn s

continued with thruster only, whose action causes the ship to drift generally in the

-

same direction as the propeller's thrust (the ship also acquires headway).

the intensity of the tension on the mooring lines is controlled by the ship-handler

Over 15 to 20 knots, however, wind effect predominates .

by operating m anoeuvri ng winches or turn-

The direction of turn is determined by the luff effect or by seeking a neutral posi-

ing the moorin g li nes around a bitt so they

tion, as we will see below, rather than by the direction of the propeller's pitch .

coun t er t he t hrust of a propel ler or overcome inertia from residual way. The way in

/'

I

\

I

I I

I I

~'

I

-

I

I

which the rope is turned contro ls its tension excessi v e te nsi on on a mooring line may cause it to snap, causing a potentially hazardou s sit uation . Doubli ng t he m ooring lin e increase s the breaking load

2.7.1 Example: moving forward against the bow spring The ship is moo red port side to the wharf. The turning effect of t he right-hand pitch propeller and th e force of an on-shore wind if pre sent, hinder the departure manoeuvre . The ship-handle r m ay no netheless leave the berth Turning a ship on the spot, with one shaft, fixed-blade propeller, right-hand pitch

by running t he engine forward and turning against a long bow spring (the length of the

2.6

The st ern thrust e r

-

good

be

spring improves its horizontal alignment) . The

maintained between bridge and

communications

must

spring is theref ore f irst taute ned by haul in g

This auxiliary device is always fitted to as-

manoeuvring

com -

on a bow lin e to prevent the stress caused by

sist the bow thruster, but it is often less

mander must always warn ma-

powerful and efficient. It is used to counter

decks.

Th e

noeuvring decks of the intention to

a jolt. At the same tim e, the port bow side is pressed against a fender to protect the hull.

drift, for turning, for checking prop walk

use the engine during a mooring

The engine is the n started in forward mo-

when the ship is in reverse or to steer with

operation generally speaking, when berth-

tion, w ith the helm hard to port. There must

same limitations as the bow thruster.

ing, mooring lines are passed to

power and the breaking strain of the hawser,

the dockside one by one, with

w hich is checking t he thrust of the propeller.

2.7

a spring, bowline and also if re-

With the combined effect s of t he turning mo-

moderate displacement

quired a breast line, taking prior-

(light ships or those less than about 120

ity. Mooring lines are then doubled

ments created by the rudd er and bow sp ri ng , the stern of the ship gradu all y moves away

the engine stopped, at low speed, with the

-

Use of mooring lines

For ships with

metres long), mooring lines are very useful aids in most manoeuvres when berth-

if necessary -

when

moving

be balance maintained between the engine

from the wharf. When the angle of t he ship is off,

the

doubled

sufficient (at least 30° ), the eng in e is stopped,

ing and getting underway.

mooring lines are released, then

then put in reverse and t he spring is ca st off.

They become essential when it is not pos-

cast off in a sequence determined

The ship then moves off with stern way on . A

sible to use tugs, when the ship has no

by the ship-handler

comparable manoeuvre can be performed by

thruster, when space is tight or when the

moving backwards on a stern spring , bu t this

wind or current are hindering the manoeu-

When the ship is close to a wharf,

vre. However, there are risks of lines snapping,

mooring

lines

attached

to

bol -

Exerted at the bow or stern of the

put the crew of the ship and the dockside

ship, these forces are applied in the

line team in danger. Safety rules must be

direction of the mooring line and the

respected: - the commander coordinates the moor-

ship-handler controls their intensity.

-

t

lards may be used to create forces .

which must be prevented since they can

ing operations.

time without using t he ru dder.

\ >\ I\ I

When circumstances

tions, depending on the manoeuvring

require it, a briefing is arranged with

scenario, and the position and direc-

manoeuvring deck officers to provide

tion of the line . Some basic principles

clear information on the purpose of the

must be kept in mind, however:

manoeuvre and the mooring plan

-

\ I

\

I

There are many possible configura-

I I

1

\ I

\

1 I

\

', ,,'

\

' \~ '

'

'

//\

•'

it is the horizontal component of

mooring lines must never be tightened

the traction applied to the line

to stop a ship which has more than re-

which is used for manoeuvring

sidual way on

(see th e chapter "Ship in m ot ion")

G) Forward against spn ng

@ Engine in reverse

3

Ship underway

3.1

Reminder about the pivot point

On the other hand, the lateral speed caused by transverse components is dif-

A ship's manoeuvre may be understood in

The pivot point sits at the tangent of the

three different ways, already covered in

turning circle and the axis of the ship and

ficult to evaluate, and transverse inertia has to be anticipated and controlled using

thus reflects the turning speed .

manoeuvring resources that are generally

previous chapters:

1.

2.

3.

The first, certainly the most demand-

At this tangent point, the direction of the

unsuited to the purpose.

ing, involves assessing all the forces

ship's speed vector merges with the ship's

It is therefore extremely important to

applied to the ship at every stage. The application of the fundamental

axis. The closer the pivot point is to the centre of the ship, the smaller the turning

have a mental picture of the intensity of the resultant of the transverse forces at the centre of gravity and of the turning

principle of dynamics then allows the

radius becomes. When the pivot point is at

translation and rotation accelerations

the centre, the turning radius is zero and

the ship experiences to be determined against a reference mark linked to the

the ship pivots on the spot . Conversely, the further the pivot point

stood and analysed .

ship . This method is used by manoeu-

moves from the centre, until it reaches

The balance of forces is complex to de-

vring simulators or in the laboratory

the ship's physical limits, ahead or astern

termine . Hull resistance in oblique motion

but is limited as regards quantifying

depending on the direction of speed, the

also exerts a force in reaction to the ship's

the forces involved . These are many

larger the radius of turn, meaning the ship

movements whose transverse component

and varied, and their duration is un-

checks the drift. This force is linked to th e

certain. Action on any one of them

is turning slowly. Any transverse force applied anywhere

also modifies the others and estab-

on the ship generates drift and turning,

to evaluate . Similarly, proximity of port

lish es a new system of forces .

the pivot point for which is placed near

structures and the displacement of masses

Test tank stud ies are used to evaluate

the point of application on the hull, itself

of water caused by the propeller rotation

this method, which still remains dif-

dependent on the direction of movement

create other transverse pressure forces on

ficult to use in practice.

and the amplitude of the lateral movement

the hull, which are also tricky to quantify.

The second method uses kinematics .

ca used by the force.

It is possible , however, at any point during

moments, and every action on the ma noeuvring resources must be well under-

square of the ship's speed and is difficult

It involves con trolling the ship's tra-

It is important to determine the location

the manoeuvre, to assume that the action

jectory using a landmark. The position

of the pi vot point since it moves anywhere

the ship-handler is preparing to perfo r m to

of the "pivot" point must be known

a long the ship's axis depending on the ac-

change the ship's trajectory w ill modify its

for this. This point moves co ntinual ly

tion taken on the engine (thus on speed),

speed, drift and rate of turn.

along the ship's longitudinal axis .

on the helm and on the thrusters. Knowing

The intensity of the forces involved and

It can be located by measuring back-

the location of the pivot point at all times

the ship's inertia affect the variation in

ground· forwa rd and reverse speeds,

trajectory. The resources ava ilable that

or else wi th plenty of experience plus

means : - the ship's trajectory can be evaluated

practical knowledge of manoeuvring.

-

generate transverse forces and turning

the points of application of transverse

moments are: - the bow thruster

It involves measuring the helm and

forces. This distance, similar to a lever

-

the rear thruster

inertia of ships in many experiments.

arm, is used to predict the amplitude of

-

the prop walk

For obvious safety reasons, this ap-

drift and turning movements

-

The third method is entirely practical.

the distance between pivot point and

proach is often restricted to reproducing previously noted situations, without necessarily understanding them .

3.2

Direction of the resultant of the forces

A great deal of time and practical ex-

the rudder (External resources such as tugs, anchors

and moorlines are covered elsewhere) Apart from the prop walk effect, which is a technical factor arising from the propel-

perience is needed to make progress

The choice of method to be used, alone or

using only this approach.

with others during the manoeuvre, gener-

ler's design, these other manoeuvring re-

ates a system of forces and of points of

sources are designed mainly to make the

The manoeuvring examples offered in this

application of these forces.

ship turn. Nonetheless they all cause drift.

chapter mainly use the second method .

Each of these forces breaks down into a

The following diagrams illustrate these

Different situations show how the pivot

longitudinal force, a transverse force and

principles. Acting on just one of these

point position varies with the actions un-

a turning moment all of which accelerate

resources creates a turning moment and

dertaken by the ship-handler.

the ship's motion within these three de-

transverse which make the ship turn in the

In this section, we will start the ship mov-

grees of freedom . Turning, for instance,

same direction as the force:

ing, explaining the phenomena that arise

is obtained by exerting a transverse force

-

during a port manoeuvre, which implies

either end of the ship, which creates a yaw

with the engine running ahead (the

moving at slow speed.

moment. When manoeuvring, the long i-

speed vector is directed towards the

There are also no disruptive effects to con-

tudinal forces generate a velocity that is

outside, the hull 's resistance R to drift

sider from wind, current, shallows or from

easy to measure and easy to control with

amplifies the rudder's turning moment)

nearby port structures. We have dealt with

the engine. Similarly, the rate of turn or

their effect in earlier chapters .

variations in the rate, are easy to quantify by eye, or with a compass.

action of the rudder to initiate a turn

(See figure next page*) -

action on bow thruster only to steer when moving forward: (speed vector is directed inside, hull resistance R to drift checks the turn) (See figure next page**)

248

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Mdlcr

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fuQ

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Coti:DIIlo

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-

.

-

VIII"

. -

---~-

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niiiP1TCH

nfr fltQi

** Thruster only

* Rudder only

Action of several resources used simulta-

-

Actions on the bow thruster and rudder

The action of several reso urces at once to

neously to cancel their turning moments

to translate the ship, moving off from

increase the rate of turn reduces or ca ncel s

increases drift if the transverse forces are

the wharf (figure below) .

drift, sin ce the transverse force s are op posed to each other.

in the same direction . - Helm in reverse movement, with bow

-

thruster to compensate for the prop

Actions of the bow thruster and th e prop walk eff ect in turni ng (f igu re below):

walk (figure below) ,

•-

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a.-

v... I ,_ I...,

1

l ~~t-

..._ 1 11...

. ...!"'·;:·~··::':""""'

w;;; " ' • .

Control surface in reverse

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~,l r... I !~o.. l.auo I "- I t.:.~~~o•

[6nol

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Lateral movement Turning with thruster

and thrusters of different power :

for turning; thrusters of the same power

Identical thruster power

..,.._

I ... I

. . . . . .. . . .

and engine

Opposing actions on bow and stern thrusters

SW

-~.:::::.:."",...._,__

-.:;: , ' ' ' I '

with thruster

-

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tw

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rGA~

._."'±:.:"''""..., -

Bow thruster more powerful

3.3

Position of pivot point in a manoeuvre

The ship-handler has to leave the ship, mentally-speaking, in order to use the concept of pivot point, and reason from a reference point on land.

3.3.1 Ship making headway When turning while making headway, the pivot point may be located approximately in the ship's forward quarter. If the bridge is set further back, the ship-handler moves sideways on an arc of a circle towards the

outside of the trajectory, which is decep tive giving the impression that the ship is drifting a great deal (figure left). It is important when turning to concentrate on the trajectory of the pivot point, which will be that of the ship. A bridge placed towards the stern third of the ship (for a contai ner vessel), gives the impression that the ship is coming to a standstill. When the bridge of the ship is further forward, the observer is thus almost on the trajectory of the ship as it turns (figure right). Although the eye may have a good impression of the trajec-

tory, it is important to reca ll that the stern of the ship sweeps through a wide zone and this movement is difficult to observe when only seeing the turn further forward . External actions, such as those of the wind for instance, also alter drift and the position of the pivot point, and amplify any errors in perception that the ship-handler may make, depending on his/her position in the longitudinal axis of the sh ip . Practical experience helps to reduce these mistakes.

::--~(~>~~----

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Observer at the stern

3.3 .2 Use of bow thruster when ship is making headway with helm ami_dships The action of the thruster causes the ship to turn by pushing the bows. It also generates an overall drift directed towards the inside of the turn (figure left). If the helm is amidships, the ship turns around a pivot point whose position changes with speed, between the middle and the transom. This point remains on the trajectory. The ce ntre of gravity of the ship moves in the direction of the thrust of the thruster. Hull resistance to drift and turn combine their effects. They are placed towards the front of the hull, on the inner side of the turn, and check the movement.

Observer at the bows

Their intensity is in proportion to the square of the ship's speed. The vector of the inertia force slightly moves the ship's axis and contributes to the turn to a small extent. The surface speed vector is directed towards the inside of the turn , because of the drift caused by the thruster's action . As soon as the ship gets underway forward, the action of the thruster, which directly checks the hull resistance, loses its effective ness . While the ship's speed increases further (over about 4 knots), the thruster's effect falls (the performance of

(It is this phenomenon that allows the increase in the thruster's efficiency curve over 4 knots to be justified. See section 1 .2.2 of the chapter on "Bow and stern thrusters".)

its propeller decreases) .

Drift towards the outside

Drift towards the inside

250

The drift caused by the thruster's action is gradually elim inated . Centrifuga l force increases w ith speed, on the other hand and causes slippage towards the outside of the turn (figure right) . Hull resistance to drift thus helps to amplify the turn giving the impression that the thruster's action is increasing .

_______

....._

CHAPTER 3 - PART 8.3 - SHIP UNDERWAY

3.3.3 Ship stopped In practice, if the ship is stopped , the pivot

Ships fitted with special Seeker rudders nonetheless create a transverse compo-

to port if it is in the opposite direction). Thrust on the st ern section causes the ship

point merges with the centre of gravity at

nent equivalent to that of a bow thruster

the middle of th e ship. A ship with a single propeller can only

and can thu s turn on th e spot by alternat-

to turn while drifting and its pivot point is forward of t he centre of gravity, m oving

ing the forward and reverse thrust to limit

further forward as the reverse speed in-

turn on this point if it has bow and stern

the longitudinal speed .

creases. The tran sverse force of the prop

3.3.4 Ship making way in re ve rse When moving in reverse , the ship is towed by its propeller but cannot move straight back because it is subject to transverse thrust (prop walk to port or starboard and deadwood effect ). This transverse thrust (around 15% of the propulsion force) makes the ship turn to starboard if it is a righ t- hand effect or

towards the in side of the turning circle (figure left) . The cen trifugal force induced

and thrusters of the same power, or if it is coupled to two tugs pulling with identical force . Otherwise, if the ship only has one bow thruster, it may only turn on its centre with the engine moving forward and the helm compensating for the bow thruster. The pivot point will then move forward once the ship gets underway.

walk causes t he ship to turn and also drift

as the ship begin s to turn implies slippage (drift) towards th e outside of the curve. If the slippage (drift caused by turning) is greater than the drift caused by the prop walk effect, hull resistance to drift ampli f ies th e t urn (f igure right) .

/

li~l

~~

~y (

'

~/. -.. r_r_cr:~

c--"L~~==-

Drift towards the inside

3.3. 5 Use of bow thruster when ship is moving in reverse

Drift towards the outside

The bow thruster may be operated to help

and is deducted from the drift caused by

If the thruster checks the prop walk be cause of its tu rn in g moment, it causes the ship to sl ip under the jo int action of its

the ship to turn in the direction of the prop

the prop walk . The hull 's resistance to drift

transverse thrust an d the drift ca used by

walk, adding the moment of its thrust to

amplifies the turn . The stern shapes, nor-

prop walk. The tu rnin g effect of th is drift

the moment caused by the prop walk.

mally rounded , nonetheless limit the turn-

amplifies the thruster's mome nt and also

The pivot point moves towards the rear

ing effect of these resistances (figure left) .

counters the prop walk t urn (figure right) .

The drift caused by the thruster is added to the slippag e cau sed by ce ntrifugal force

third, in this case.

Drift towards the outside

Drift towards the inside

------------------~ 251 1

3.3.6 Berthing, port side to the wharf without thruster; berthing at a dockside In this example, the berthing side is said to be favourable, since when moving in re verse, the ship's poop naturally approaches the wharf with the prop walk effect.

The approach to the wharf is at an angle of about 20° to 25° to the side, considering the place where the pivot point is placed (forward third), at low speed, from 2 to 3 knots for a coaster and adapted to suit the ship's inertia for the larger ones. When the bow is sufficiently far from the wharf to suit th e inertia and speed (about

1.5 ship's lengths), the eng ine is put half astern then slowed as a function of th e turn caused by prop walk and the redu ction in speed. The prop walk effect acts like the rudde r, and the trajectory has to be controlled wi th the engine so the ship approaches the wharf while rounding it.

----- ...

I

/

(

- ----- - --- - -

- -- Berthing, port side to the wharf without thruster

The slower the speed, the more noticeable is the prop walk. If the ship stops too early it will still be away from the wharf. If

3.3.8 Berthing, port side to the wharf: ship fitted with a bow t hrust er

the prop walk effect is only slight, the turn has to be made using the helm and the engine running forward. The stern of the ship approaches the wharf while the port side remains at a constant distance from the wharf, and almost alongside. At this stage, residual speed is very low and the engine is stopped.

In this case, the bow thruster is started up facing to the port side, w hich counters the turning effect of the prop walk. A latera l drift is set up for the ship to port. Operating the engine forward , with the helm to port and the starboard thruster,

A kick ahead with the helm hard to starboard is needed to reduce the aft approach speed just before it comes alongside, until the ship stops turning. This kick ahead gives the ship a little more speed, and this is why the initial speed should be fairly low.

3.3.9 Berthing, starboard side to the wharf without thruster: berth-

The engi ne is then reversed to stop the ship at the wharf side. Throughout the final approach, care will be taken to ensure the bow spring does not tauten before the ship stops, since as it becomes tight it would pull the bow v iolently towards the wharf and push the stern away.

3.3.7 Berthing, port side to the wharf; berthing in a lock In a lock, the angle of approach is limited by the width of the channel and can be no more than about fifteen degrees, and the sto pping distance must allow for several kicks ahead to stabilise the stern turning towards the lock wall . Further resistance to forward progress (piston effect) helps to reduce speed. The restricted volume of water around the propeller reduces the latter' s efficiency. This is a tri cky manoeuvre.

252

cancels this drift to port.

ing directly or in a lock In this example, the prop walk effect is a handicap because it moves the stern away from the wharf and the water streams from the propeller running in reverse create a water wedge effe ct between the ship and the dockside, which causes first the stern then the whole ship to move away when the water streams move forward. It is difficult to overcome this phenomenon in order to prevent these effects, but they can be reduced by using the engine and helm . The term "direct" means the ship is approaching while moving forward . The approach to the wharf is at a slight angle, some ten degrees, with regard to the point where the middle of the ship is located . The engine is put into reverse to

At this point, the ship moves away from the wharf again at an angle, and the engine has to be started forward again with the helm to starboard to bring the stern ro und, and m ove back slowly to stop the turn. In this type of berthing, countering the movement with a sma ll ship, a head rope is quickly carried to the stern to hold it while slowing the ship down . What is more, if the sh ip is too far from the wharf and its stern is brought closer using a kick ahead action, with the helm to starboard there is a risk that the absorbent effect of the water stream and the prop walk would be lost. The stern could then strike the wha rf.

3.3.10

Berthing, starboa rd side to the wharf: ship fitted with a

bow thruster If the ship has a bow thruster, the best method is to bring the stern alongside first, and steer using the thruster. There are two examples: 1. The ship arrives in the berthing direction, 2 . The ship arrives from the opposite direction to that for berthing , so it ha s

keep the speed down, and once the prop

to turn . If it arrives in the berthing direction, the approach takes place slowly, parallel at one width from the berth . When the ship passes in front of its berth, the helm is put

walk effect appears, it is stopped then started in forward motion for a kick ahead with the helm to starboard. This action brings the stern towards th e wharf. The engine is then put in reverse to dampen the turn when the ship comes parallel with the wharf.

ship turning, then the engine is reversed with the thruster to port. Slippage compensates for drift caused by prop walk and the thruster. The significant prop walk should be overcome by the thruster working to port, to preven t the ship turning .

hard to port with a kick ahead to start the

When the ship is perpendicular to the wharf, at a position forward of its berth, and it begins to move in reverse, the thruster is stopped and the ship begins to

('

I

turn to starboard with the prop walk as it approaches the wharf. The thruster is used to control the trajectory of the pivot point and keep the stern as close as possible to the wharf. Finally, the engine is run forward to stop the ship, while controlling the stern with the helm and the bow with the

Berthing, unfavourable starboa rd side to the wharf with thruster

thruster. (figure right) When presenting in the opposite direction

the ship is port side to the wharf, mov-

to that for berthing, the approach is paral-

ing in reverse generates a prop walk effect

and heavier ships if they have 2 propel-

lel to the berth, at a half sh ip's length and

that pushes the stern towards the wharf.

at low speed .

The method in this case consists adopting

lers. In this case, both propellers are controlled separately with equal speeds, port

When the bow passes ahead of the berth,

the angle to the wharf by moving forward

in reverse and starboard forward, with the

the turn to starboard is begun with a kick

on the bow spring, helm hard to port after

helm to port.

ahead, helm hard to port, then the engine

releasing all the other mooring lines. As

The propeller driving forward , w hich is al-

put in half reverse, and the port thruster

it takes the angle, the port side presses

ways more effective t han that in reverse,

used. When the ship is stopped, the

against the dockside fenders.

creates tension on the spring that may re -

turning torque generated by the prop walk

Care must be taken to avoid any obstacles on the wharf as the bow passes over the

main moderate .

effect and the thruster around the mid-

This method is po ssib le on much larger

die pivot point is at its maximum, giving

dockside. When the ship is at least 30° to

Se con d type : ship fitted with a bow

the ship a significant kinetic rotation en-

the wharf, the engine is put slow astern

ergy, which then has to be reduced by

with the helm amidships, and the spring

checking it with the bow thruster.

is let go. The ship moves away from the

thru st e r If the ship ha s space ahead, all mooring lines are let go, an d the ship m oves parallel with th e wharf, r un ning the engine forward with t he helm to port and the bow thruster to starboard. As speed increases, the thruster loses its effectiveness; the lever arm of th e force it applies ,

The engine and thruster are controlled so

wharf while coming to starboard with the

the ship's turn can be stabilised, and be-

prop walk effect. When the ship begins to move back paral-

gins to move in reverse when it is perpendicular to the berth . The rest of the ma-

lel to the wharf, the engine is stopped then

noeuvre then proceeds as before.

started in forward motion with the helm to starboard to adopt the angle and be ready for the ship moored ahead.

3.3.11

Getting underway, port side to the wharf

This

manoeuvre,

which

relies

on

with respect to the pivot point, is much less than that of the rudder. The engine

the

or helm must be re duced so the ship can

spring, is only possible for ships whose

adopt the angle. This met hod can be used

The technical choice made for moving off

engine can be controlled to reduce the risk

on all types of ship f itted wi th bo w thrust -

depends on several factors:

of breaking the mooring line (the propeller

ers (figure below).

1.

Whether or not there is a bow thruster

2.

The space available in front of the

thrust can be estimated at one tonne of thrust per 100 hp and depending on the

ship, and the port layout

type of rudder system, around 30% of this force is absorbed by the rudder's drag) .

First case: ship without bow thruster If the ship has space ahead, the easiest solution for moving off is to let go all mooring lines from the bow, then draw in the stern mooring lin e, lengthening

Generally, this applies to ships less than 120 metres long . It is also important to ensure the spring is tightened carefully or the slack on the mooring line taken up, to reduce the risks of breakage.

the stern spring so that the ship moves away at an angle. Once the angle is wide enough, the mooring lines are let go and the engine started up forward with a slight starboard helm . This method is only valid for small ships (up to about lSOm) whose stern shapes are rounded enough for the ship to take up the angle while ensuring that the stern overhang does not strike the dockside bollards. If the ship does not have enough space ahead, with another ship present for instance, or if its stern shapes do not allow it to open up, there is

Berthing, unfavourable port side to the wharf with thruster

no choice but to move off in reverse . Since

253

If the ship is restricted for space ahead, it

moves off with the engine running ahead on the bow spring, helm hard to port, and the thruster running to starboard to hold the bow away from the wharf. As the thruster opposes the action of the helm, more power is needed on the engine so the ship can take up the angle. When the ship is at an angle of about 30° to the wharf, the thruster and engine are stopped and the helm put to midships. Once the spring is let go, the engine is put slow astern, and the thruster to port as soon as possible to reduce prop walk. When the ship has cleared its berth completely, the engine is stopped and the thruster put to starboard . As the pivot point is towards the stern, the thruster's lever arm is almost as long as the ship, increasing its effectiveness. The engine is then run forward, the helm and thruster set as required.

3.3.12 Getting underway, starboard side to the wharf; ship without bow thruster If the ship has space ahead and can move off directly, the same procedure is followed as for moving off, port side to the wharf, hauling on the stern head line . If the ship has to move off in reverse, after taking up the angle on its bow spring, the prop walk makes it turn to starboard as it leaves its berth . There are then two examples: - the departure heading is opposite to that of the berth. The starboard turn , started by the prop walk effect, is favourable and it is then enough to start the engine run-

-

ning forward and the helm hard to starboard to complete the manoeuvre the departure heading is in the same direction to that of the berth. There are two options, depending on the space available to manoeuvre: •

if the space around the berth allows, the ship can turn itse lf through 360° alternating the prop walk effects with the engine in reverse, and that of the rudder hard to starboard on the engine running forward,

•

if there is less available space, then after taking up the angle with the spring, a port chain has to be attached before moving backwards . The ship will begin to turn to starboard with the prop walk effect, then as it tightens, the chain will hold the bow and the ship will move straight back. When the ship is far enough away from the berth, the engine is started forward with the helm to port at the same time as the chain is hauled in .

254

Berthing, unfavourable starboard side to the wharf with thruster

""'""'~" "'"'"""" 11\tiii iii iii\ IIUI\ 11

'''" "''11'11

.:::~;~~;, % 3.3.13 Getting underway, starboard side to the wharf; ship with bow thruster If the ship can move off directly, the same

procedure is fo llowed as for the port side to the wharf, moving the ship away in parallel with the thruster and the engine running forward with the helm hard to port. Otherwise, the ship is sprung off on the bow spring with the helm hard to starboard with the engine running forward, while putting the thruster to port to move the starboard side away from the wharf. In this case, a smaller angle is needed before letting go since the prop walk effect, when the engine is running backward, will help to open up the angle (figure above). When the angle is wide enough, the ma chine is started up in reverse and the bow thruster reduced, but kept to port to overcome the pivoting of the ship about its centre as caused by prop walk.

3.4

Estimate of speed

The ship- handler's skill is linked generally to his/her capacity to measure longitudinal, transverse and angular speeds by observation, and con trol them contin ually in order to adjust speed to inertia as well as the trajectory plans determined for th e manoeuvre.

% Passing of single, nearby objects such as the beacons along a jetty, or objects along the wharf, gives a different, lower estimate of the speed . During a turn, speed is difficult to evaluate just by observing the environment. On the other hand, passage of foam on the surface of the water along th e hull gives a very good idea of speed. When moving in reverse to stop the sh ip during a turn, it is clear precisely that the ship has stopped when the foam is stationary with respect to the deck stanchions, while observing how the eddies from the propeller move towards the middl e of the ship is still an approximate method. The ship-handler must learn to estimate speed by calibrating the estimate made by sight against an instrument measurement . This tra ining allows speed to be observed to an accuracy of around half-a- knot, to an experienced eye. The other essential aspect of the manoeuvre resulting from the ship's speed is the concept of inertia. Inertia is proportional to the weight of the ship, but increases with the square of the speed. Hence, a heavy ship as it picks up speed stores considerable kinetic energy that must then be dissipated by the engine alone, while sti ll maintaining the trajectory. Just as it is necessary to calibrate the eye for speed, it is also important to learn to estimate the ship' s inertia , to apply "the right speed at the right place".

3 .4.1 Longitudinal speed Longitudinal speed is easy to acquire and generates kinetic energy (V2 m.V 2). The longitudinal speed is adjusted taking into account two aspects of the ship-handler's

This can only be learned by manoeuvring ever larger ships, using a low manoeuvring speed for each tonnage category, which is gradually increased while observing and memorising distan ces needed to

problem: - the inertia acquired by the ship which

eliminate the in ertia associated with the various speeds .

has to be eliminated so it can stop - the accuracy of the trajectory The longitudinal speed is judged by observing from the wing of the bridge how objects in the port environment nearby

A manoeuvre is successful when the ship is placed at the chosen point, with a positive, zero or negative speed essential to

move in front of other objects further away in the background.

ensure that the ship can follow the chosen trajectory.

When manoeuvring speed is too low with respect to displacement, the ship has a lee helm, it is unstable with an uncertain pivot point and insufficient inertia to start the turn effectively, and it moves away too quickly, too far ahead of the desired position. Conversely, too great a speed leads to overlong stopping distances, a trajectory that is distorted by prop walk, made worse by using the engine too much, at too high a power in reverse, increased slippage, and thus a lack of control and accuracy for the trajectory.

3. 4 .2 Tra nsverse speed w he n ship f oll ows a head ing Difficult to detect and estimate, this is caused by slippage during the turn, with any transverse force caused by manoeuvring resources or wind drift. Transverse speed is often low, but inertia gained during this transverse movem ent is significant and hard to eliminate since the ship's manoeuvring resources are not really designed for this, except for the transverse thrusters. The problem there fore is to identify transverse drift then quantify it. When the ship is moving fast enough (about 2 knots, variable depending on the type of ship and its draft), the transverse drift creates resistance to oblique movement on the bow side, which is sensitive enough to generate a turn . Regular helm adjustments are needed to combat this tendency, in order to maintain the heading . At low or zero speed, the turning phenomenon disappears ; but if the water surface is calm, the drift generates a typical chop on the side of the drift, while the water surface remains smooth on the opposite side . Drift can be detected by observing the passage of objects either side of the ship with respect to the background : - if an object on one side is fixed with respect to the ship, the ship if drifting on -

a collision course with this object if two objects situated either side of the ship are seen to move past at different speeds, this shows a drift towards the object passing more slowly against the background

The two-mast alignment systems are used to detect lateral drift with respect to the direction they show. Conical systems formed of two masts to the front and one behind, defining the limits of a passage zone by means of two alignments, provide useful information on the direction and speed of drift when the ship is following a stable heading, and by observing the

movement of the rear mast towards one of the two forward masts . When there is no alignment, one must be created by choosing separate reference marks placed ahead and parallel to the axis of the ship by eye, and observing how their angle with the eye changes, while ensuring that changes in the appearance of the alignments caused by a change in the ship's heading are not interpreted as drift. This technique applies when the ship is approaching port structures offering many suitable landmarks, but cannot be used when passing through into a pool where there are no close reference marks and those in the background are t oo far away. If the reference marks on the axis are useless, the radar can be used as far as possible in apparent movement by observing the plot of a fixed object, such as the echo from a buoy or a fixed object inside or outside the pool. The electronic sight, adjusted parallel to the plot of this object, shows the ship's true heading.

3 .4 .3 Estim at e of drift when ship is turning For ships with aft superstructure , care must be taken not to confuse the ship's drift with the trajectory of an observer watching from the outside of the turning curve. In fact, during a turn , the longer the sh ip, the higher the speed at which an observer situated astern of the ship approaches objects on the outer side of the turn , but the observer is deceive d by the optical illusion that the bows of the ship are pivoting around him/her and incorrectly interprets the approach of objects as caused by the whole ship drifting towards the outside of the trajectory. For this reason, during a turn, drift can only be assessed from inside the turn, observing the variation in distance between a point located towards the front third of the ship, and a reference mark on its beam . Similarly, if a turning ship is stopped, drift must be observed from a point placed towards the middle of the ship with respect to a fixed mark on its beam and from the inner side of the turn. So to sum up, it is difficult to estimate drift. It is best to treat it as a random factor for which only the direction can actually be known (outside the turning curve and wind direction in a straight line).

Underestimating the drift means the ship approaches the middle of the channel, while if it is overestimated, the helm angl e and rate of turn have to be reduced, to re turn to th e chosen trajectory. It is actually always easier to slow down a turn than to speed it up.

3 .4.4 Angul a r speed It is very easy to visualise the angular

speed (or rate of turn) when there are several waymarks ahead of t he ship and an experienced observer ca n detect a very small change of heading. When ma noeuvring, the main problem w ith angular speed is the inertia it causes and making a correct estimate in order to control it in advance . The GPS velocity measurement during a turn is therefore often misinterpreted. It depends on the position of the antenna with respect to the pivot point. The reading of th e velocity is actually a combination of surface speed and linear speed . The linear speed, according to the GPS antenna of a ship 290 metres long with a bridge set at the stern, is calculated at 1.2 knots when the turn is 12°/ min, and 0 .8 knots if it were turning when stopped, assisted by its tugs, with the same angu lar speed. Conversely, when the sh ip is reversing, the antenna approaches the pivot point and the speed given by the GPS is much more accurate. The port positioning units (PPU ) calculate the speed of the cen tre of gravity and the rate of turn . They help with short-term projection of the ship's movements. The resources used to control inertia during a turn are those that apply a transverse force on the ship and that are located the furthest away from the pivot point. The intensity of the transverse force and the duration of its application depend on many factors, including length, length/ beam ratio, surface and angular speeds , displa cement, the force or forces generating the turn, the resources used to stop it. When the ship is turning, the most suitable way to slow its turn is of course to use the rudder, which has the best lever arm with respect to th e pivot point, as long as the ship 's surface speed is lower, and no more than that matching the engine speed. The maximum turning speed depends on the size of the ship, the surface speed, the under-keel clea rance , type of rudde r and

In order to cross a narrow channel while turning for instance, the ship-handler aims not at the middle of th e channel, but at a point to one side towards the inside of

the power delivered at the propell ers . The following table gives some angular speed values as a function of size of ship. The initial surface speed corresponds to an engine setting of Full ahead, with the helm

the turn.

hard to port.

255

Sh ip lenght

Deep water

Limited depth of water

60°/min

I I I I

230 m.

36°/min

32°/min

290 m.

30°/min

26°/min

110 m .

90°/min

Ferry 180 m.

90°lmin

145 m .

70°/min

180 m .

All things being equal, the maximum turning speed drops with surface speed . For comparison's sake, the turning speed w ith helm at 35° , slow ahead, for a ship 290 metres long is no more than 10°/min and increases to 15°/ min when the engine speed is increased to Full ahead. In practice, in order to stabilize the heading of a ship running at its maximum turning speed , the helm is set amidships, between 20 to 25° before reaching the heading, then the helm hard over between around 10° and 15° of the final heading. The helm angle is then adjusted to stop the turn and the turning effects of slippage . When manoeuv ring it is safer to keep a reserve of power on the engine of one or t wo points, depending on circumstances (luff effect, current), to be able to give a kick ahead on the helm if necessary. When a ship is turning under its own power, its maximum angular speed is mainly limited by the power of its bow thruster and that of its stern thruster, if present. These same resources should then stop the turn of a ship pivoting on its centre, which has conserved some inertia. If a ship turning at its maximum angular speed wants to use its own resources for control, all trans-

3.5

3.5.1 Point of application of wind force: the centre of effort The position of this application point of the wind force varies depending on the distribution of exposed surfaces and their angle of incidence w ith respect to wind direction. Distribution of the surfaces exposed to the wind and their size determine typical groups of ship silhouettes, which produce similar wind behaviour characteristi cs within each group. These are: - ships with low freeboard; this family includes river coasters , crude carriers and loaded bulk carriers . These ships all share a low freeboard compared to their draft, a stern superstructure and open bridge. These low-profile ships

-

Qatar Flex and Qatar Max: design standards for the new methane carriers intended to optimize operations at the Ras Laffan port in Qatar. Q-Max: capacity from 260,000 to 270,000 m3. Q-Fiex: capacity from 220,000 m 3.

256

210,000

to

tend to drift very little, but react to the effects of drift: they luff sharply ships with high freeboard; this type of ship includes ro-ro ferries, loaded North Sea crude carriers, light container ships, gas tankers, various cargo ships and coasters and light bulk carriers. These ships have a large lateral surface area because of their greater freeboard, or moderate freeboard with greater surface area of their underside revealed because they are light. They also have a stern superstructure. These ships drift easily, and naturally try to recover their neutral position beam on

trol the comp letion of the turn than were used to perform it. Hence maximum an-

turning continues on inertia, blocked at about 15° before the final heading.

These ships drift easily. The significant stresses linked to their mass and surface area exposed to the wind may also quickly exceed their own abil ity to manoeuvre and that of the tugs used to help them. Safety limits must therefore often be observed in order to ensure safety of port manoeuvres . Problems associated with passage through confined waters and berthing and departure manoeuvres are taken into account by pilots and masters in order to evaluate these limits.

formance at sea in the wind.

verse action must stop about 30° before the final heading and check the movement about 20° ahead. A very large ship is turned with the help of tugs, and fewer may be allocated to con-

gular speed may be very high with regard to the resources allocated afterwards to counter the inertia, and stopping the turn must be planned even further ahead . The tugs' action stops 45° ahead, and

Disruptive effects: wind

Wind is normally the prime generating factor for forces that disrupt the ship's behaviour. Evaluating the ship's windage must allow for all the aspects described in the section on wind. In order of importance, these are first the wind, then the shape and the surface area the ship presents to the wind, depending on its heading. Assessing displacement (mass) and the drift plane formed by the wetted longitudinal surfaces in the vertical plane gives further indication of the ship's per-

-

to the wind ships with a large lateral surface area; these include loaded ULCS ( Ultra Large Container Ship), Q-Fiex and Q-Max methane carriers, light North Sea crude carriers, liners and ferries and vehicle carriers. These ships have an almost rectangular silhouette, since the sterncastle is incorporated into the superstructures or the deck loading . On ULCSs or the larger liners, the exposed surface area may be as much as 14,000 m 2 , a significant amount. This is added to their smaller draft compared to the exposed windage area.

3.5.2 Manoeuvring scenario according to wind direction A detailed study of the stresses exerted by wind shows the direction of the movements communicated to the ship: wind causes the ship to drift, turn, and move forward or back. Its trajectory then becomes an oblique movement. Hull resistances will thus have a turning effect and in turn will influence the ship's trajectory. The squa re of the apparent wind speeds and of the ship's speed mainly determine the intensity of these aerodynamic and hydrodynamic stresses. The ship-handler keeps the ship under control by maintaining the minimum speed throughout the manoeuvre compatib le with drift and good helm control. The situation is understood more easily by calculating all the con straints exerted on the ship. This complex calcu lation is not absolutely necessary, however, in order to develop a coherent manoeuvring plan. Considering a specific example of a ship arriving at a port to berth at a wharf helps us to appreciate the difficulty of a manoeuvre and to develop a scenario. First question : which side is to the

1.

wharf? Berthing, starboard side to the wharf: The wind direction, slightly onshore, reduces drift and helps in the approach to the wharf. However, pushed by the wind from astern, it will be very difficult, even impossible to control the ship's heading when the engine is started in reverse to bring the ship to a standstill (prop walk). It is also possible to consider an approach with the wind, dragging the anchor on the port side to improve helm control. The final phase of the approach, when the ship has to come to a halt at its berth, is still very difficult since the neutral position with the wind from astern is unstable.

The slightest change in wind direction will produce a hazardous swing. The skill of the crew in quickly putting out a stern line and a bow spring can make this manoeuvre safe, by creating forces allowing the ship to be brought alongside with its heading under control. The ship-handler should also hope that the wind direction remains steady during the manoeuvre . It therefore seems very tricky to berth 2.

....

starboard side to the wharf. Be rt hing, port side to the wharf (see figure): Presentation seems more complicated , given the geogra phy of the pool. The confined space downwind of the berth means that it is not possible to approach close to wind , with a slightly onshore wind direction . It may be difficult for the ship to manage to reverse into its berth, upwind, keeping its heading under control (very difficult for a ship without a bow thruster) . It is also possible to present beam onto the wind, then come to a halt at the berth, limiting the transverse drift speed. Intuitively

600 m

= - - -- - ------~-----

--__... ...,---

%

120 meter ship, half-loaded 1 fixed-pitch propeller, right-hand turn

%

No thruster

Example of dockside manoeuvre

%

this solution seems dangerous, however, especially if the ship is light. The final, chosen solution is to drop anchor starboard upwind of the berth then pay out enough chain to bring the head into the wind. The ship- handler therefore needs to present upwind of the berth, with wind on the starboard side, at low speed, to lower the starboard anchor to around lOOm upwind of the final position of the ship's bow. When the chain tautens under the effect of drift, the ship pivots to bring its head into wind. With its bow held by the chain, the ship is safe to pass out its mooring lines and berth. The bow line and spring create a fixed point with the starboard chain that secures the ship to the wharf. This manoeuvre and choice of the side to present to the wharf seem safest in the end.

Second q uestion: presentation How should th e ship present itself at the berth so that the starboard anchor can be lowered at a precise spot? The speed and heading mu st be perfectly controlled to ensure th e necessary accuracy for thi s manoeu-

Finally, considerin g each prob lem ind ividually st artin g from the last one, the shiphandl er ca n design the manoeuvring scenari o. Kno wing th e ship's turning capabilities thorough ly, the geography of the body of water and an y loca l features of wind direc-

vre. The ship must therefore arri ve at a heading close to the neutral position, which does not need particular steering capability. The easiest way is to present beam on to the wind, with a very slight head -

t ion and force will ensure the manoeuvre can be carried out safely. Despite all the ship- handler and crew's precautions, it ta kes on ly one random factor, even a tin y one, to upset even t he most carefully design scena ri o.

way to arrive at the mooring point . A ship that has entered the channel with the wind on the port side must first prepare to turn, in order to pre -

It is important t he refore to remain vigilant and alert through out the manoeuvre.

sent beam on to the wind with the wind on the sta rboard side . The ship must therefore come far enough upwind to create a space in which it can slow as it comes to starboard , then as soon as the wind is on the starboard side, come to a halt .

CHAPTER3-

...

----...:~~"1"

~

Manoeuvring training .., resources: simulation

1

Training on a manoeuvring simulator

Today, some 80% of marine accidents, out at sea or in a port environment, are caused by human action . Standardization of procedures and training of mariners at various levels helps to correct these errors and improve safety at sea. Electronic simulation has become an essential tool in this process for training crews, as well as evaluating and maintaining their skills. Th ere are a number of areas this can cover : navigation rules, ship dynamics, ma noeuvrability, port manoeuvring, as well as all the associated human factors and management of a bridge crew. The combination of "knowledge, skills and attitude" forms the essential alliance needed for training a merchant navy of-

Apart from purely visible and audible aspects, it seems clear that the essential element of the simulator is the ship's mathematical movement model which combines the various values of propulsion and helm controls and their interaction with sea and wind, or with the environment in general. This model is very complicated to develop. It should effectively provide a faithful representation of the ship's responses using step by step calculations of the fo rces by

These equation s, developed along t he ship 's three axes of m ove me nt, thus govern its behaviour w hen it is subject to extern a I forces: gravitational force, hydrostatic force, hydrodynamic forces exerted on the hull and its appendages and aerodynamic forces. Man oeuv rab ility m ode ls used by manoeuvring simu lators are based on solutions to these equation s, expressing the forces appli ed to the ship exp licitly using empirical

which the sea, wind and ship-handler control the ship. All these various forces are

formulas ext ra polated from results of tests on m ock- ups, or using theoretical formulations. There are a number of effects reproduced in a more or less realistic form in this way : thru ster po wer, control surface effect, hull

interlinked . The application of t he basi c principle of dynamics is therefore compl ex since the resultant of the external forces applied to the ship varies with its move ments . It is even trickier since the hydro dynamic forces applied to the hull include a term in phase with acceleration (concept of added mass). These forces are also cal-

resistance, effects of sha ll ows, interaction between ship and port structures, forces fro m win d, forces generated by anchors or mooring lines, etc. Many parameters are needed in order to quantify these f orces . They are measured directly on the ship, or extrapolated using tank expe rim ents on models .

standard equipment found on commercial vessels : radar, AIS, Ecdis, VHF, depth sounders, telegraph, helm, etc. Th e sound environment reproduces the communications between ships, tugs, wharf team and harbour master's office) .

culated in a frame of reference linked to the ship, while its trajectory is calculated in a Galilean frame of reference . In order to simulate the ship 's movement, the manoeuvrability calculation code used to quantify the hydrodynamic force s has to be paired with the dy namic model of th e ship, so its movements can be calculat ed. This is a complex exercise, especially in "real time" simulations that require very

The water surface display is provided by projectors or screens generating an im-

fast calculations . The dynamics are to be understood on the

The limitation s of extrapola ti ng the measurements made on a model into real situation s, and th e impossibility of reproducing all t he vari ous situations t hat could arise, mean that ship reactions are someti m es still appro xi mate, even though they are also con sistent . Sim ulators can therefo re still not be used to validate feasibility of a

age in a wide visual field, around 210° or more. There is a realistic atmosphere . Databases are also used to create images of the largest ports in the world, for vari ous types of ships. The instructor man-

basis of a frame of refe rence linked to the ship's centre of gravity.

manoeu vre , or set up a precise procedure, but only to encou rage the exchange of ex-

ficer. Th e simulator is designed to look exactly like a real ship's bridge. It contains all the

ages the marine traffic from his/her desk, acting as the harbour master and other participants. It is possible to create interactions with nearby ships, reduced visibility, and many other situations that are im portant in terms of marine and port safety. Exercises are recorded using cameras and are followed by a debrief each time . For sailors, the real benefits of a simulator above all are the provision of useful exercises and sharing true - life experiences. The simulator is therefore an overview tool. Nonetheless, whatever the intended objective - avoiding collisions, navigation, manoeuvrability studies or training in ma noeuvring - realism is the main quality expected of a simulator. It allows a constructive exchange . Two main assemblies must then be considered when evaluating a manoeuvring simulator: -

the ship, its propulsion systems and helm control on the one hand,

-

en v ironment (wind, current, landscape, etc.) on the other.

CHAPTER

depth,

perience and comp are different scenarios.

I.~+w/\w=M dt m: mass of ship , VG : velocity of the centre of gravity expressed in the ship's frame of reference, instant vector of rotation of the ship expressed in the ship's frame of reference, F : external forces applied to the ship in

w:

the ship's frame of reference, M: external moments applied to the ship in the ship 's frame of reference, I : inertia matrix of ship. The dynamic torsor defined by the force vector and the moment vector of the force can be used to describe the movement of the ship.

Torsor: mathematical tool used to describe movements of solids, and the mechanical actions they undergo from an external environment.

However, theoretical kn ow ledg e abo ut ships continues to grow. Work carried ou t in test t anks and hydrodynamic research laboratories has led to development of th eo reti ca l formul a. These are computation al tool s of viscous flow around hulls, usi ng " Navier-Stokes free surface code" (for insta nce, the !CARE code developed by Nantes cen tral college, CNRS and th e DGA; or Eol e, by the Principia laboratory ). These num erica l models that incorporate concepts of viscous flow and formation of waves accompanying ship movements can help determine the torsor of hydrodynami c fo rces by integration of pressure and friction forces exerted on the hull and its appendages. These models are a priori the most effective . They open the way to a new generation of simulators able t o calculate forces that are exerted on a ship w ithout having to refer t o experiments on models.

2 59

Nonetheless, the complexity of the instability (Unstable flow: flow variable as a function of time) and viscous aspects of flow, as well as multiple interactions between environment, waves, hull, propeller and rudder when the ship is drifting or turning (frequently in port manoeuvres, for instance) , make the formulas more complicated and involved even greater calculation times to give a "real tim e" sim ulation . In addition, comparison of hull re sistances between test results in the tank on models , and the results from these digita l formulations, show significant diffe rences, especiall y on quant ifica t ion of transve rse fo rces .

Measures in test tank

"

10

H---+--...4"~ -+.'j;l~----l - 15

Contrary to other areas of study into ship behaviour (resistance to forward movement and sea-handling), there is still no real digital method suitable for simulation of manoeuvring, since the representation of physical phenomena from knowledge of the ship and the environment is particularly difficult. Simulation cannot therefore rely on resolution of a hydrodynamic problem, but on overlapping behavioural mod els describing, more or less realistically, the ship interacting with its environment. Digital

simulation

Measurement by calculation for a turn

Navier- Stokes

is presently a tool mainly developed to help naval arch ite ct s and shipbuilders and therefore covers only some aspects of ma noeuvring . Even though initial published results are encouraging, there is a great deal of work to be done in order to extend its scope and especially to simulate open water ship tests or port manoeuvring, which is even more complicated. Future developments however will almost certainly include these specifi c mathematical models of manoeuvri ng in Navier-Stokes equations, which

The simulators used provide " real tim e" simulation, which puts th e 3D ship into its port environment (figure below). Reproduction of the navigation brid ge and display of the port environment m ean that the immersion experience is striking. Computers can reproduce the move ments of ships using simpl e digital mod els as mentioned above, combining software for the ship dynamics with 6 degrees of freedom and hydrodynamic databases . These models need precise configurati on for each ship, normally obtained from tan k tests on models .

integrate the sh ip in a port envi ronment and improve the reali sm of th e simulators .

Th e real ism of th e simulat ion s thus ma inly depends on the quality of th ese pa ram -

Navier-Stokes equations use non-linear, partial derivati ves to give approx imate descriptions of the movement of fluids in continuous environments . They allow the >- eq uation of the quantity movement for flu-

eters (numerous tests of hull resistan ce, aerodynamic resistance, manoeuv rabil ity and propulsi on are needed in order to un derstand thoroughly all the coefficients for quantifying forces and moments exerted on the ship). In some case s, parameters are not accurate enough and ship move -

ids t o be solved:

:r Forces = Mass x ' - - - 1 ---1·25

using

Solving these equations also allows a th eoretical study of flow along the hull of a ship . Hull resistances can then be possibl e. Their complexity, however, means th at they often have to be simplified (modelling turbulence for instance) and only a few experts, using powerful compute rs can solve them . At the moment, therefo re, they cannot be used in real time du ring a manoeuvre, nor is on - board computer support possible .

Acceleration

Solving these equations allows acceleration of fluid parti cles to be quantified, along with pressure, friction and gravity forces applied at each point on the hull.

ments are only reproduced approximately. During simulator exercises it is therefore important to preserve a critical attitud e and sea sense in order to validate t heir results . Before accepting the result of an equation, it is always necessary to kn ow the basic assumptions used to write it. A firm lesson may nonetheless be drawn from training on simulators, especially if care is taken in selecting ship models and the exercises are limited to simple ma noeuvring phases , similar to situations that have been tested in the tank .

Full Mission bridge on the manoeuvring simulator at ENSM, Marseille centre

260

ENSM mainly relies on simulation to train future sailors . The Marseille centre with simulators for manoeuvring, navigation, radar, Ecdis, engine, SMDSM, dynamic positioning, oil, gas and tank, VTS, Ice-Navigation is one of the largest marine simulation facilities in Europe.

Simulated ship

PTER 3 ·PART 9.1 ·TRAINING ON A MANOEUVRING SIMULATOR

This is generally the case for standard sailing, approach, and port transit situations. Other than that, when the ship movements are significantly accelerated, e.g . if the wind is blowing in strong gusts, if the engine power is varied frequently, or even when manoeuvring in "catastrophic" scenarios, simulators are still not realistic enough. It is then generally preferable when considering the limits of the feasibility of a manoeuvre to refer to similar, real situations and thus to the professional experience of masters and pilots. Similarly, to maintain a rational approach, training in berthing or moving off manoeuvres is a special ca se in which care is often needed. It is still difficult to quantify all interactions between the ship and the environment, and thus to programme simulators to reproduce perfectly the most complex port situations. It is also difficult to simulate the ship precisely in all sail-

The increasing size of ships and ever greater safety dema nds have also led m arine t raining schools and most pilot stations in French ports t o purch ase simulators.

The plan of the manoeuvring simulator installations at ENSM,

ing, trim and loading conditions, or even adjust to its changes over time (wear on

2

th e engine, distortion of the hull, etc .) . The bases are nonetheless useful and already allow a really consistent initial train ing, although performance of the different simulators available on the market varies considerably

In Europe there are several train ing centres for manoe uv rin g on sailing models . These centres offer courses at wh ich trainees manoeuvre smaller models, representing different types of sh ips, which are placed in small-scale port environments . Channels, locks and wharfs are reproduced, along w ith swell , w ind , current or the work of tugs. It is therefore possible to highlight reactions of models in different sit uations found in port

Tra ining on sa iling model

environments . Interactions between models passing each othe r or between a model and the banks or bottom of a canal are shown realisticall y. Berthi ng and moving off manoeuvres are also undertaken in different wind and current configuration s, to study the difficu lties of the most relevant manoeuvres and scena ri os. Special efforts are made to ensure that the beha vio ur of th e models resemb les as closely as possible that of a real ship. Nonetheless, the compl ex nature of the rul es on similarity mentioned in the chapter about knowledge of the ship sho ws t hat models sho uld not be seen as simulators but rather as realistic resources fo r stu dying t he behaviour of the ships. The more t he size of th e m odel differs from tha t of the actual sh ip, the more difficult it is to determine references in terms of the speed of th e ship, the wi nd force and intensity of current.

AHTS (Anchor-Handling Tug Supply vessel for oil platform) . Bourbon simulator The computer simulation therefore appears to be a good approach with development opportunities, and is in some ways more realistic than the "simulation"

Model at Port Revel

Model in channel at Port Revel

on sailing models, especially because it allows a rational approach to speed in ma-

Finally, sailing and manoeuvring a ship is not a matter for improvisation. Solid t ra ining is needed, especially before taking responsibility for a manoeuvre . In its resoluti on A960 ,

noeuvring (speed is fundamental, since its square is used for calculating inertia, lift and drag of the ship) . Realism of the visual and sound ambiance recreated in the sim ulators also allows stressful human situations to be reproduced, which are very useful in te rms of training and behaviour studies (Bridge Resource Management) .

(IMO Resolution A960- January 2004; see chapter, "Coop eration with pilots ") the IMO perfectly summarises the procedure to be used when training pilots. The same approach can be followed by masters and officers. This resolution recommends a training programme including pra ctical ex perie nce, ~a i ne d under the close supervision of experienced pilots . This pra ctical ex perience (a few hundred manoeuvres, or thousands of hours training for most ports ) , gained on board ship in real piloting situations, may be supplemented by computer si m ulati ons and smal lscale models of piloted ships, by theory courses, or through other train ing m eth ods.

261

1.1.2 Using automatic pilot and steering gear

1.2

There are numerous national and interna-

Manufacturing and usage standards for

The International Mari t ime Orga nisation has

tional legislative bodies in the area of ship

the control surface gear, set by Solas and

adopted the follo wi ng recommendations re-

manoeuvring. They cover ship-building,

the classification companies, should comply with a number of criteria:

-

Regulations

1

design of gear, regulations for manoeuvring capabilities and their limits. They

-

control of the helm in zones of heavy

on equipment and ship operation . These

traffic and in reduced visibility,

include: -

-

Solas (International Convention for the Safety of Life at Sea), an international curity, safety and operation of ships ,

-

about ship man oeuvrability, endorsed by resolution MSC 137 (76) 4

-

service in hazardous zones, all helm systems must be checked and

-

tested 12 hours before movi ng off, an exercise should be run every three

prov isions: provisional standards on ship manoeuv rabili t y :

-

months to set up the emergency helm, the ship must have a main control sys-

-

resolution A.75 1 (18) 4 November 1993 Dece mber 2002, replacing the annexed

-

MS C/Circ. 1053 of 16 December 2002: explanatory notes on cond iti ons for sh ip

the IMO whose resolutions define the

tem and an auxiliary, or two identical

manoeuv rability tests as specified in MSC

manoeuvrability required of a ship and

systems,

resolutio n 137 (76).

its limitations, -

resolution A.60 1 (15) 19 November 1987 : presen t ation and display of information

the helm must be trimmed after an exthe ship must have two helm motors in

for mooring lines, and monitoring ship operation.

noeuvring performance in ship design, -

tended use of the automatic pilot,

certification and con t rol bodies, defin ing for instance th e number of fittings

MSC/Circ . 389 of 10 Ja nuary 1985: provisional directives for estimating ma-

-

treaty defining the various rules on se-

lating to ship man oe uvrabi lity :

it must be possible to resume manual

also set rules relating to functional checks

IMO Recommendations on ship m a noeuvrability

-

the main control system must be ca-

national authorities (in France , "Marine

pable of moving from 30° on one tack

1.2.1 MSC/Circ .3 89

Affairs") applying the policy of the State

to 35° on the other within 30 seconds,

Circular 389 def in es the shi p manoeuvrin g

concerned, delivering sailing licences

when the ship is running at her maxi-

characteristics.

and certificates and monitoring ships.

mum weight. This procedure must have

estimate m anoeuvrabi lity and handling

been tested , the auxiliary control system must take

fully- laden ship in deep water.

scope of this work, but we note the most

less than 60 seconds to move from 15°

It also descri bes the real-life tests to be per-

important, especially IMO resolutions that

on one tack to 15° on th e othe r, w ith

form ed in ord er to confirm a ship's manoeu-

give the boundaries within which a ship

the ship having maximum draft and

vring performances.

running at half speed or 7 knots, the control gear and the shank must be

These specific m anoeuvres include : - Turn ing circle

designed to tolerate maximum re verse

-

The Z- manoeu vre: ship's yaw checking

-

Initial turni ng t est on 10° helm,

A survey of all the regulations linked to ship manoeuvrability would be outside the

-

should move . All ship - handlers should be aware of these limitations, which are spe-

-

cific to each ship.

Safety regulations (Sol as)

Except fo r helm tests running at ma x imum

are

-

to

manoeuvres to t est the sh ip's capacity to

speed in reverse, all other limitations are

stabili se its headin g:

subject to mandatory testing .

•

pull-out test,

•

direct and reve rse spira l t ests,

The engine started up in reverse must be

used

ab ilit y - zig zag test ),

1.1.1 Using engine in reverse able to stop the ship's forward movement

data

properties . The y are determined for a

speed without damage .

1.1

Th ese

-

Emerg ency stopping tests:

1.1.3 Visibility on the bridge

within an appropriate period and over a

Ships over 45 metres long must comply

1.2.2 Resolution A.60 1 ( 1 5)

reasonable distance, and this should be

with the following provisions:

Resolution A.601 (15) specifies the reg ula-

tested. For ships with twin-shafts, the ship

-

itself must have measurements available of time and stopping distances using one

of the bows from the bridge shall be un-

tion documentation and its conten t linked to ship's characteristics and manoeuvrabil -

obstructed for over two ship's lengths

ity. This documentati on must be availa bl e to

shaft line, made during the commission-

or over 500 metres . The horizontal field of view from the

ship-handlers and to th e pilot:

ing tests.

-

The view of the surface of the sea ahead

-

blind area angle of 20° (lifting gear) . -

The horizontal field of view from the

Pilot card showing th e characteristi cs of the ship and its equipment in normal

bridge shall be 225° with an acceptable

operating and loading conditions , -

the Wheelhouse Poster is displayed in t he

main helm control must be 60° either

wheelhouse giving detail s of the manoeu -

side of the ship's axis.

vring capabilities of th e loaded and li ght ship within a deep and shallow water en vironment, -

Manoeuvring booklet contains the characteristics of the ship that affect its manoeuvrability. It cov ers all th e information noted in the Pilot Card and th e Wheelhouse Poster. Most of this information come s from studies into ship-bui lding, as well as from the tests. This booklet is filled in throughout the ship 's service life.

263

1.2.3 Resolution A.751 (18)Resolution MSC 137 (76); Forward

Resolution A.751 (18) applies to ships over lOO metres long, to gas carriers and chemical product tankers . It defines regulations and their limits for the following manoeuvres: - Turning ability, - Initial turning ability, - Ship's yaw checking and course keeping abilities, - Stopping ability.

< 4,5L

/ / Transfer

/1.-----..1 I

I

< SL

I

1.2.4 MSC I Circ 1053 Circular 1053, adopted by A.751 (18) is intended to give administrative authorities specific directives on definition and

''

execution of manoeuvres as well as rules relating to ship manoeuvrability. It is intended for both research offices and

test tanks, as well as for ship-handlers.

IMO rules on manoeuvring defined in resolution A.751 (18)

1.3

Resolution A.751 (18) defines the manoeuvra bility limits a ship must not exceed in each of the following manoeuvres: - Turning ability determines (figure above right) : 1. The distance between the moment

2.

-

where the ship sets the helm 35° to starboard , and the moment it is at 90° of its initia l course shall be within 4.5 ship's lengths . The distance between the moment where the ship sets the helm 35°

Turning ability

-

The yaw checking and course keeping abilities of the ship, zigzag test, define the limits by which the heading is overshot after two successive changes of helm angle. The two regulatory tests are to be carried out with one helm change of 10° and a second of 20°. For the test with a helm angle change of 10° the first ove rshoot should be less than 10° if L/V < 10 seconds ; and less than 20 ° if L/V > 30 seconds . The second overshoot should not be greater than 15° of the same values of the first one . For the test with 20° helm angle, the first overshoot should not exceed 25° .

4J ,c5 Port 1st eading over hoot

to starboard, and the moment it is at 180° of its initial course shall be within 5 ship's lengths (tactical diameter). The initial turning ability requires the distance travelled to be less than 2.5 ship's lengths between the moment

Helm angle

"6"

the helm is at 10° and the moment the ship's heading changes by 10°: I

10°1

Change of head ing

Starboard

Yaw checking ability 10°/1 0 °

Distance travelled < 2,5L

Initial turn ing abilit y

264

CHAPTER 3- PART 10 ·REGULATIONS

Lateral deviation -

The stopping ability indicates the distance travelled during a crash stop. The ship with an initial speed VT, hav ing put its engine in Full astern, should stop over a distance less than fifteen times the ship's length (figure 4).

Fo rward

Reverse propeller

Full astern order - H-t- - -- -- - - - - - - ' Stopping distances

1.4

Manoeuvring tests

Relying on rules defined by IMO, the organisations cove ring test tank research centres, such as ITTC 2002 have developed and standardised their own procedures . All the tests quantified in the test tanks comply with the same specifications, using terminology and measurement conditions common to all laboratories. The table summarises the tests recommended in the I MO resolution, and those developed by ITTC. !MO A601

IMO A751

Tur n ing circle

X

X

X

Z-manoeuvre test

X

X

X

Modified Z-manreuvre Test Z-manoeuvre at low speed test

ITTC 2002

X X

Direct spiral test

X X

Reverse spiral test

X

X

X

Pull-out test

X

X

X

Stopping test

X

X

X

Stopping inertial test

X

X

Man-overboard test

X

X

Parallel course manreuvre test

X

Initial turning test Accelerating turning test

X

Thruster test

X

Crabbing test New c ourse keeping test Acceleration

I deceleration test

X X

X X X X

X X

Cras h s top ahead test

X

Minimum revolution test

X

265

Helm order

2

Helm orders are instructions for the heading, and t he regulat ion he lm movements the ship-hand ler gives to the he lm officer to perform a manoeuvre . They are standardised to ensure the settings are properly understood by the helmsman or woman. It is essential to maintain th is formal language, to avoid any ambiguity about the type of the order to be fo ll owed . "Thanks" or "ok", "heading to be followed by he lmsman only using a designated landmark", are absolutely forbidden. It is essential to verify that the order given has been executed. A helmsman or woman who puts the helm hard to starboard when the setting was hard to port is putting at risk the safety of the ship wi th possibly irreversible consequences.

The sh ip-hand ler must always keep a close eye on t he he lm angle repeater, to check on the helm movements and notice how the ship behaves . The helm commands are: 1. Helm commands are given using the words: "starboard", "port" corresponding to the direction towards which a ship making headway should come. The control gear must be installed so that for instance w hen a ship going ahead has to bear to starboard, the control device and helm repeater also turn to starboard. The use of the words: "right", "left" is for-

2.

bidden . The terms to use for these commands are: - "Starboard" (or "Port") meaning: put the helm over to the right (or

-

when necessary, the terms "starboard", "port" are followed by the number of degrees indicating the angle the helm must make with the shi p's longitudina l plane, - The commands "starboard" and "port" preceded by the words "hard to" indicate that the he lm must be placed at the extreme angle to right or to left. 3. "Helm amidships", meaning : put the helm in line with the ship's longitudi na l plane .

4.

5.

"Steady", meaning: keep the heading as it is. With this latter command, the he lm is manoeuvred in order to keep the vessel at its present heading. Commands are repeated by the person steering, at the time the order is given; this person then checks that the command has been carried out. "

to left),

2.1

Examples Instructions: Ship-handler

"Starboard 15" (*)

Change heading with 15° of helm angle . Initial course 000°; final course 055° . The Ship-handler controls the turn to bring the ship round to 055°.

comments

Helmsman/woman "Starboard 15" (*)

The helmsman/woman repeats the command given by the ship-handler to indicate it has been properly heard.

"Wheel is 15 starboard" (**)

The helmsman reports that the helm is 15° to starboard.

"010, 020, 030, ... "

The ship-handler collates the information . The helmsman "sings out" headings every 10°.

The helmsman moves the helm 15° to starboard.

"Good" "Ease helm to 5"

"Ease helm to 5"

During the turn, the ship-handler may reduce the helm angle .

"helm amidships"

"he lm amidships"

The ship-handler orders the helm to be put to 0°. The helmsma n repeats and moves the helm to 0 °

"Helm amidships" . "Good".

The he lmsman reports when the helm is amidships . The ship-handler collates .

"Steer to 055"

The ship-handler orders a heading of 055° . "Steer to 055"

The helmsman collates and adjusts the heading to come to 055°.

"Steer on 055 "

The helmsman reports when the helm is at 055 °.

"Good"

The ship-handler collates. Change heading with 15° of helm, giving the new head ing to follow directly( ***). Initial course 000°; final course 055° . The ship-handler instructs the he lmsman/woman to go to the heading indicated.

"Starboard 15, steer to 055"

"Starboard 15, steer to 055"

The helmsman repeats the command given by the ship - handler. The helm sman moves the helm 15° to starboard .

"Helm is at 15 starboard" (*)

The helmsman reports that the helm is 15° to starboard.

"Good"

The ship-handler collates the information. "Steady on 055".

"Good" "Steer at this course"

The he lmsman reports when the he lm is at 055° . The ship- handler col lates.

"Steady as she goes"

The helmsman stops immediatly the giration and steers to the present course"

**

The words are "Starboard fifteen", or else "Helm starboard 15" The ship-handler first gives the direction of turn, then its angle . Once the helm is at 15° starboard, the helm sman/wo man reports, first giving the angle then the side to which it is turned . This distinction is made in order not to confuse the command given by the ship-handler with the instruction carried out by the helmsman . This procedure is valid for a change of heading less that 090° .

266

3

STCW 2010

The required skills demanded by STCW 2010 to exercise the role of ship-handl er responsible for manoeuvring a ship are as follows: 1. to be able to manoeuvre when approaching pilot stations to emba rk and disembark the pilot, taking account of weather, tide, 2.

distance to travel and stopping distances, to be able to navigate a river, estuary and confined waters, allowing for the effects of current, wind and shallows on the action

3. 4. 5. 6. 7. 8. 9.

of to to to to to to to

the helm, be able to turn the ship at a constant rate, manoeuvre in shallow water, allowing for the reduction in under-keel clearance caused by squat, roll and pitch , handle interaction s between ship and banks (bank effect), be able to berth or move off under all wind, tide and current conditio ns , with or without a tug, handle interactions between ship and the tugs, be able to use propulsion systems and manoeuvring systems, be able to choose a mooring; to moor using one or two anchors in restricted spaces; to assess the various factors that deter-

mine the length of anchor warp to drop, 10. to be able to notice an anchor dragging, and know how to release a fouled anchor, 11. to be able to enter a dry dock, when damaged or undamaged, 12. to be able to sail and manoeuvre in bad weather, including when coming to the aid of a ship or aircraft in distress ; to be able to carry out emergency towing operations, and take the steps needed to make a ship safe if it is difficult to steer in the hollow of a wave, and reduce drift, 13. to be able to manoeuvre with care in order to launch lifeboats and to release survival gear in bad weather, 14. be able to winch survivors on board from rescue craft or liferafts, 15. to be able to assess the manoeuvring characteristics of the main types of ship (stopping distances, turn ing circle at different speeds and drafts), 16 . to be able to sail at reduced speed to avoid damage being caused to other sh ips by bow wave or wake, 17. to be able to take the necessary precautio ns for sailing near to ice, or when ice is accumulating on board , 18. to be able to sail close to or within traffic separation zones, and in zones covered by the VTS. STCW 2010: Standards of Tra ining, Certifications and Watchkeeping . VTS:

Vessel Traffic Service

CHAPTER 3 ·PART 10.3 · STCW 95 _ _ _ _ _ _ __ _ _.....;,o;

267

,..

Appendices

Appendices Anchoring preparation sheet

1.

2.

Pilot Card Ferry 11 046T

3.

Wheelhouse Poster Ferry 11,046T

4.

Pilot Card VLCC 1 Load 159,584T

5.

Wheelhouse Poster VLCC 1 Load 159,584T

6.

Pilot Card Container Ship 32,025T

7.

Wheelhouse Poster Container Ship 32,025T

Fiche N°

ANCHORING

Designation A

Yes

No

Nu

Has an anchoring plan been prepared taking into account : Speed reduction in ample time '! Direction/strength of wind and current? Tida l stream when manoeuvring at low speeds'! Depth of water, type of sea bed and the scope of anchor cable required Have the engine officer on duty and anchor party been informed of the time of 'stand-by' for anchoring'! Are the anchors, lights I shapes and sonnd signalling apparatus ready for use? Has the anchor position of the ship been reported to the port authority'? Make confirm by anchor party that swivel, anchor shackle, anchor ring and all pins in good order and position '!

While dropping the anchor Record on GPS the anchor position as the ccntcr of swingi ng circle Comments

ISSUED : MASTER

CHAPTER 3-

Filled by

CONTROLLED:

DAT E OF ISSUE :

Signature :

Date:

REFERENCE: INDICE:

269

Pilot Card Ferry

11046T

Date Year built

Passenger car feny (Dis.ll 046t) v41 jCallSign I Full load and 50% bunkers and Forward deckh.ouse 11046 tonnes 1Draft forward NJA tonnes IDraft forward extreme NIA Draft after 38.68 m I 127ft 2in I Draft after extreme

!ship nan1e jiMONumber I Load Condition I Displacement I Deadweight

~~acity Air draft

)2.12.2008

I

5.05 m I 16ft 7 in 5.05 m I 16ft 7 in 5.32ml17ft6in 5.32m I 17ft 6in

Ship's Particulars

I

overall r·Le~h ..--

Typeofbow !Bulbous 1145 m i25 .2 m - - I~-~~===[t,r~~d l t4 shackles !14 shackles (1 shackle =27.5 m/ 15 fathoms) ]NJA shackles

!

:Breadth I'AnchO!chain(Port) I Anchor Chain(Starooard) [Anchor Chain(Stern)

140 112

33

ru·--------1 L -

J~c=== - -- -==- ==~:r> !_ ____ ____ _____

~-J

4G .3

~ ~----~~. -------~--Steering 1 £J!~~~(;t~ristic~--~---· -------------1 ;Semis~f'ended /2 : 35 i 0 ~s :20.5 seconds

Ruddel{s) (type/No .) Maximum angle [Rudder angle for neutral effect [Hard over t~~l{2_ E_U!!\f'S) I

St~p!!!gs Description FAHtoFAS HAHtoHAS SAHtoSAS

I

Number ofbow thrusters

I

Number ofstern thrusters Power

_ 12 . ·c:c:-:-.-:::~:·.-::·c::···-- - - - - - J T590 kW 1590 kW 11-=.Nc::/Ac.:.____ _ _ _ _ _ _--1 NIA I

Tmuing circle

I

.-- Full Time 123.5 s 138.5 s 147 s

1

IPower

Head reach !3.03 cbls !2.65 cbls !2.11 cbls

Ordered Engine : 100%, Ordered rudder: 35 degrees 12.17 cbls Advance 10.85 cbls Transfer -r2.22 cbls Tactical diameter

lvlain Engine( s) IYf'~Ofl\o!a!n_~--., Number of Main EngUle(s) Maximum power per shaft Astell!_power Time limit astern

~cl!um ~eed diesel _

2 2x9556 kW 60 %ahead NIA

Number of }l!I)Eellors Propellor rotation Propellor tVPe Min. RPM --- -- - - Full Ahead to Full Astern

2 Outward CPP 170 30 seconds

Engine Telegraph Table ~order

I - - - loO-~.

80 % 60 % t-------------- 40 % 20 % -20 % ! - - - -· -40 % -60 % -80% -100 % ~ -· -·-······--

270

~peed,

knots 20.92 18.57 15.08 11.07 5.54 -3.77 -6.15 -7.41 -8.85 -9.9

~ower,kW

9556 6304.7 3747.3 2193.3 1276.7 1249.7 1489.1 1739 2193.3 2604.6

Pitch ratio RPM 170 1.4 170 1.16 170 0.87 o 5~ 170 170 0.15 1.7Q__ ,____:Q.:.! J_ 170 -0.3 170 -0.42 110 .o.5-r--

i7o -~--=offf-

---

Wheelhouse Poster Pilot Card Ferry

11046T

WHEELHOtTSE POSTER

TURNING CIRCLES

Deep Water

Shallow Water*

Emergency Manoeuvers(DW)

STOPPll\/G CHA.R.ACTERlSTICS Track DW R ach SW

st.Q posilionmar1cs e-wery mint.t~~tOfpos:sib lc.)

4

mhs:

-2

0

~ 14r.ots:

Emergency Manoeuvers(SW"') ~idErs:tnJtt · [Tr.~ckruch .c bls]

4

[fin:!ltirne.rnin-s]

[Finalspeo

......

"' "'

""'

...

0

2

No. Rudd. •Fllg F11ll time Head reach Sidl read 35 100 128s l.'lll~ls 2.51cbls I 2 -35 lOO 12Bs U ll cbls -25l~ls 3 35 -80 104 ~ 255cbls 0.32 cbls 4 104s 255cbls .n 32~ls. ~5 -80 Ocbls -80 124s 3.(J6cbls 5

r

MAN OVERBOARD RESCUE MANOEUVRE

I~EQUENCEOF ACTION TO BE TAKEN ·

TO CAST A BUOY • TO GIVE THE HELM ORDER • TO SOUND THE ALARM ·• TO KEEP THE LOOK OUT ApproxilllateM~rP=

145 112

ActiOn Time I jSet ruddtr 35 STBD. WMI 0 • ill ship cowse altered

33

o 31 degrees from initial Set ruddtr 35 PORT. Wail till o=e eltored to -170 dog190s romll'Jtial jTumAPOll diffemu:~ botw.en AP 163 s and w lial rour.;e . liS! be 180 c!epe's Partictdars [Length overall!Breadth ~znchor chain(Port) Anchor Chain(Stalboard) ]Anchor Chain(Stem)

]261.3 m [48.3 m ]15 shackles l-15 shackles j NIA s}lackles

i

]Bulbous [V-shap_ec!

Typeofbow TY,Ee of stem

(1 shackle =27.5 m I 15 fathoms)

209.5

: 51 .8

r-[1---- -------·r ~ L ~ ___~

{....___[ [J_l- - - - - - ' )

00.4

;_ .~~~p_e_:~~~~~~~~- ~~:~ers

lRudder(s) (tyeefNo.) Maximum angle

I

77.3

lB~_-_____________________

1

I

jl

145 I Power - - -r-e!N:.:-'IA':"-_ _ _ _ _ _ _ __ Rudder angle for neutrel effect !QJ2 de~es I Number of stem tlUUSters j NI_!. ____ _ Hard ove_!_!£! over(~1J!Cl.E~)__ _ _ 34 seco~ _ __jf.C!~!_ i NIA i 1

StOt>t>ing DescriE!_ion FAH to FAS HAHtoHAS t---· ~oSAS

t-FullT~-

666 s 7&95 s 1133.5 s

TtUlung_cir~!e__________

I

Ordered~ : 100%, Ordered rudder: 45 de~es Head reach--+ . -] 4.07 cbls 10.95 cbls Advance 10.75 cbls ]Transfer I1.72 cbls 10.93 cbls ITactic_al diameter 1 4,Q~_ £b~--------

11ain En~ine(s) Type ofMainEngine Number of Main Engine(s) Maximum power per shaft Astern power Time limit astern

Slow S.('eed diesel 1 1 x15500 kW 40 %ahead NIA

] Number of propellers IPropeller rotation IPropeller type [Min. RPM [Full Ahead to Full Astern _

[1 ]Right FPP 27 ! _0 seconds

~ Tele~h Table

Engme order Full Sea Ahead Full Ahead Half Ahead Slow Ahead Dead Slow Ahead Dead Slow Astern Slow Astern Half Astern ___ _ Fu]lAstem "'Modd:

272

; VS'IIO:l:;

S.('eed, knots 15 12.5 10.1 7.3 4.7

-1.7 -25 -3.7 -4.4

RPM Engine I'ower, kW 15429.2 9035 75.25 900&.5 57.22 39095 1256.4 3&.41 27 530 553.7 -2&.4 1494.& -39.73 3924.2 -55.92 ~~~~ :9 _____ c___"'65 .96

~i!~Et~~ ~ 0.& 0.& 0.&_ _ 0.& 0.&-0.& _ 0.& 0.& 0.& --- ·-·-

Wheelhouse Poster VLCC1 load 159584T

Ship's name VLCC 1 CDis.159584t) v54 . Call sign NIA , Gross tonnage NIA , Net tonnage NIA , Load Condition Full load , Displacement 159584 tones , Deadweight NIA tones

TURNING CIRCLES Shallow Water*

Deep Water

· ~.tr~~ :..~~) fj 0 . .. .. ... .. ........

!}""" cbls

·4

0

. Ellg !l:.udd . Advance : Trensfer , Tact. I;! final RoT_. FiJ>o!speed __F~ time · tOO 35 . 4.95_c_l>ls 2.t2 cbJ:; :4.57 ~ ..2_8_deglmin; 5 kJ1Dts . 661 s . : tpQ. :~5 _i