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Stability Guide for Smaller Vessels

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Stability Guide for Smaller Vessels STABILITY GUIDE FOR SMALLER VESSELS The EU’s Ministry of Food, Fisheries Fund Agriculture and Fisheries The project is funded by the Ministry of Food, Agriculture and Fisheries and the EU 2 All photos and texts in this publication are protected by Danish copyright law. All rights belong to or are managed by the Danish Fishermen’s Occupational Health Service. It is not allowed to copy or use texts and pictures from this publication without written permission. Preface 3 Over the years, a number of vessels in the category 0 - 15 metres have capsized or sunk. Several of these losses were due to vessel instability. A number of fishermen have lost their lives because of this. Failure in stability can be the result of repeated rebuilding of a vessel, where a number of small changes suddenly has a great impact on stability. It may also be due to changes in fishing patterns as well as fishing in other waters where external influences are different. So it is important that the captain has a good knowledge of the vessel’s stability and a thorough understanding of how it can change during fishing and in connection with rig- ging to another kind of fishery. In particular when rebuilding or making other changes to the vessel, it is extremely important to be aware of the impact this has on the stability of the vessel. This reference book illustrates the basic principles of stability and how to calculate stabil- ity. It is a guide to understanding and interpreting vessel stability calculations. Stability problems are highlighted in the form of case studies with examples of both improving and deteriorating changes in stability. The main intention is for this publication to be instrumental in reducing the death toll as- sociated with these accidents. We also expect that the material will help to reduce dangerous situations related to stabil- ity issues. It is important that both fishermen and service companies who work on vessels help to increase safety in the fishing sector and thereby prevent sinking, where lack of stability is a contributing factor. We hope this guide will give you a greater knowledge and understanding of what stability is and make you more aware of how important it is in your daily work We hope you enjoy reading this booklet! Flemming Christensen Danish Fishermen’s Occupational Health Service 4 Contents Chapter 1: What is stability? 5 Lightweight, deadweight and displacement 5 Buoyancy 6 Centre of gravity G 7 Centre of buoyancy B 7 The centre of buoyancy B moves when the vessel heels 8 Metacentric height GM 8 Righting arm GZ 9 GZ curve 10 Chapter 2: Determining a vessel’s stability 12 Line Drawing 12 Preparations for the heeling test 13 Carrying out the heeling test 13 Additional data collection under the heeling test 15 Calculation of the vessel’s lightweight and centre of gravity 16 Calculating the vessel stability 20 What are the rules? 21 What can the stability report be used for? 22 Dynamometer Test 26 What about the very small vessels with a scantling number under 20? 27 Chapter 3: Measures to improve stability 30 Measures to improve stability 30 Freeboard 31 Weight critical review of the vessel 32 Buoyancy Cases / side building on the side of the wheelhouse aft 32 Buoyancy Cases / side building on the side of the wheelhouse in front 33 Added buoyancy in the gunwale 33 Raised deck 33 Shelter deck 34 Closing the pond 35 Installation of ballast 35 Extension amidships prior to the installation of ballast 35 Rebuilding the stern 35 Net trunk 36 Tackle box 36 Roll damping tank and bilge keels do not improve stability 37 New heeling test 37 Chapter 4: What influences stability 38 Is the vessel watertight? 38 Are you sailing with free surfaces? 40 Heel 41 Leech trim 42 Hauling fish / lifting 43 Towing with gear 44 Dirty nets on deck 45 Icing 46 Heavy sea 47 Small changes over time 48 Routine check 49 Convergence of several factors 50 Terms and abbreviations 51 Chapter 1 5 What is stability? Stablilty ... ? Stability is a measure of the vessel’s ability Centre of gravity ... Freeboard ... to get back on an even keel after having suf- How was that again ??? fered a heel. Different factors affect a vessel’s stability. Basically it is the ratio between the centre of gravity and the distribution of a vessel’s buoyancy that determines the vessel’s abil- ity to get back on an even keel. Lightweight, deadweight and displacement The vessel’s own weight and the distribution of the weights on the vessel are essential for determining the vessel’s stability. The vessel’s weight is composed as follows: Lightweight This is the weight of the unrigged vessel without gear, fuel oil, water, ice, boxes, crew, provisions, catch, etc. Lightweight changes e.g. when the ves- sel is fitted with optional equipment, when switching engines, winches or other fixed components. Deadweight This is the term for all the weights the crew takes on board in order to fish or during fishing. Deadweight includes equipment, fuel oil, water, ice, boxes, crew, provisions, catch, etc. 6 Chapter 1 Displacement Displacement is the term for the vessel’s total weight. That means displacement = lightweight + deadweight. During fishing, the vessel’s displacement changes constantly as a result of oil con- sumption and the weight of the fish caught. A fishing vessel has a tonnage certificate that displays a certain tonnage. This tonnage has nothing to do with the vessel’s weight. The tonnage on the tonnage certificate is an expression of the vessel’s volume. The tonnage has therefore nothing to do with the vessel’s stability. Buoyancy A vessel floating on the water will displace an amount of water equal to the weight of the vessel (displacement). This is illustrated by the figure to the right: 1. A large container filled with water up to an overflow. 2. A vessel with a weight of 10 tonnes is lowered into the tank. 3. Exactly 10 tonnes of water flow over the edge. The experiment can be extended, for ex- ample by loading a vessel with 2 tonnes of fish, as illustrated in the figure to the left. The vessel is thus 2 tonnes heavier and pushed deeper into the water. An additional 2 tonnes of water flow out of the big container, so that now the small container alongside it contains 12 tonnes of water, equal to the combined weight of the vessel and the fish. What is stability? 7 Centre of gravity G The centre of gravity is a rather theoretical concept. It is composed of all weights on board, including the vessel’s own weight (lightweight). For example, if the total weight of the vessel (displace- ment), including deadweight such as gear, catch, etc. is 10 tonnes, all the small weights can be replaced by one total weight of 10 tonnes located in the centre of gravity. One can say that the centre of gravity is the average location of all the weights. • For most fishing vessels, the centre of gravity is usually just above the waterline. • Vessels may become unstable if the centre of grav- ity is positioned too high. • Fish and gear on deck pull the centre of gravity up. • Installation of new equipment on deck or in the wheelhouse pulls the centre of gravity up. • Replacement of a heavy diesel engine with a lighter engine pulls the centre of gravity up. • A high centre of gravity makes the vessel roll more slowly and can be a danger signal. Centre of buoyancy B All parts of the hull under the waterline contribute to the vessel’s overall buoyancy. The total buoyancy can, just like the centre of gravity mentioned above, be merged in one single point called the centre of buoyancy and this is indi- cated by the letter B. The centre of buoyancy B is the average location of the total buoyancy. B is not fixed; it changes all the time depending on the vessel’s draft, heel and trim. 8 Chapter 1 The centre of buoyancy B moves when the vessel heels When the vessel is upright, and not tilted, the centre of gravity G is in the vessel’s centre line. In a straight line below is the centre of buoyancy B, and the vessel is in balance. If the vessel is heeled, the buoyancy centre moves imme- diately off to the side of the vessel. See the adjacent illus- tration, where B is moved to one side and called B1. If the gear and catch are stowed away safely, there is no weight on board that can move during the roll. So the centre of gravity G remains in the same position. Metacentric height GM Under a light small heeling, the vertical line of buoyancy intersects with the vessel’s centre line at a point called meta centre, which is indicated by the letter M. The distance between the centre of gravity G and meta centre M is called the metacentric height GM. The GM value is a measure of the vessel’s stability under small heeling, also called initial stability. The higher the GM value, the better the vessel’s initial stability and the harder it is to get the vessel to heel. A vessel with a large GM value can be described as a rigid vessel that rolls fast on sea. What is stability? 9 Righting arm GZ When the vessel suffers a heel, the centre of gravity G and the centre of buoyancy B are no longer on the same vertical line above one another.
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