Coxwain and Master 5 Stability resource.pdf

PUBLICATIONS VISION DIGITAL ONLINE MAR048 MARITIME STUDIES Learner’s Guide STABILITY A Coxswain Level...

PUBLICATIONS VISION DIGITAL ONLINE MAR048 MARITIME STUDIES Learner’s Guide STABILITY A Coxswain Level Master 5 Level w w w. w e s t o n e. w a. g o v. a u MARITIME LEARNING RESOURCE STABILITY A COXSWAIN LEVEL MASTER 5 LEVEL LEARNER’S GUIDE Copyright and Terms of Use © Department of Training and Workforce Development 2016 (unless indicated otherwise, for example ‘Excluded Material’). The copyright material published in this product is subject to the Copyright Act 1968 (Cth), and is owned by the Department of Training and Workforce Development or, where indicated, by a party other than the Department of Training and Workforce Development. The Department of Training and Workforce Development supports and encourages use of its material for all legitimate purposes. Copyright material available on this website is licensed under a Creative Commons Attribution 4.0 (CC BY 4.0) license unless indicated otherwise (Excluded Material). 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Excluded Material may not be licensed under a CC BY license and can only be used in accordance with the specific terms of use attached to that material or where permitted by the Copyright Act 1968 (Cth). If you want to use such material in a manner that is not covered by those specific terms of use, you must request permission from the copyright owner of the material. If you have any questions regarding use of material available in this product, please contact the Department of Training and Workforce Development. Training Sector Services Telephone: 08 6212 9789 Email: [email protected] Website: www.dtwd.wa.gov.au First published 2003 ISBN 0 7307 9203 X © WestOne Services 2003 The Open Training and Education network of the NSW Department of Education and Training participated in developing this product. 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, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of WestOne Services. Whilst every effort has been made to ensure the accuracy of the information contained in this publication, no guarantee can be given that all errors and omissions have been excluded. No responsibility for loss occasioned to any person acting or refraining from action as a result of the material in this publication can be accepted by WestOne Services. Published by and available from WestOne Services Prospect Place West Perth WA 6005 Tel: (08) 9229 5252 Fax: (08) 9229 5292 E-mail: [email protected] Website: www.westone.wa.gov.au  STABILITY − PART A COXSWAIN LEVEL, MASTER 5 LEVEL INTRODUCTION TO STABILITY A AND STABILITY B This Maritime resource contains all the requirements for Stability at the Coxswain, Master 5 and Master 4 levels. It is accordingly divided into 3 sections. Stability A contains the material for Coxswain and Master 5. Stability B is for Master 4 students and has been printed separately. Within each section there are activities for you to work through and most contain self-test questions for you to check that you have understood what you have just been studying. At the end of several Sections, in the Master 4, are Mastery Assignments. These are longer and will take some time to work through. The worked answers are given for these. As the final assessment will contain calculations, it is important that they are practised. TEXTS For Coxswain and Master 5: the booklet entitled ‘Fishing Vessel Stability’ (reproduced by kind permission from the National Fishing Industry Training Council) Simplified Stability Information for MV Twosuch, WestOne Publication (reproduced) For Master 4: D.R. Derrett, Ship Stability for Masters and Mates, 5th edition, Stanford Maritime, London Simplified Stability Information for MV Twosuch, WestOne Publication (reproduced) Simplified Stability Information for MV Onesuch, WestOne Publication (reproduced) i STABILITY COXSWAIN  STABILITY − PART A COXSWAIN LEVEL – LEARNER’S GUIDE CONTENTS – Coxswain Level Page SECTION 1 Vessel Stability 1.1 UNIT 1.1 Basic Principles of Stability 1.1 1.2 Forces Involved 1.2 1.3 Centre of Gravity 1.3 1.4 Where is the Centre of Gravity? 1.4 1.5 Terminology 1 1.5 1.6 Buoyancy 1.6 1.7 Terminology 2 1.9 1.8 Free Surface 1.9 1.9 Summary 1.13 1.11 Study ‘An Introduction to Fishing Vessel Stability’ 1.15 1.12 In Conclusion 1.15 Check-your-progress Assignment 1 1.21 Check-your-progress Assignment Answers 1.23 Appendix A: An Introduction to Fishing Vessel Stability 1.27 iii  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE SECTION 1 VESSEL STABILITY A simple understanding of what makes a boat stable is critical to the safe navigation of any craft – in small craft this is more so. The tolerances are much smaller and small movements of weights can have BIG effects as we may have found out in small tenders when a passenger has stepped on the gunwale. Stability is calculated for a vessel in ordinary trim and without a list. Vessels with an excessive trim will have considerably reduced stability. A list always reduces stability. The first Section is an introduction to the subject and so needs to be studied carefully so that the basic terms are understood. This first Section covers two of the training package requirements. Learning Outcomes: Describe the principles of stability and trim in a small vessel and the disposition of passengers and cargo required to maintain stability and trim within safe limits. Explain the effects of loss of watertight integrity. Unit 1.1 Basic Principles of Stability The question is, "What is stability?" What does it mean when we say a vessel is stable? How often have we heard statements like: "Great little tender, very stable" "Most stable boat I have been on" "She is so stable you can sit down for a meal in a force 7" "Really dry boat" "Doesn't roll much" "Lots of freeboard, really stable" "Has a good centre of gravity" "Really stable, you would never get sick on her" (Are seasickness and stability related!!) What do all these statements tell us? That there are many ideas about what stability is. So we will use a very simple definition: 1.1 SECTION 1 – VESSEL STABILITY Stability is the ability of a vessel to return to her initial position. A vessel rolling in a seaway rolls around her initial position (usually upright) and in calm water returns to that position. A yacht sailing along at some angle will roll around that angle as the wind gusts. When the wind stops gusting, she will return to her original angle. In the transverse or athwartships direction, stability is more easily endangered and easier to understand than in the fore and aft or longitudinal direction. Unit 1.2 Forces Involved The most obvious thing about a boat is that it floats. The boat, using its own weight, pushes some water out of the way and creates a hole in the water in which it can sit. Lift the boat out, the water goes back. When we lower the boat into the water it floats at the point where the weight of the boat pushing down is equal to the force of the water pushing up. So, when a boat is floating, there are two forces acting equally and in opposite directions. Now the force acting down is obviously the weight of the boat and the force acting up is called buoyancy. When they are equal and opposite the boat floats. This simple fact applies to every object floating. Weight acts vertically down Ð and buoyancy acts vertically up Ï. 1.2  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Stability is all about the interaction between these two forces. So, to understand stability properly, we need to clearly understand these two forces. Q. What is the difference between a beachball and a dinghy? A. The beachball will float upside down. Unit 1.3 Centre of Gravity If you have ever taken some of the uninitiated out in a small dinghy, you will know how important it is for them to sit in the centre to keep it upright. What about vertically? What happens when they stand up? Weight has moved up in the boat and it is now less stable and feels it! We know this in practice. Let's have a look at the theory behind this. Fig 1.1 If we take a plank of wood – it has a point about which it balances. This point through which we could consider the weight is acting, vertically downward, is called the centre of gravity. The centre of gravity actually lies at the centre of the plank, the geometric centre, and the plank will rock around point P without falling off. The sum of the weights of everything in the vessel such as structure, stores, fuel, water, fish, and cargo is the vessel's weight and can be considered as one force acting downward through a single point, called the Centre of Gravity – G. The centre of gravity stays in the same point independent of the vessel's motion. Fig. 1.2 Figure 1.3 shows how moving the centre of gravity upwards reduces stability (just like when the person stands up in the dinghy). 1.3 SECTION 1 – VESSEL STABILITY Fig. 1.3 Fig. 1.4 Principle: when a weight is added, the centre of gravity moves towards the added weight. In (1.) upwards, in (2.) downwards and, when the weight is discharged, it will move back to its original position; that is, away from the weight discharged. There is a third situation … If you were sitting in a dinghy, you and the dinghy combined would have a certain centre of gravity. What happens to the centre of gravity if you move sideways? The centre of gravity moves sideways; that is, parallel with your movement. G is no longer on the centre line and the dinghy has a list. Unit 1.4 Where is the Centre of Gravity? Of course boats are not like blocks of wood. Not only are they all different shapes and sizes, but will have weights in all sorts of different places. So, where is the centre of gravity? The answer to this is in two parts: 1. When the boat is brand new – fresh off the shelf, as it were – with no fuel, oil, water, stores, passengers, crew, etc, then it is in what is called the "light ship" condition. For this condition the designer (naval architect) has worked out where the centre of gravity will be. 1.4  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 2. You come along as the skipper and start putting things in (fuel, water, stores, etc) and where they go will determine where the centre of gravity will move to. For some things this is more or less fixed (fuel, water) and for others it is very much up to you – for example, how many passengers you have on board and more importantly where they are. For example if some, or all, of your passengers are on the flying bridge the centre of gravity will be higher than if they are all on the main deck. In other words the final position of the centre of gravity is determined by you, the skipper. To summarise so far: 1. Stability: the ability of a vessel to return to its initial position. 2. Centre of Gravity: the point through which a weight is said to act vertically downwards. 3. The centre of gravity will move towards a weight added. 4. The centre of gravity will move away from a weight discharged. 5. The centre of gravity will move parallel with a weight moved. 6. Don't stand up in dinghies. Unit 1.5 Terminology I Just a few terms to note: Heel: When an external force acts on a boat it will incline to some angle. This is called the angle of heel (remember yachts heel) and G has not moved. List: When a weight is moved off the centre line the centre of gravity will also move and the vessel will now be inclined at some angle. This angle is called a list. (See Fig. 1.5.) Heel: an inclination caused by an external force. List: an inclination caused by an off centre weight. Fig. 1.5 1.5 SECTION 1 – VESSEL STABILITY Generally the freeboard should be sufficient to prevent the decks becoming immersed as shown in the sketch below: W L W L A vessel with low freeboard may dip her decks underwater when heeled, resulting in a loss of stability. UNIT 1.6 Buoyancy Now, what about buoyancy? Let us go back to our block of wood, which is now floating. We saw before that G is the point through which the weight was considered to act vertically downwards and that for the block to float there must be another force, buoyancy, acting vertically upwards through a point which must be in line with G acting vertically downwards. This point, the centre of buoyancy B, is at the centre of the underwater volume. For the block of wood, this is easily found by drawing diagonals and getting the centre of the under- water volume. In ship shape boats, this is worked out by the naval architect. Fig. 1.6 1.6  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Fig. 1.7 The Centre of Buoyancy B is the point through which the total force of buoyancy acts vertically upwards. For a vessel at rest it acts in the same vertical line as G but in the opposite direction. As the boat rolls in a swell so the underwater section changes and the centre of buoyancy moves. (See Fig. 1.7.) The force of buoyancy What happens when a yacht is heeled due to a wind? The yacht is inclined to some angle and, provided the strength of the wind does not change, it will stay at that angle. So if there is some force trying to heel the boat, then there must be an equal and opposite force holding the boat at that angle and preventing it from heeling right over. In Fig. 1.8 we can see G and B1 are in the same vertical line. Fig. 1.8 In Fig. 1.9 the centre of gravity has not moved as there has been no change or movement of the weights on board. What about B1? The underwater shape of the vessel has changed so the centre of buoyancy now moves to the new centre B2. Fig. 1.9 1.7 SECTION 1 – VESSEL STABILITY So in a vessel which is heeled, it is the buoyancy acting through the centre of buoyancy which tries to bring the vessel back to the initial position. This, of course, applies to every floating object from a dinghy to a supertanker. The final point to note from Fig. 1.9 is the horizontal distance between G acting down and B acting up. This creates a lever (GZ) for the buoyancy to push up. In the heeled position the force of buoyancy acting upwards is not in line with the force of gravity acting down and creates a lever called the righting lever. (Fig. 1.10) Fig. 1.10 Heel and list in combination Now if some weight W had moved on board so that G1 was now at G2, (that is, vessel has a list) whilst the buoyancy at B2 is the same, the length of the lever has been reduced and consequently the ability of the buoyancy to right the vessel is reduced. What does this mean out on the water? Simply, a boat with a list (G off the centre line) is less stable – remember the definition – than one which is upright. So to increase stability the list must be reduced. Fig. 1.11 1.8  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Unit 1.7 Terminology 2 Draft: The height between the keel and the waterline. Trim: The difference between the fore and aft draft. Either called positive when the forward draft is greater than the after draft or negative when the after draft is greater than the forward draft. Freeboard: The distance from the waterline to the lowest watertight deck. The buoyant volume is the volume of the watertight part of the hull. The part below the waterline is called buoyancy and the part above is called reserve buoyancy. When additional weight is put into the vessel, reserve buoyancy is used up. The reserve buoyancy largely determines the ship’s stability and the freeboard gives an approximate measure of the reserve buoyancy. Fig. 1.12 UNIT 1.8 Free Surface There is an effect called free surface which creates a lot of problems on vessels. Get a washing up bowl and put in a litre of milk (still in the carton), pick up the bowl... Now put a litre of water in the bowl and pick it up. How does it feel? 1.9 SECTION 1 – VESSEL STABILITY Quite difficult to keep it horizontal... Why? When the liquid cannot move as shown in Fig. 1.13, when the bowl moves the carton of milk makes a whole unit with the bowl and the centre of gravity does not move. Fig. 1.13 When the liquid is free to move [Fig. 1.14(a) and (b)] we have a double problem: The centre of gravity is free to move independently of the movement of the bowl. The moving liquid carries its own momentum - so it does not stop moving when the bowl stops moving. The combined effect of these two will cause G to rise making GZ shorter and reducing stability. Fig. 1.14(a) Fig. 1.14(b) The effect of free surface What does this mean on a boat? If the fuel or water tanks are partially full then there will be a reduction in GZ. Generally this will make very little difference to a boat's stability. However, if the water is trapped on deck there will be a rise in G (G moves towards the weight added) with a reduction in GZ. The importance of keeping decks dry, for this reason, cannot be over-estimated. Large quantities of fish landed on deck will have a similar effect and need to be contained as soon as possible. 1.10  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE (a) (b) Fig. 5.14 Free surface (either water on deck or a (c) partly filled tank) will cause the centre - of gravity G to move off the centre when the vessel is heeled and effectively rise, thus reducing the GZ, making the vessel less stable. Fig. 1.15 1.11 SECTION 1 – VESSEL STABILITY Fig. 1.16 We can see that as G becomes lower in the vessel the GZ increases G moves down to G1 and GZ increases to G1Z1 making the vessel more stable (able to return to the upright). As weight moves higher GZ reduces to G2Z2 and the vessel becomes less stable. This is why survey requirements control the number of passengers on upper decks. Any excess can lead to reduction in stability. Underwater Shape Fig. 1.17 We can see in the above that beam plays an important role in the stability of a vessel. In Fig. 1.18, the vessel having the larger beam allows the centre of buoyancy to move out further when she is inclined. This causes an increase in the GZ, making the vessel more stable. Note: As soon as the deck edge immersion is reached then B will not move out any further. This, in theory, will give the maximum GZ. In practice, this is not so for various fairly complex reasons to do with the underwater shape. 1.12  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Once deck edge immersion has been reached, if all watertight doors and openings are not shut down then it is possible for down flooding to take place. As the vessel is already inclined, more weight (water) will rapidly cause the vessel to capsize. Fig. 1.18 Unit 1.9 Summary 1. Stability - ability of a vessel to return to its initial position. 2. Centre of Gravity - a point through which a weight is said to act vertically down. 3. The centre of gravity will move towards a weight added. 4. The centre of gravity will move away from a weight discharged. 5. The centre of gravity will move parallel with a weight moved internally. 6. Don't stand up in dinghys. 7. Heel - inclination caused by an external force. 8. List - inclination caused by an internal force. 9. Centre of buoyancy is at the centre of the underwater volume and is the point through which the force of buoyancy acts vertically upwards. 10. The underwater shape of the vessel changes as the vessel moves, so the centre of buoyancy continuously moves. 11. Righting Lever - when a vessel is heeled the horizontal separation between G (acting down) and B (acting up) is the righting lever, GZ. Generally, the lower the centre of gravity the larger the GZ will be. 12. A vessel with a list is less stable than a vessel which is upright. 13. Water trapped on deck (free surface) will reduce the vessel’s stability. Always keep freeing ports clear. 1.13 SECTION 1 – VESSEL STABILITY. 1.14  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Unit 1.11 Read and then study the booklet ‘An Introduction to Fishing Vessel Stability’ which is reproduced at the end of this section, as Appendix A, by kind permission of the National Fishing Industry Training Council. Unit 1.12 In Conclusion This section has introduced you to the principles of Vessel Stability. What you need to do now is look at how you, the Coxswain, can control the stability of your vessel. Important concepts were discussed; they included, Beam, Centre of Buoyancy, Centre of Gravity, Freeboard, Free Surface Effect, and the righting lever GZ. Let’s look at each and see which the Coxswain has direct control over. Beam is a design feature of the vessel; the Coxswain has no control over Beam. Centre of Buoyancy is controlled by the vessel shape. For any displacement (total weight of the vessel) the Centre of Buoyancy is fixed by that design; it doesn’t change as you move those weights around. Therefore the Coxswain does not control Centre of Buoyancy. Centre of Gravity is decided by where weights are loaded. A Coxswain is responsible for this, and therefore controls Centre of Gravity. Freeboard is a design feature but is then affected by the total weight added. A Coxswain is responsible for this, and therefore controls Freeboard. Free Surface Effect is liquid that has space to move such as slack tanks and bilge water. The Coxswain is responsible for this, and therefore controls Free Surface Effect. The righting lever, GZ, is controlled by the Centre of Buoyancy and the Centre of Gravity. As noted, we do not control the Centre of Buoyancy but we do control the Centre of Gravity. Therefore, the Coxswain controls the GZ but only through control of the Centre of Gravity. 1.15 SECTION 1 – VESSEL STABILITY To summarise this, the Coxswain directly controls: 1. Centre of gravity 2. Freeboard, and 3. Free Surface Effect. So what comes next is not very surprising! As a Coxswain, whenever you think stability you should ask yourself three questions: 1. What effect does this have on the Centre of Gravity? 2. What effect does this have on Freeboard? 3. What effect does this have on Free Surface Effect? Then review these questions: Lowering the Centre of Gravity or moving it back to the centreline is good. Raising it or moving it off centre is bad. Increasing the Freeboard is good, decreasing it is bad. Removing Free Surface Effect is good, introducing it is bad. One of the best illustrations of this is water on deck. Think about how this affects stability. Write down your thoughts, and then compare your notes to the following: 1. Water on deck is weight added high, therefore the Centre of Gravity will move up towards the added weight. Raising the Centre of Gravity - bad for stability. 2. Water on deck is an added weight, increasing Draft therefore the Freeboard will be reduced. Decreasing the freeboard - bad for stability. 3. Water on deck is not contained, therefore it has Free Surface Effect. Introducing Free Surface Effect - bad for stability. Water on deck is bad in all three cases, it is very bad for stability, thus the importance of your freeing ports (often called scuppers) to allow the water to drain quickly. Another example: A suction line for the engine cooling splits and you start taking water into the engine room bilge. Again, think about how this affects stability, write down your thoughts, and then compare your notes to the following: 1. Water in the bilge is weight added low, therefore the Centre of Gravity will move down towards the added weight. Lowering the Centre of Gravity - good for stability. 2. Water in the bilge is an added weight, increasing Draft therefore the Freeboard will be reduced. Decreasing the freeboard - bad for stability. 3. Water in the bilge is not contained, therefore it has Free Surface Effect. Introducing Free Surface Effect - bad for stability. You may think that lowering the Centre of Gravity, in this case, should increase stability; it does but the benefit is more than outweighed by Free Surface Effect and the loss of Freeboard. 1.16  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Free Surface Effect is a bit of a wild card in the stability pack. Its effect can be anywhere from good to devastating and it can go between the two extremes in seconds (as the liquid moves). Therefore we always consider it at its worst. Remember that the extent of Free Surface Effect is controlled, not by the amount of liquid moving, but by the space that it has to move in. In the two examples above there is lots of room: from one side of the deck to the other and from one side of the engine room bilge to the other. You will notice that tanks built into boats are not normally very wide; this is to help control Free Surface Effect as a vessel will always have some slack (partially full) tanks. Think: Stability 1.17 SECTION 1 – VESSEL STABILITY 1.18  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.19 SECTION 1 – VESSEL STABILITY 1.20  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Check-your-progress Assignment 1 Question 1. When a weight is added, the centre of gravity moves ……………….. the weight added. Question 2. When a weight in a boat is moved horizontally, the centre of gravity: (a) moves up (b) moves down (c) moves horizontally Question 3. Which items are not part of the light ship condition? (a) fuel (b) engine (c) stores (d) crew Question 4. Don’t ……….….. ….….. in dinghies. Question 5. Heel is caused by objects being moved on board. T/F Question 6. List is produced by an external force – ie a sailing boat listing with the wind. T/F Question 7. Centre of buoyancy is: (a) at the centre of the waterplane area (b) at the centre of the underwater volume (c) the same as the centre of gravity 1.21 SECTION 1 – VESSEL STABILITY Question 8. When a boat is inclined by an external force, it comes back to the upright because of the: (a) centre of buoyancy acting up (b) centre of gravity acting down (c) small GZ (d) low centre of gravity The answer is: (a) a and d (b) a and c (c) b and a Question 9. Trim is positive when the aft draft is greater than the forward draft. T/F Question 10. A boat is heeled. Draw a cross section showing the relative positions of B. G. GZ. B2 If the centre of gravity was lowered, show on your diagram what would happen to GZ. Question 11. Draw a cross section of 2 box-shaped barges, the one being wider than the other. Show which one is more stable. Question 12. Your 11.5 metre fishing charter vessel is going offshore with 15 passengers. As soon as she is clear of the land she starts to roll violently. List 5 steps you would take to correct this if you assumed it was a stability problem. NOW CHECK YOUR ANSWERS AGAINST THOSE PROVIDED ON THE FOLLOWING PAGES. 1.22  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE Check-your-progress Assignment Answers Question 1. When a weight is added, the centre of gravity moves towards the weight added. Question 2. When a weight in a boat is moved horizontally, the centre of gravity… (a) moves up (b) moves down (c) moves horizontally Question 3. Which items are not part of the light ship condition? a. fuel b. engine c. stores d. crew Question 4. Don’t STAND UP in dinghies. Question 5. Heel is caused by objects being moved on board. T/F Question 6. List is produced by an external force – ie a sailing boat listing with the wind. T/F Question 7. Centre of buoyancy is: (a) at the centre of the waterplane area (b) at the centre of the underwater volume (c) the same as the centre of gravity 1.23 SECTION 1 – VESSEL STABILITY Question 8. When a boat is inclined by an external force, it comes back to the upright because of the: (a) centre of buoyancy acting up (b) centre of gravity acting down (c) a high centre of buoyancy (d) low centre of gravity The answer is (a) a and d (b) a and c (c) b and a Question 9. Trim is positive when the aft draft is greater than the forward draft. T/F Question 10. A boat is heeled. Draw a cross section showing the relative positions of B. G. GZ. B2 See Fig 1.10. If the centre of gravity was lowered, show on your diagram what would happen to GZ. See Fig 1.16. Question 11. Draw a cross section of 2 box-shaped barges, the one being wider than the other. Show which one is more stable. See Fig 1.17 and Fig 1.18. Question 12. Your 11.5 metre fishing charter vessel is going offshore with 15 passengers. As soon as she is clear of the land she starts to roll violently. List 5 steps you would take to correct this if you assumed it was a stability problem. 1.24  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE FILL UP ANY SLACK TANKS. PUT WEIGHTS LOW IN THE VESSEL. LOWER WEIGHTS HIGH UP IN THE VESSEL. ALTER COURSE TO REDUCE EXCESSIVE ROLLING. PUT PASSENGERS BELOW. 1.25 SECTION 1 – VESSEL STABILITY 1.26 APPENDIX A  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.27 SECTION 1 – VESSEL STABILITY 1.28  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.29 SECTION 1 – VESSEL STABILITY 1.30  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.31 SECTION 1 – VESSEL STABILITY 1.32  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.33 SECTION 1 – VESSEL STABILITY 1.34  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.35 SECTION 1 – VESSEL STABILITY 1.36  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.37 SECTION 1 – VESSEL STABILITY 1.38  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.39 SECTION 1 – VESSEL STABILITY 1.40  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.41 SECTION 1 – VESSEL STABILITY 1.42  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.43 SECTION 1 – VESSEL STABILITY 1.44  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.45 SECTION 1 – VESSEL STABILITY 1.46  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.47 SECTION 1 – VESSEL STABILITY 1.48  STABILITY A – COXSWAIN LEVEL– LEARNER’S GUIDE 1.49 SECTION 1 – VESSEL STABILITY 1.50 STABILITY MASTER 5  STABILITY − PART A COXSWAIN LEVEL – LEARNER’S GUIDE CONTENTS – Master 5 Level TOPIC 1 BASIC PRINCIPLES OF STABILITY Section 1 Basic Stability TOPIC 2 LONGITUDINAL AND TRAVERSE STABILITY Section 2A List and Trim Section 2B Stability in Seaway TOPIC 3 FACTORS AFFECTING STABILITY Section 3 Factors Affecting Stability Section 4 Twosuch Guide Notes Appendix A: Simplified Stability Information for M.V. ‘Twosuch’ Assignment 1 Assignment 2  STABILITY A − MASTER 5 LEVEL − LEARNER’S GUIDE TOPIC 1 BASIC PRINCIPLES OF STABILITY Syllabus Learning Outcome On completion of this learning outcome the learner will be able to describe the basic principles of stability relevant to small vessels. Assessment Criteria Explain the relationship between weight and buoyancy in relation to floating bodies Explain the meaning of terms commonly used in relation to the stability of a vessel Explain why a vessel’s draft would change when moving from fresh water to salt water Explain the relationship between light displacement, load displacement and dead- weight tonnage Text National Fishing Industry Training Committee: An Introduction to Fishing Vessel Stability (included in the Coxswain’s section of this learning resource)  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE SECTION 1 BASIC STABILITY Introduction Stability is a word which is put to several uses: the steering of a vessel can be stable, meaning that it responds nicely to the helm and does not heel over excessively when making a tight turn directional stability refers to the vessel’s ability to maintain its course with very little helm required a vessel is said to be stable in a seaway - meaning that it will not capsize under the forces of the waves and wind a vessel is also said to be stable when it tends to right itself after being heeled by an external force. Although all the above uses of the term “stability” are related, it is primarily with the last meaning listed above that we will be concerning ourselves in the remainder of this course. Objectives By the end of this Section you should be able to: explain what is meant by the term Equilibrium state the law of flotation and show how a body is in equilibrium when floating freely in a fluid explain what is meant by the term Centre of Gravity (C.G.) and show how the C.G. of a body can be located explain what is meant by the term Centre of Buoyancy (C.B.) define the following terms: draft freeboard displacement deadweight (DWT) trim tonnes per centimetre immersion (T.P.C.) fresh water allowance (F.W.A.) loadline relate the above definitions to a block of wood floating in fresh water and salt water 1.1 SECTION 1 – BASIC STABILITY Unit 1.1 Archimedes’ Principle Many years ago a Greek scholar named Archimedes became intrigued by the rise in water level every time he had a bath. He investigated this phenomenon and came up with a principle which has stood the test of time. Archimedes’ Principle When a body is wholly or partially immersed in a fluid, it appears to suffer a loss in mass equal to the mass of fluid it displaces. This relationship between weight and volume is called density. It is expressed as a ratio of the weight of a substance for a unit of volume. In our metric system, one metric tonne of fresh water has a volume of one cubic metre. It has a density of one tonne per cubic metre. Salt water on the other hand is heavier. One cubic metre weighs 1.025 tonnes and has a density of 1.025 tonnes/m3. Frequently we use the term Relative Density, which is simply a comparison of the density of a substance with the density of fresh water. It is also expressed as a ratio: Density of Substance RD = Density of FreshWater This is a pure number and has no units. The R.D. of sea water is therefore 1.025. READ and then STUDY Archimedes Principle on page 1 of your text, An Introduction to Fishing Vessel Stability (F.V.S). Weight vs Buoyancy Now let us refer back to Archimedes. Suppose we have a body or block that measures 1 cubic metre and weighs 3000 kg. If we now lower the block into fresh water, it will displace 1 cubic metre of fresh water – which, as we now know, weighs 1000 kg. In other words, there is a force acting upwards of 1000 kg and a force acting downwards of 3000 kg; the resultant force has to be 2000 kg downwards. That is, the block will sink. Let us now take the same 3000 kg block and re-mould it into a hollow box with a volume of 3 cubic metres of fresh water - which also weighs 3000 kg. In this case the upward force now equals and is opposite to the downward force. That is, the box will now (just) float. If we now take the original 3000 kg block and mould it into a hollow box with a volume of 5 cubic metres and then place it in fresh water, it has sufficient volume to displace 5 cubic metres of fresh water. If the box were now completely submerged it would experience an upward force of 5000 kg. However, the downward force of the box is still only 3000 kg, thus the downward resultant force will be 2000 kg upwards. In this case the box will rise out of the water to a level where the forces are equal and opposite. This box will now have a draft equal to 3/5 of the maximum depth. 1.2  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE If one cubic metre of iron is immersed in fresh water, it will displace one cubic metre of the water which we know to weigh 1 tonne. As 1 m3 of iron weighs 7.8 tonnes, it is clearly not displacing its own weight. Now consider the same weight of iron with an enlarged volume, say 2 m3 (an air space of 1m3 having been introduced in the centre of the iron). If this enlarged block of iron is immersed in fresh water, 2 m3 of fresh water is displaced. This will cause an apparent reduction in weight of 2 tonnes - still not enough to cause it to float. When the volume of the block and air space reaches 7.8 m3, the block will just float as it is displacing its own weight of the liquid in which it is floating. If the volume is further increased, it will float with a certain amount of freeboard; i.e. Force of Buoyancy = Weight Consider a rectangular wooden log with a relative density of 0.6. It has a volume of 2.5 m3 and weighs 1.5 tonnes. In Figure 1.1, the weight of the log acts directly downwards through its centre of gravity (G), and the force of buoyancy acts vertically upwards through the centre of buoyancy (B). Fig. 1.1 Weight vs Buoyancy Unit 1.2 Centre of Gravity, Centre of Buoyancy STUDY the definitions of ‘buoyancy’ and ‘centre of buoyancy’ on page 6 of your text, An Introduction to Fishing Vessel Stability. STUDY the definition of ‘gravity’ and ‘centre of gravity’ on page 5 of your text. STUDY the following notes. The centre of gravity of a body is the point about which a body will balance. 1.3 SECTION 1 – BASIC STABILITY There are many ways to find this balance point. In a homogenous body the centre of gravity is found at the centroid. The material of which a homogenous body is made is uniformly distributed throughout its volume; i.e., if you take 1m3 of the material from one part of the body it will weigh exactly the same as 1 m3 of the material from any other part of the body. In the shapes in Figure 1.2 the ‘G’ represents the centroid. If each area was suspended from this point the shape would balance: Fig. 1.2 Centroids To find the centre of gravity of a rectangular wooden log is relatively simple; you would do it by locating the centroid as shown in Figure 1.2 for a rectangle. It is more difficult to exactly locate the C.G. of a non-uniform wooden log. One could however quite easily find the vertical line through which ‘G’ acts. Fig 1.3 Lift up one end of the log and place a roller underneath it, as shown in Figure 1.3. Now push the log towards the roller and the log will slide on top of the roller. When the log balances on the roller, the centre of gravity is directly above the centre of the roller and approximately in the centre of the log as shown in Figure 1.4. 1.4  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 1.4 Consider a rectangular log floating in fresh water. The volume of water it displaces weighs the same as the log. The force of buoyancy acts vertically upwards through the centre of buoyancy. Because the log is homogenous and of uniform dimensions, the waterline will be parallel to the top and bottom edges of the log. The centre of buoyancy and the centre of gravity will be in the same vertical line. If we were to disregard all above the waterline, the centre of buoyancy will be the centre of gravity of the underwater portion as shown in Figure 1.5. Fig. 1.5 In a ship shape body, the external dimensions are not nice and simple to measure and more complex methods are used to calculate the position of the centre of buoyancy. The principle however is the same, and B is always located at the geometric centre of the underwater part. Unit 1.3 Draft, Freeboard and Trim STUDY the definitions of ‘draft’, ‘freeboard’, ‘list’ and ‘trim’ on pages 2, 3 and 4 in your text book. A few new terms are introduced here. The centre of flotation is the centroid of the water plane area. Consider the block in Figure 1.6. In this case the block is floating on an even keel with no list and no trim. 1.5 SECTION 1 – BASIC STABILITY Fig. 1.6 C.F. is the position of the centre of flotation. This is sometimes known as the ‘tipping centre’ since it is the point about which a floating body pivots. If a small weight was placed on the left-hand end of the log, this end would sink slightly, causing the log to have a trim. The draft underneath the weight would increase and the draft at the other end would decrease. The difference between the two drafts would be the trim as shown in Figure 1.7. Draft 1 minus Draft 2 = Trim. Fig. 1.7 1.6  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE NOTE that the left-hand side has been called the after end and the right-hand side the fore end. When drawing diagrams of ships, this is the usual way it is done. NOTE also that a trim by the stern is by convention, in Australia, designated as negative, and a trim by the bow, positive. Remember that a block floating in fresh water floats at a deeper draft than in salt water. Example Consider a rectangular, homogenous block of relative density 0.8 and with the following dimensions: length 10 m, breadth 2 m, depth 1 m. What will be its draft in: (a) fresh water? (b) sea water, density 1.025? Answers Weight of Block = Volume × Density Weight of Water Displaced = Volume × Density ∴ 1 × b × d × density (block) = 1 × b × draft × density (water) ∴ depth × density (block) = draft × density (water) depth×density( (log) ∴ Draft = density water This formula is good for all rectangular shaped floating objects. (a) In fresh water, density = 1 1m × 0.8 ∴ draft = 1 = 0.8 m (b) In salt water, density = 1.025 1m × 0.8 ∴ draft = 1.025 = 0.78 m Unit 1.4 Displacement, T.P.C., F.W.A., Load Line and Dead- weight STUDY the definitions of ‘displacement’, ‘load displacement’ and ‘deadweight’ on pages 1 and 3 in your text. STUDY the following notes... Displacement When we refer to the displacement of a vessel, we mean the weight of water that it displaces. We know this to be the weight of the vessel (Archimedes’ Principle), but it is 1.7 SECTION 1 – BASIC STABILITY easier to measure the volume of water that a floating body displaces than to actually weigh the body, especially when the body is a ship weighing several thousand tonnes. Example If a block of wood with R.D. of 0.75 weighs 1.5 tonnes: (a) what will its volume be? (b) what will be the volume of water it will displace in (i) fresh water? (ii) sea water, density 1.025? Answers Weight (a) Volume = Density 1.5 ∴ Volume = 0.75 = 2 m3 (b) (i) Volume × Density(water) = Volume × Density(wood) Volume × Density(wood) ∴ Volume(water) = Density(wood) 2 × 0.75 = 1 = 1.5 m3 Volume × Density(wood) (ii) Volume(water) = Density(water) 2 × 0.75 = 1.025 = 1.463m3 From this, it is clear to see that the volume of water displaced by a floating body in fresh water is greater than the volume of water displaced by the same floating body in sea water. We saw (in Unit 1.3) that a block of wood floating in fresh water would have a deeper draft than it would have if it was floating in salt water. This difference is known as the Fresh Water Allowance (F.W.A.). F.W.A. is a useful quantity to know. It is usually expressed in millimetres. In the example in Unit 1.3 the F.W.A. would be 20 mm. F.W.A. is the number of millimetres by which the mean draft changes when a ship passes from salt water to fresh water (or vice versa) when floating at its loaded draft. LOAD DRAFT is the draft at which a vessel floats when loaded to her load displacement. 1.8  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Tonnes Per Centimetre Immersion (T.P.C.) The T.P.C. for any draft is the weight which must be loaded or discharged to change a ship’s mean draft, in sea water, by one centimetre. To find T.P.C. all that will be necessary is to calculate the weight of the additional water displaced when the draft is increased 1 cm. To do this we would have to calculate the volume of the additional water displaced and multiply it by the density (1.025). Fig. 1.8 Consider the block in Figure 1.8. If it had a length of 6m and a breadth of 3m its T.P.C. would be found as follows: T.P.C. = Length × Breadth × 1 cm × Density NOTE: 1 (i) 1cm = 100 (ii) Density of Sea Water = 1.025 (iii) Length x Breadth = Area of Waterplane (Aw) When Aw is in square metres (m2) this formula is good for all vessels. For the block in Figure 1.8: A w = 6 × 3 =18 m 2 1.025 ×18 ∴ T.P.C. = 100 = 0.185 tonnes per centimetre 1.9 SECTION 1 – BASIC STABILITY Unit 1.5 Self-test Questions 1. (a) What is Archimedes’ principle? (b) What is Relative Density and how can you express it as a ratio? 2. Calculate the relative density of salt water whose density is 1,025 kg/m3. 3. Calculate the density of a fuel oil whose relative density is 0.92. 4. When a double-bottom tank is full of fresh water on a vessel, it holds 12 tonnes. Calculate how many tonnes of oil of relative density 0.84 it will hold. 5. Fig. 1.9 Figure 1.9 shows a homogenous block of wood heeled over by an external force. (a) Reproduce Figure 1.9 and show clearly the positions of G and B on your drawing. (b) Sketch the same block floating in equilibrium when the external force has been removed. Show on your sketch the positions of G and B. On both sketches draw arrows showing clearly the directions in which the forces of buoyancy and weight are acting. 6. A box-shaped vessel 10.5 m long, which has a 3 m beam and a 2 m height, is floating upright in fresh water. If it displaces 19.5 tonnes calculate the volume of reserve buoyancy and mean draft. 7. When a double-bottom tank is full of fresh water, it holds 12 tonnes. Calculate how many tonnes of oil of relative density 0.84 it will hold. 1.10  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE ANSWERS TO SELF-TEST QUESTIONS Unit 1.5 1. (a) Every floating body displaces its own weight of the liquid in which it floats. (b) Relative Density refers to the comparison of density between a substance and the density of fresh water. It can be expressed in the following ratio: Density of Substance RD = Density of FreshWater Density of salt water in kg per m3 2. R.D. = 1000 1025 = 1000 ∴ Relative Density of salt water = 1.025 3. Density in per m3 = 1000 × S.G. = 1000 × 0.92 ∴ Density = 920 kg/m3 Mass of Oil 4. Relative Density = Mass of Fresh Water(F.W.) ∴ Mass of oil = Mass of F.W.× Relative Density = 12 × 0.84 tonnes 5. (a) The centre of buoyancy is the centre of the underwater section. 1.11 SECTION 1 – BASIC STABILITY 5. (b) G = Centre of Gravity found by dividing the block (corner to corner) B = Centre of buoyancy found by dividing the under section of the block NOTE: G and B are vertically in line 6. Volumeof vessel = length × breadth × depth = 10.5× 3 × 2 = 63 m3 Underwater volume = draft × length × breadth 19.5 m3 = d ×10.5× 3 19.5 d= 10.5 × 3 ∴ draft = 0.6 m Reserve buoyancy is volume of enclosed space above water = 63 – 19.5 = 43.5 m3 7. In case of fresh water: Mass = volume × density = 12 cubic metres ×1 (relative density of fresh water is1) = 12 tonnes In case of oil: Mass = volume × density = 12 m 3 × 0.84 = 120.08 tonnes 1.12  STABILITY A − MASTER 5 LEVEL − LEARNER’S GUIDE TOPIC 2 LONGITUDINAL AND TRANSVERSE STABILITY Syllabus Learning Outcome On completion of this learning outcome the learner will be able to apply stability principles to the safe operation of a small vessel. Assessment Criteria Differentiate between transverse and longitudinal stability and explain the causes of list and trim Describe the movement of a vessel’s centre of gravity and metacentric height when weights are loaded to, discharged from or shifted onboard a vessel Describe the conditions of stable, neutral and unstable equilibrium and explain the significance when a vessel is disturbed from the upright List the steps to be taken to bring an unstable vessel to a stable condition Describe the factors which will affect the rolling period of a vessel Describe the information contained in simplified stability data supplied to small vessels and explain how it is used to maintain the vessel in a stable condition during operations Text National Fishing Industry Training Committee: An Introduction to Fishing Vessel Stability (included in the Coxswain’s section of this learning resource)  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE SECTION 2A LIST AND TRIM In this Section we will be looking at List and Trim in greater detail than is given in your text. Objectives By the end of this Section you should be able to: explain what is meant by the terms Longitudinal and Transverse, and relate these to the centres of gravity and centres of buoyancy in ship-shaped vessels predict the direction of the change in position of the centre of buoyancy with a change in draft explain the association between list and movement of the centre of gravity and centre of buoyancy, off the centre line of a vessel explain the association between trim and the movement of the longitudinal centres of gravity and buoyancy with respect to each other. Unit 2A.1 Longitudinal and Transverse Centres of Gravity and Centres of Buoyancy We frequently refer to the Longitudinal Centre of Gravity (LCG) or the Longitudinal Centre of Buoyancy (LCB). The dictionary defines Longitudinal as “pertaining to longitude or length”. We use LCG and LCB when studying Trim. Figure 2A.1 shows a longitudinal profile of a vessel. The position of LCB and LCG are shown relative to the length of the vessel: FP is the Forward Perpendicular. AP is the After Perpendicular. It must be remembered that LCB and LCG are in exactly the same position as the transverse centre of buoyancy and the transverse centre of gravity of the vessel. The dictionary defines Transverse as “lying or being across, or in a crosswise direction; athwart”. When considering the list of a vessel we frequently study the locations of the Transverse Centre of Gravity (TCG) and the Transverse Centre of Buoyancy (TCB) (see Figure 2A.2). In both cases, transverse and longitudinal, it is usual to simply label the positions of the centres of gravity and buoyancy as G and B respectively. 2A.1  SECTION 2A – LIST AND TRIM 2A.2 Fig. 2A.1 Longitudinal centre of buoyancy and longitudinal centre of gravity  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 2A.2 Transverse centre of gravity and transverse centre of buoyancy Unit 2A.2 Changes of B with Draft In order to be able to predict the change in B with draft, it is necessary to have an appreciation of the underwater shape of the vessel. In nearly all cases, the bow of a vessel is finer than the stern, and the underwater volume of the vessel is greater aft of midships than forward. Figure 2A.3 shows a lines diagram of a small, round-bilge hull form vessel. The line plans show the contours of the vessel at regular stations along its length (body plan), its breadth (profile plan) and its depth (half-breadth plan). Studying the half-breadth plan, it is easy to see that, as the draft increases from Waterline 1 (WL 1) to Waterline 2 (WL 2) and on to the Load Waterline (LWL), the water plane area not only increases, but its geometric centre moves aft. It follows therefore, that the geometric centre of the underwater volume (Centre of Buoyancy) should also move aft with an increase in draft. This, in fact, is true for nearly all vessels. Figure 2A.4 shows the change in the position of the Longitudinal Centre of Buoyancy with a change in draft. As the draft increases from WL 1 to WL 2, the position of B moves from B1 to B2. 2A.3 SECTION 2A – LIST AND TRIM Fig. 2A.3 Fig. 2A.4 Movement of B with change in draft Transverse Centre of Buoyancy The TCB behaves in a similar manner, but instead of moving upwards and aft with an increase in draft (as does the LCB), the TCB only moves upwards and downwards while the vessel is upright. In Figure 2A.5, once again, as the draft changes from WL 1 to WL 2, the geometric centre (B) moves from B1 to B2 and vice versa. 2A.4  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 2A.5 Change of B with draft Unit 2A.3 List For a vessel to float upright, both the centre of gravity and the centre of buoyancy must be on the transverse vertical centre line (see Figure 2A.6). 2A.5 SECTION 2A – LIST AND TRIM Fig. 2A6 Upright freely floating vessel Imagine what would happen if G was not on the centre line (see Figure 2A.7). 2A.6  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 2A.7 Displacement of G required to produce a list The weight acts directly downwards, the force of buoyancy vertically upwards, and the vessel will experience a clockwise turning moment (the listing moment) which will cause it to roll over. 2A.7 SECTION 2A – LIST AND TRIM Fig. 2A.8 Resultant list As the vessel rolls, the low side of the vessel becomes submerged and the high side emerges from the water. This results in a change in the shape of the underwater volume. The entire volume of the emersed wedge shown in Figure 2A.8 is effectively relocated in the position of the immersed wedge between WL1 and WL2. Since B is still located at the geometric centre of the underwater section, it begins to move to the right. When B eventually reaches a position where it is directly underneath G, equilibrium is reached (the forces of B and G are equal and opposite) and the vessel will float at an angle of list, 0°. If the situation arose where B never reached a position directly underneath G, the vessel would capsize. The vertical line passing through B2 and G intersects the centre line at M. For small angles of heel this position is regarded as the metacentre. The height of the metacentre above G is used in determining the stability of a vessel. More about this will be discussed in Section 3. Unit 2A.4 Trim For a vessel to float at equilibrium on an even keel with no trim, the centres of buoyancy, flotation and gravity must be in the same vertical line (see Figure 2A.9). 2A.8  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 2A9 Vessel floating on even keel Trim may be regarded as a type of longitudinal list. Consider Figure 2A.10. When the vessel is on even keel, G and B are not in the same vertical line. The vessel will experience an anti-clockwise turning moment (trimming moment) causing a negative trim. Fig. 2A.10 Displacement of G to produce a trim 2A.9 SECTION 2A – LIST AND TRIM As a result, the stern will begin to sink and the bow to rise. B will begin to move aft as the underwater volume increases aft and decreases forward. When B reaches a position where it is directly below G, equilibrium has been reached and the vessel will float with an angle of trim, 0°. We measure trim, however, by the difference between the fore and after drafts: TRIM = FORE DRAFT – AFTER DRAFT If the answer is negative (−, or minus) the trim is by the stern; if positive (+, or plus) the trim is by the head. The vertical line passing through B2 and G intersects the old vertical line passing through B1 at ML. This is the longitudinal metacentre. The height of ML above G is commonly referred to as GML (See Figure 2A.11). Fig. 2A.11 Resultant trim 2A.10  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Unit 2A.5 Self-test Questions The answers to these questions are to be found at the end of this Section. 1. Explain what happens to the position of the centre of gravity when block A is joined to block B to form one unit. They are rectangular blocks with identical cross-sectional dimensions. 2. A vessel is floating upright and on an even keel. A weight is loaded on the after deck. Show with a sketch the positions of the old and new centres of gravity, flotation, buoyancy, and the old and new waterlines. Show also a possible position of ML and the angle of trim 0. 3. In Question 2, is this a positive or a negative trim? 4. Is this a desirable state of trim? 2A.11 SECTION 2A – LIST AND TRIM 5. Using the sketch provided, show what would happen to the vessel if a weight was loaded on the deck in the position indicated. Show the positions of: (a) G1 (b) B1 (c) angle of list The weight is not heavy enough to capsize the vessel. Also indicate the new waterline. 6. A vessel is floating on an even keel with the positions of B, G and F in the positions indicated. A weight is loaded on the fore deck as shown. Indicate on the sketch: (a) (i) The new positions of B, G and the new waterline (ii) The angle of trim (b) Is this trim positive or negative? (c) Is this desirable? Explain why. 2A.12  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE 2A.13 SECTION 2A – LIST AND TRIM 2A.14  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE ANSWERS TO SELF-TEST QUESTIONS Unit 2A.5 1. GA is located 1.5 m from the ends of block A. GB is located 4.5 m from the ends of block B. The final position of G is located at G1, 6 m from either end. Conclusion: When a weight is added to a body, the position of G will shift to G1. The line GG1 is along the line connecting the two centres of gravity – the original body and the added weight. 2A.15 SECTION 2A – LIST AND TRIM 2. 3. The vessel has a negative trim; ie, trim by the stern. 4. Yes. It is generally more desirable to be trimmed by the stern than by the head. Your rudder and propeller are more effective. 2A.16  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE 5. 2A.17 SECTION 2A – LIST AND TRIM 6. (a) (b) Trim is positive. (c) Not a good trim. Vessel likely to take water over bow. More tendency to broach in a seaway. Rudder and propeller less effective. 2A.18  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE SECTION 2B STABILITY IN SEAWAY We have so far discussed vessels in calm waters. Stability as related to wooden blocks has also been considered. While the wooden block analogy has its uses, very few of us will ever put to sea on a wooden block. The time has come to leave behind the wooden blocks and dead calm seas and consider stability in a more realistic sense. In this Section, we will be discussing the factors affecting a vessel’s stability in a seaway, as well as the way she would behave. Objectives By the end of this Section, you should be able to: define, with respect to ship-shape vessels, the following terms: (a) metacentre (b) metacentric height (c) righting lever define stable, unstable and neutral equilibrium explain with the aid of diagrams the significance of three conditions of equilibrium when a vessel is disturbed from rest explain with the aid of sketches the relationship between metacentric height and righting lever explain the terms ‘range of stability’, ‘flooding angle’ and ‘loll’. Unit 2B.1 Righting Lever (GZ) It is now necessary to consider the actual forces that work together to give a vessel the ability to return to the upright position when forcibly inclined by an external force. We are already familiar with the terms G (Centre of Gravity) and B (Centre of Buoyancy); it does not matter if G is above or below B. The two important facts are that G is fixed (for a particular condition of loading) and, providing weights cannot move, will remain fixed even when the vessel is forcibly inclined. B, however, changes position every time the underwater volume of the vessel changes. Let us now consider this phenomenon. Figure 2B.1 shows a vessel in the upright position; Figure 2B.2 shows the same vessel when forcibly inclined by an external force: Because the underwater volume has changed, B moves to B1 (the new Centre of Buoyancy). If we now further study Figure 2B.2, we should be able to see that G will continue to act vertically downwards, and B1 will continue to act vertically upwards. However, they no longer act in an equal and opposite direction (Figure 2B.3). 2B.1 SECTION 2B – STABILITY IN SEAWAY Fig. 2B.1 Fig. 2B.2 In effect, a righting couple has been formed. If we construct a line from G at right angles to the force of G, it will meet the upwards force of B1 at a point which is called Z. In effect, GZ is the righting lever between G and B1. If we draw in the upwards force of buoyancy from B1 at some point, it will cut the centre line of the vessel. This point is known as M (the Metacentre). For small angles of heel (up to 15°) M can be considered to be a fixed point. Fig. 2B.3 Thus the forces of G and B combine together through the righting level (GZ) to produce a righting moment which will return the vessel to the upright position and the forces of G and B once again cancel each other out. 2B.2  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE The terms GZ, GM and KM should be committed to memory as they constitute the basics of ship stability. Unit 2B.2 Equilibrium From the foregoing unit, it is possible to now define ‘upright’. In general terms, it can be stated that for a vessel to be in an upright condition the points G, B and M must be in the same vertical line, but does this necessarily mean that the vessel is stable? In other words, has the vessel the ability to return to the upright after being forcibly inclined? In reality, there are three possible conditions that a vessel can be loaded to: Stable Equilibrium Neutral Equilibrium Unstable Equilibrium (a) Stable Equilibrium The term stable equilibrium refers to a vessel when forcibly inclined, having the ability to return to her original position (see Figure 2B.4). Fig. 2B.4 If the vessel in Figure 2B.4 is now forcibly inclined, B will move to B1 and the righting moment will return the vessel to its original position (see Figure 2B.5). Fig. 2B.5 2B.3 SECTION 2B – STABILITY IN SEAWAY This will apply whether B is above or below G and also if the vessel is listed already. (b) Neutral Equilibrium The term neutral equilibrium refers to a vessel, when forcibly inclined, tending to remain at that angle of heel until another external force is applied. For this to occur the position of G must be the same as M; that is, zero GM. Figure 2B.6 should clarify this point for you. Fig. 2B.6 (c) Unstable Equilibrium The term unstable equilibrium refers to a vessel, when forcibly inclined, being inclined to heel over further; that is, the righting moment wants to capsize the vessel altogether. For this to occur, G must lie above M (see Figure 2B.7). Fig. 2B.7 (Figure 2B7 should be compared to Figure 2B.5.) Thus, for a vessel to have unstable equilibrium she must have what is called ‘negative GM’. Fortunately in this instance as will be seen in a later unit, the vessel will not continue to capsize although unstable equilibrium and the resultant state are extremely dangerous. 2B.4  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Unit 2B.3 Metacentric Height (GM) It should now be apparent that there is a direct relationship between the position of G, M and GM or the metacentric height. The larger the GM, the larger the righting lever GZ will be for any given angle of heel. The larger the GZ the larger the righting moment. Thus a vessel with a large GM, when forcibly inclined, will return to its original position quicker than a vessel with a small GM. The motion of a vessel with a large GM can be violent, after being forcibly inclined, as she returns to her original position. Such a vessel is said to be “STIFF”. A vessel with a small GM will have a much smoother, slower motion and will be very comfortable. Although comfortable this condition is not desirable and it is very close to neutral equilibrium. A vessel in this condition is said to be “TENDER”. Neither condition is desirable and a compromise between the two must be reached. Unit 2B.4 It was stated earlier that the position of M was only considered to be fixed for small angles of heel (up to 15°). It was also stated that, for a vessel with a negative GM, the vessel would not necessarily capsize. It is because of the fact that M is not fixed at larger angles of heel that an unstable vessel will not necessarily capsize. If we consider the underwater volume of a vessel as she is inclined further, so B will continue to move towards the low side. At some angle of heel B will again lie below G (see Figure 2B.8). Fig. 2B.8 As the vessel is forcibly inclined, so B will move to B1 and then B2. When it is directly below G, the vessel is once again in neutral equilibrium. The angle of heel at which she regains stability is known as the “ANGLE OF LOLL”. Any further inclination by external forces will once again produce a righting moment (Figure 2B.9). 2B.5 SECTION 2B – STABILITY IN SEAWAY Fig. 2B.9 This righting moment will cause the vessel to return to the point at which she became stable (the angle of loll). In effect she will now oscillate around the angle of loll. It should be emphasised that, if the centre of buoyancy cannot move out far enough to get vertically under G, then the vessel must and will capsize. A further source of danger is that of the external forces being strong enough to rotate the vessel back through the upright position. Should this happen, the vessel will flop heavily onto the other side and in all probability capsize, as any righting moments produced will not be strong enough to overcome this movement. From the above it should be obvious that not only is an angle of loll undesirable but it is also very dangerous. There are only two reasons why your vessel would suddenly heel over while at sea, and then remain heeled over. The first is a list caused by shifting cargo or weights. You should be able to determine quickly if that is the case. The other is loll caused by a negative GM. If this is the case you must waste no time in taking steps to lower your centre of gravity. If you have slack water in your ballast tanks, pump them up, filling the tanks on the low side first. The last thing you want is your vessel to flop over to the other side - which will happen, if you fill the high side’s tanks - with the possibility of disastrous consequences. If you don’t have slack tanks, or you have no ballast tanks at all, you must re-stow your deck cargo as low down as possible. Have all your crew sit as low as possible in the boat. If there is no way you can restow your deck cargo so that it is lower in the boat, then you have no alternative but to jettison it. This may be a painful decision to a fisherman whose gear on deck is causing the stability problem, but the gear is no use to him when he’s dead and it cannot be used when his boat is upside down! Factors affecting the rolling period There are two main factors affecting the movement of a vessel in a seaway: the sea state and the vessel’s roll period. A vessel disturbed in still water will have a natural roll period. Whilst the size of the roll will decrease the period will stay the same. This is the natural roll period of the vessel, like the period of a pendulum. 2B.6  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE The roll period depends on the beam of the vessel, height of G and can be modified by moving weights outboard. In practice, only the centre of gravity is adjustable by the master through the use of tanks, positioning of cargo and the movement of passengers. Once in a seaway, the situation becomes complex as there is a natural period for the sea waves and the swell waves which are also interacting with each other. As the vessel moves through the sea there is now an apparent wave period, similar to the true/relative (apparent) wind. When the apparent wave period becomes similar to the vessel’s own natural period then synchronous rolling takes place where the movement of the sea reinforces the rolling of the vessel. This can cause the vessel to go to large angles and may capsize. The roll period is also an indicator of a vessel’s stability. The metacentric height can be found approximately by the formulae: GM = (0.8 × Beam/ Roll 2 period). The roll period is the time from upright to upright going in the same direction. It should be done over several rolls and averaged. This formulae depends on the.8 which would vary from.88 for a small cargo vessel in ballast to.75 for a fully laden cargo vessel. Unit 2B.5 Turn to page 15 of your text and STUDY the segment on stability curves. A lot of information can be found from a GZ curve. Let’s construct a GZ curve for a vessel with the following righting levers: Angle of Heel 15 30 45 60 75 90 Righting Lever (GZ) 0.35 m 0.95 m 1.16 m 0.90 m 0.10 m −1.0 m Initial GM was 0.8 m Figure 2B.10 shows the curve. The following information can be gained from the GZ curve: 1. The range of stability. This includes all the angles of heel for which the vessel has a positive GZ. In this case it is from 0 − 77½° 2. The angle of vanishing stability. This is the maximum angle of heel at which GZ becomes zero. In this case it is also 77°, but you should note that the angle of vanishing stability and the range of stability are not always the same. 3. The maximum GZ lever and the angle at which it occurs. In this case it is 1.16 m at 45°. 4. The point at which the deck edge immerses. This is known as the point of contraflexure; i.e., where the shape of the curve changes from concave to convex. In this case it is approximately 23°. 5. The approximate GM. The tangent drawn to the slope of the curve at its origin, extended to intersect the vertical line through 57.3° (1 Radian), will give the GM. In this case it is 0.8 m. Usually the initial GM is known and the tangent is drawn in to find the original slope of the curve. 2B.7 SECTION 2B – STABILITY IN SEAWAY Fig. 2B.10 Stable equilibrium 2B.8  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE There is one other important angle to be considered, and this is the angle of flooding. Angle of flooding occurs when the vessel heels to a point when water enters the hull through openings that cannot be made watertight. Usually when this happens the vessel sinks. The angle of flooding should not be confused with the potential angle of flooding. The potential angle of flooding is the angle to which the vessel must be heeled to submerge the external watertight doors and openings. The potential for flooding would exist at this angle if the watertight doors and openings were not sealed. Unit 2B.6 Self-test Questions 1. What does the term Loll describe? 2. There are two main reasons why a vessel will suddenly list over while at sea. What are they? Give a brief description of any remedial action necessary. 3. What is the main precaution that should be taken into account when correcting an angle of loll by the filling of tanks? 4. What is the difference between a stiff and a tender ship? Use diagrams to explain your answer. 5. Sketch and describe stable, unstable and neutral equilibrium. 6. What information can be obtained from a GZ curve? 7. Figure 2B.11 shows a fishing trawler in the process of swinging a laden net on board. Where does the centre of gravity of the suspended weight act and what effect does this have on the centre of gravity of the vessel? Fig. 2B.11 2B.9 SECTION 2B – STABILITY IN SEAWAY 2B.10  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE ANSWERS TO SELF-TEST QUESTIONS UNIT 2B.6 1. Loll The term loll describes the state of a ship which is unstable when in an upright position and therefore floats at an angle of heel to one side or the other. If disturbed by some external force caused by wind or waves, the vessel will lurch to the same angle of loll on the opposite side. Loll is quite different from list, being caused by different circumstances and requiring different counter-measures to correct it, and it is therefore most important that the seaman should be able to distinguish between the two. 2. The two reasons are: (a) Shifting of cargo or weights. (b) Loll caused by a negative GM. A may be rectified by distributing the weights more evenly. B may be rectified by lowering the centre of gravity. 3. Where possible the tank on the low side should be filled first to prevent the vessel flopping heavily onto the other side as it will probably capsize the vessel. 4. ‘STIFF’ and ‘TENDER’ Ships When a weight is added to a vessel, the centre of gravity of the vessel always moves in the direction of the added weight. Weight added at deck level results in the vessel’s centre of gravity rising, causing a decrease in the vessel’s metacentric height. A vessel with little or no metacentric height is said to be TENDER. 2B.11 SECTION 2B – STABILITY IN SEAWAY Weight added low down in the vessel lowers the centre of gravity and consequently causes an increase in the vessel’s metacentric height. A vessel with a large metacentric height is said to be a STIFF ship. A stiff ship tends to be comparatively difficult to heel and will roll from side to side very quickly and perhaps violently. If this condition is thought to be a problem it can be corrected by raising the vessel’s centre of gravity. A tender ship will be much easier to incline and will not tend to return quickly to the upright position. The time period taken to roll from side to side will be comparatively long. This condition is not desirable and can be corrected by lowering the centre of gravity. 5. (a) Stable Equilibrium The term stable equilibrium refers to a vessel, when forcibly inclined, having the ability to return to her original position. (b) Neutral Equilibrium The term neutral equilibrium refers to a vessel, when forcibly inclined, tending to remain at that angle of heel until another external force is applied. 2B.12  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE (c) Unstable Equilibrium The term unstable equilibrium refers to a vessel, when forcibly inclined, being inclined to heel over further; that is, the righting moment wants to capsize the vessel altogether. For this to occur, G must lie above M. 6. A GZ curve gives the following information: (a) the range of stability (b) the angle of vanishing stability (c) the maximum GZ lever and the angle at which it occurs (d) the point at which the deck edge immerses. 2B.13 SECTION 2B – STABILITY IN SEAWAY 7. The centre of gravity of a suspended weight can be considered to be at the point of suspension. Therefore, a trawl cod end, when being lifted clear of the water, has the same effect on the vessel’s centre of gravity as if the weight were actually the head of the boom. It also exerts a heeling force upon the vessel. G moves along the line connecting G and the point of suspension. The new G becomes G1 and θ is the angle of heel. B moves to B1. 2B.14  STABILITY A − MASTER 5 LEVEL − LEARNER’S GUIDE TOPIC 3 FACTORS AFFECTING STABILITY Syllabus Learning Outcome On completion of this learning outcome the learner will be able to recognise factors that have an adverse effect on the stability of a small vessel and describe appropriate action to ensure the safe operation of the vessel. Assessment Criteria Describe the effect of suspended weights on the stability of a vessel when using cargo gear or fishing gear to load and discharge heavy weights Describe the precautions to be taken on board a fishing vessel when clearing a net which is caught fast on an underwater obstruction Describe the causes of free surfaces on the stability of a vessel Describe onboard arrangements inlcuding safe working practices to reduce free surface effects Describe the effect of water on deck on the stability of a small vessel and the means of reducing that effect Differentiate between list and loll and describe the actions to be taken to correct an angle of loll Explain the term ‘Resrve Buoyancy’ Describe the effect on the stability of a vessel that has been ‘bilged’ List the actions to be taken to contain flooding in the event of underwater damage to the hull Explain the precautions required when making alterations to a vessel that may affect stability Text National Fishing Industry Training Committee: An Introduction to Fishing Vessel Stability (included in the Coxswain’s section of this learning resource) WestOne Publication: Simplified Stability Information for M.V. Twosuch (included as an appendix in this learning resource)  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE SECTION 3 FACTORS AFFECTING STABILITY These are the last two Sections that you will study on stability. Thus far we have discussed the principles of ship stability. You should be acquainted with the terms relating to ship stability and have a general idea of the changes that occur when a vessel is rolling or listing. In Section 3, we will consider the various aspects of stability that are under your control and for which you, as a Master, are directly responsible. Objectives By the end of this Section you should be able to: show how the loading, discharging and moving of weights on board a vessel affects its stability show how the free surface effect of liquids or fish affect the stability of the vessel explain what effect bilging has on trim and reserve buoyancy show how sudden and constant loads on gear affect a vessel’s stability show how structural changes to a vessel may be regarded as weights added or removed, and explain the need to develop new stability data for a vessel after structural changes. Unit 3.1 Loading, Discharging and Shifting Weights STUDY pages 11, 12, 18 and 19 in your text book for this Section, An Introduction to Fishing Vessel Stability, then STUDY the following notes. The Master has total control over the position of the centre of gravity of his vessel, and so has total control over the stability of his vessel. A centre of gravity too far forward will result in a positive trim (trim by the head). This would result in difficulty in manoeuvring and reduce the vessel’s ability to ride over head seas. There is also a reduction of her reserve buoyancy forward and seas coming on deck may overcome her. She would also be very sluggish and slow to respond to helm. A centre of gravity too far aft will result in excessive trim (negative) by the stern. Such a vessel will heel over excessively when helm is applied, and will be over responsive to her helm. In a large following sea she will be almost impossible to control and will have a tendency to broach. When the centre of gravity is off centre, the vessel will have a list. With a centre of gravity too high – she is tender; too low – she is stiff. With careful consideration to the stowage of cargoes and the amount of ballast on board, all of the above problems can easily be avoided. 3.1 SECTION 3 – FACTORS AFFECTING STABILITY Adding Weights Whenever a weight is added to a ship, the centre of gravity moves in the direction of the added weight. When we refer to the position of G, we mean the position of G with respect to the ship plus its cargo (see Figures 3.1(a) and 3.1(b)). Fig. 3.1(a) 3.2  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 3.1(b) Once a weight has been loaded, we no longer consider the weight on its own, but rather the effect that it has had on the centre of gravity of the vessel as a whole. When a weight is removed, the centre of gravity of the vessel moves directly away from the removed weight (see Figures 3.2(a) and 3.2(b)). When the weight is moved, the centre of gravity moves parallel to the line connecting the initial and final positions of the centre of gravity of the moved weight. (See Figures 3.3(a) and 3.3(b)) In Figures 3.1(a) and (b) a weight is loaded on the Port side on deck. This causes a list to Port. G moves to G1 and GM is reduced to G1M. 3.3 SECTION 3 – FACTORS AFFECTING STABILITY In Figures 3.2(a) and (b) a weight is discharged from the starboard side. This also causes a Port list. G moves to G1 and GM is increased to G1M. In Figures 3.3(a) and (b) a weight is moved from the after deck to the fore deck. This results in a change of trim and G moves in the same direction as the weight. Figures 3.4(a) and (b) show a weight being moved from the port side on deck to the starboard side below deck. This causes a starboard list, even though GM is increased. Fig. 3.2(a) Before discharging a weight 3.4  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 3.2(b) After discharging a weight 3.5 SECTION 3 – FACTORS AFFECTING STABILITY Fig. 3.3(a) Before shifting weight Fig. 3.3(b) After shifting weight 3.6  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Fig. 3.4(a) Before weight is removed 3.7 SECTION 3 – FACTORS AFFECTING STABILITY Fig. 3.4(b) After weight is removed A final word about loading weights and cargoes on deck should be made. Apart from reducing GM, there is the added danger of deck cargoes trapping water that comes on deck. If the water does not drain away, the added weight high up could be very serious. Deck cargoes of pipes are especially liable to cause water entrapment. So remember, when carrying deck cargoes that could trap water, make sure that you have a large GM. It is not easy to estimate the effect that a wave on deck would have on your vessel’s stability, but you can be sure that it is always detrimental. Unit 3.2 Free Surface Effect STUDY pages 13 and 14 of your text under the heading “Free Surface Effect”. Any cargo that is free to move has a ‘free surface’ and will produce the same effect on the stability of the vessel as liquids in a partially filled tank. This includes fish on deck or in a hold. They move around quite easily with the rolling of the vessel. Once they are frozen solid, the problem doesn’t exist, but until then and unless the fish hold is longitudinally subdivided, a dangerous situation could exist. 3.8  STABILITY A – MASTER 5 LEVEL – LEARNER’S GUIDE Let’s examine what happens to a vessel that is rolling and has a large transverse tank, partially filled. Figures 3.5(a), 3.5(b) and 3.5(c) show two vessels being heeled, one with a filled tank and one with a half filled tank.