The Right Way Up - Evolution and use of the Small Vessel Stability Book (First Edition)

This book has been revised and updated following publication of the MCA’s 2018 Second Edition of the Workboat Code.

It provides an introduction to ship stability, weight, centre of gravity and hydrostatic information, and then guides the reader through the calculations necessary to maintain buoyancy and stability. It discusses how to conduct inclining experiments and roll tests and how to use the results.

Detailed information is provided on loading condition and free surface effects, together with calculation of the associated flotation and stability characteristics.

Further chapters cover stability acceptance criteria, trim, the downflooding angle, damage survivability, the use of lifting devices and the stability of sailing vessels.

The metric system of measurement is used throughout and the author’s original hand-drawn sketches have been retained to illustrate the text.

The author hopes that the reader, who develops an understanding of the subject matter, will then be able to follow the Safety Codes and the ISO Standards, and will begin to understand some of the important basics of naval architecture that keep ships and small vessels The Right Way Up.

These notes on the Stability of Small Vessels do not give a definitive study of the subject but have been written to explain, for the layman, the parts of the subject that are covered in the MCA’s Small Vessel Safety Codes.

It is intended that this book will assist skippers to understand and apply the requirements and information contained in their vessels’ Stability Information Book and when preparing for and undertaking their mandatory stability course, prior to assuming command.

The book is also intended to enable Code examiners and surveyors to understand the reasoning behind the requirements of Section 11 and to exercise intelligent judgement on the continuing safe relationship between a vessel’s Stability Book and its current condition during inspections.

With the recent development and approval by the Government of an apprenticeship for marine surveyors, this book will provide a useful introduction to stability for young persons coming into this profession without prior experience at sea.

The stability design standard supporting the EU Recreational Craft Directive (RCD),

ISO 12217 – Parts 1 and 2, is mentioned in places as alternatives to some sections of the Codes. These are extremely useful additional instructions on small vessel stability. However, care must be taken when considering the stability of RCD vessels – see the Cautionary Note that follows this Introduction.

The metric system of measurement is used throughout and the author’s original hand-drawn sketches have been retained to illustrate the text.

The author hopes that the reader, who develops an understanding of the subject matter, will then be able to follow the Safety Codes and the ISO Standards. Thus, the reader will begin to understand some of the important basics of naval architecture that keep ships and small vessels The Right Way Up.

Ted Penny

MCA Note

At the beginning of the 21st century, there were several ‘Codes of Practice’ covering the Safety of Small Vessels (up to 24 metres in length), published and enforced by the MCA, including:

The Safety of Small Vessels in Commercial Use for Sport or Pleasure Operating from a Nominated Departure Point (NDP) (the Red Code)

The Safety of Small Commercial Sailing Vessels (the Blue Code) The Safety of Small Commercial Motor Vessels (the Yellow Code) The Safety of Small Workboats and Pilot Boats (the Brown Code).

Due to the then Government’s drive to simplify regulation, it was considered necessary to ‘harmonise’ these Codes. This was done by condensing them all into one volume and referring to it as the ‘Harmonised Code’, published as an Annex to Marine Guidance Notice MGN 280 (M), Small Vessels in Commercial Use for Sport or Pleasure, Workboats and Pilot Boats – Alternative Construction Standards, which is applicable under the original UK Statutory Instruments as a revision of the technical standards required.

The MCA published a new ‘Second Edition’ of the Workboat Code in December 2018. Its Section 11 is applicable to and draws from the original Red, Blue and Yellow Codes, pending their revision, which is currently in hand.

The above information was correct at the time of this document’s publication.

Contents

Other Books in this Series iii
Foreword v
Second Edition vi
Introduction vii
Recreational Craft Directive – Cautionary Note – ISO 12217 Stability Assessment ix

Chapter 1 Introduction to Stability 1
1.1 Background 3
1.2 Definitions 3
1.3 Symbols 4
1.4 Initial conditions of stability 6

Chapter 2 Weight and Centre of Gravity 9
2.1 Initial estimate of Lightship Weight and Centre of Gravity 11
2.2 Estimate of vessel weight (Displacement) and CG in operation 11

Chapter 3 Hydrostatic Information 13
3.1 Hydrostatics 15
3.2 Righting Levers 15

Chapter 4 Keeping the Vessel Upright 17
4.1 Metacentre (from the Greek μετα: ‘After’ – i.e. calculated fro, known factors) 19
4.2 Stability at small angles and GM 23
4.3 Stability at larger angles and the GZ curve 24

Chapter 5 The Inclining Experiment 27
5.1 Why is it done? 29
5.2 How is it done? 29
5.3 How do you use the results? 32

Chapter 6 Alternative Stability Checking by Roll Moment 39
6.1 Background 41
6.2 Principle and procedure 41
6.3 Roll moment method – summary 42

Chapter 7 Loading Conditions 45
7.1 Loading Condition 47
7.2 Free Surface Effects 48
7.3 Typical GZ curve 52
7.4 Maximum permissible KG curve 53

Chapter 8 Stability Acceptance Criteria 55
8.1 Normal acceptance criteria for a vessel 57
8.2 Acceptance criteria for a catamaran or multihull vessel that cannot meet the requirements of 8.1 58
8.3 Acceptance criteria for motor vessels complying with Annex Section 11.1.1.3 58

Chapter 9 Trim 61

Chapter 10 Downflooding Angle 67
10.1 What is the Downflooding Angle? 69
10.2 Critical Angle of Downflooding 69

Chapter 11 Damage Survivability 71
11.1 Floodable length curves 73
11.2 Damage stability 76
11.3 Criteria for damage stability assessment 77

Chapter 12 Cranes and Suspended Weights 79
12.1 Fitting a crane 81
12.2 Stability considerations 81
12.3 GZ and heeling curves 81
12.4 Criteria for acceptance of vessel with lifting device or crane 83
12.5 Crane testing 83

Chapter 13 Stability of Sailing Vessels 85
13.1 Background 87
13.2 Wind curves 88
13.3 Gusts and Squalls – definitions 89
13.3.1 Preventing downflooding in Gusts 89
13.3.2 Preventing downflooding in Gusts and Squalls 91
13.4 Sailing multihulls 93
13.5 STOPS Numeral 94
13.6 Stability and safety screening: the SSS Numeral 96
13.7 The stability index: STIX 96

References 99

Appendices 103
Appendix 1 105
Appendix 2 108
Appendix 3 109
Appendix 4 110