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merchant ship stability PDF

212 Pages·2008·4.9 MB·English
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MERCHANT SHIP STABILITY (METRIC EDITION) A Companion to "Merchant Ship Construction" BY H. J. PURSEY EXTRA MASTER Formerly Lecturer to the School of Navigation University of Southampton GLASGOW BROWN, SON & FERGUSON, LTD., NAUTICALPUBLISHERS 4-10 DARNLEY STREET Copyright in all countries signatory to the Berne Convention All rights reserved First Edition 1945 Sixth Edition - 1977 Revised 1983 Reprinted - 1992 Reprinted 1996 ISBN 085174 442 7 (Revised Sixth Edition) ISBN 085174 274 2 (Sixth Edition) ©1996-BROWN, SON & FERGUSON, LTD., GLASGOW, G41 2SD Printed and Made in Great Britain INTRODUCTION D URING the past few years there have been considerable changes in the approach to ship stability, so far as it affects the merchant seaman. The most obvious of these is the introduction of metric units. In addition, the Department of Trade have already increased their examination requirements: they have also produced recommendations for a standard method of presenting and using stability information, which will undoubtedly be reflected in the various examinations. This revised edition has been designed to meet the above-mentioned requirements. The basic information contained in the early chapters has been retained for the benefit of those who are not familiar with such matters. The remainder of the text has been re-arranged and expanded, as desirable, to lead into the new material which has been introduced; whilst a new chapter on stability information has been added to illustrate the Department of Trade recommendations. The theory of stability has been covered up to the standard required for a Master's Certificate and includes all that is needed by students for Ordinary National Diplomas and similar courses. This has been carefully linked-up with practice, since the connection between the two is a common stumbling block. Particular attention has been paid to matters which are commonly misunderstood, or not fully appreciated by seamen. H. J. P. SOUTHAMPTON, 1982. V CONTENTS CHAPTER I-SOME GENERAL INFORMATION PAGE The Metric System .. .. . . .. .. ·. .. .. 1 Increase of pressure with depth ·. .. ·. ·. .. 2 Effect of water in sounding pipes .. .. ·. 2 The Law of Archimedes .. ·. . . .. 3 Floating bodies and the density of water .. ·. 4 Ship dimensions ·. 4 Decks .. .. · . 4 Ship tonnages 4 Grain and bale measurement 5 Displacement and deadweight 5 Draft .. ·. ·. 6 Freeboard 6 Loadlines 6 CHAPTER 2-AREAS AND VOLUMES Areas of plane figures 8 Surface areas and volumes 8 Areas of waterplanes and other ship sections 9 Simpson's First Rule · . 10 Simpson's Second Rule 12 The 'Five-Eight Rule' 12 Sharp-ended waterplanes 13 Unsuitable numbers of ordinates 13 Volumes of ship shapes 15 Half-intervals 16 Coefficients of fineness 17 Wetted surface 18 CHAPTER 3-FORCES AND MOMENTS Forces 19 Moments 20 Centre of gravity 23 Effect of weights on centre of gravity ·. 25 Use of moments to find centre of gravity 27 To find the centre of gravity of a waterplane 28 To find the centre of buoyancy of a ship shape 30 The use of intermediate ordinates 31 Appendages .. 32 Inertia and moment of inertia 33 Equilibrium .. 36 CHAPTER 4-DENSITY, DEADWEIGHT AND DRAFT Effect of density on draft .. ·. 37 Tonnes per centimetre immersion .. .. ·. 39 Loading to a given loadline .. .. 40 Vll VIl1 CONTENTS CHAPTER 5-CENTRE OF GRAVITY OF SHIPS PAGE Centre of Gravity of a ship-G .. ·. ·. ·. ·. .. .. 42 KG .. ·. ·. ·. ·. ·. ·. ·. ·. 42 Shift of G ·. .. ·. ·. .. .. · . 42 KG for any condition of loading .. ·. ·. .. · . 43 Deadweight moment .. ·. ·. · . 45 Real and virtual centres of gravity · . 46 Effect of tanks on G .. .. · . ·. ·. ·. ·. 47 CHAPTER 6-CENTRES OF BUOYANCY AND FLOTATION Centre of buoyancy-B .. .. ·. ·. ·. · . ·. ·. . . 49 Centre of flotation-F 49 Shift of B ·. · . ·. ·. ·. .. ·. · . 50 CHAPTER 7-THE RIGHTING LEVER AND METACENTRE Equilibrium of ships .. ·. ·. · . ·. .. .. .. ·. 53 The righting lever-GZ .. ·. · . ·. 55 The metacentre-M ·. ·. .. .. ·. ·. ·. ·. · . 55 Metacentric height-GM ·. ·. ·. ·. .. ·. ·. .. 55 Stable, unstable and neutral equilibrium .. .. 55 Longitudinal metacentric height-GML .. ·. 56 CHAPTER 8-TRANSVERSE STATICAL STABILITY Moment of statical stability ·. .. 57 Relation between GM and GZ ·. · . 57 Initial stability and range of stability ·. 57 Calculation of a ship's stability ·. 58 Calculation of BM .. .. ·. 58 The Inclining Experiment .. ·. ·. ·. 60 Statical stability at small angles of heel 62 Statical stability at any angle of heel 62 GZ by the Wall-Sided Formula ·. 64 Loll, or list .. ·. ·. .. ·. 64 Heel due to G being out of the centre-line ·. 65 Loll due to a negative GM .. ·. ·. 68 CHAPTER 9-FREE SURFACE EFFECT The effect of free surface of liquids ·. .. ·. 70 Free surface effect when tanks are filled or emptied 72 Free surface in divided tanks ·. 73 Free surface moments ·. ·. ·. .. .. ·. 75 CHAPTER 10-TRANSVERSE STATICAL STABILITY IN PRACTICE Factors affecting statical stability .. · . 76 Placing of weights · . 78 Stiff and tender ships 78 Unstable ships ·. 80 Ships in ballast .. .. 81 The effect of winging out weights .. 82 Deck cargoes .. .. 83 Free liquid in tanks .. ·. 84 Free surface effect in oil tankers 85 CONTENTS IX. CHAPTER 11-DYNAMICAL STABILITY PAGE Dynamical stability .. .. .. .. .. 86 Dynamical stability from a curve of statical stability 86 Calculation of dynamical stability 88 CHAPTER 12-LONGITUDINAL STABILITY Longitudinal metacentric height-GML ·. 90 Calculation of EM L 91 Trim .. .. 92 Change of mean draft due to change of trim 94- Displacement out of designed trim ·. 96 Moment to change trim by one centimetre 98 The effect of shifting a weight 99 Effect of adding weight at the centre of flotation 101 Moderate weights loaded off the centre of flotation 103 Large weights loaded off the centre of flotation 106 To obtain special trim or draft 108 Use of moments about the after perpendicular .. 113 CHAPTER 13-ST ABILITY CURVES AND SCALES Hydrostatic curves 117 The deadweight scale 118 Hydrostatic particulars 118 Curves of statical stability 119 Cross curves .. 120 Effect of height of G 122 KN curves .. 123 The Metacentric Diagram 123 CHAPTER 14-BILGING OF COMPARTMENTS The effect of bilging a compartment .. 126 Permeability .. .. · . 126 Bilging an empty compartment amidships 127 Bilging an amidships compartment, with cargo 128 Bilging an empty compartment, not amidships 129 Effect of a watertight flat 131 CHAPTER IS-STABILITY AND THE LOAD LINE RULES Stability requirements .. ·. 133 Information to be supplied to ships ·. 134- The Stability Information Booklet 134 The use of maximum deadweight moments .. 139 Simplified stability information 140 CHAPTER 16-MISCELLANEOUS MATTERS Drydocking and grounding 143 The effect of density on stability ·. 145 The effect of density on draft of ships 146 Derivation of the fresh-water allowance 147 Reserve buoyancy .. 147 Longitudinal bulkheads 147 Bulkhead subdivision and sheer 148 Pressure on bulkheads .. 149 x CONTENTS CHAPTER 17-ROLLING PAG£ The formation of waves 150 The Trochoidal Theory 150 The period of waves 150 The period of a ship 151 Synchronism .. 151 Unresisted rolling 152 Resistances to rolling 152 The effects of bi1ge keels 153 Cures for heavy rolling 153 CHAPTER 18-SUMMARY Abbreviations 154 Formu!ae 156 Definitions 161 Prob]ems 164 DEADWEIGHT SCALE, HYDROSTATIC PARTICULARS AND HYDROSTATIC CURVES Insert at end of book Ship Dimensions.-The following are the principal dimensions used in measuring ships. LIoyds' Length is the length of the ship, measured from the fore side of the stem to the after side of the stern post at the summer load-line. In ships with cruiser sterns, it is taken as 96 per cent of the length overall provided that this is not less than the above. Moulded Breadth is the greatest breadth of the ship, measured from side to side outside the frames, but inside the shell plating. Moulded Depth is measured vertically at the middle length of the ship, from the top of the keel to the top of the beams at the side of the uppermost continuous deck. The Framing Depth is measured vertically from the top of the double bottom to the top of the beams at the side of the lowest deck. Depth of Hold is measured at the centre line, from the top of the beams at the tonnage deck to the top of the double bottom or ceiling. Decks.-The Freeboard Deck is the uppermost complete deck, having permanent means of closing all openings in its weather portion. The Tonnage Deck is the upper deck in single-decked ships and the second deck in all others. Ship Tonnages.- These are not measures of weight, but of space: the word "ton" being used to indicate 100 cubic feet or 2·83 cubic metres. For instance, if the gross tonnage of a ship is 5000 tons this does not mean that she weighs that amount, but that certain spaces in her measure 500,000 cubic feet or 14150 cubic metres. SOME GENERAL INFORMATION 5 Under Deck Tonnage is the volume of the ship below the tonnage deck. It does not normally include the cellular double bottom below the inner bottom: or, in the case of open floors, the space between the outer bottom and the tops. of the floors. Gross Tonnage is under deck tonnage, plus spaces in the hull above the tonnage deck. It also includes permanently enclosed superstructures, with some exceptions, and any deck cargo that is on board. Nett Tonnage is found by deducting, from the gross tonnage, certain non-earning spaces. These "deductions" inc1ude crew accommodation, stores and certain water ballast spaces: also an "allowance for propelling power" which depends partly on the size of the machinery spaces. Under the 1967 Tonnage Rules, some ships may now have a Modified Tonnage. This means that they have a tonnage which is less than the normal tonnage for a ship of their size, but are not allowed to load so deeply. Other ships may have two Alternative Tonnages: normal tonnage for use when they are loaded to their normal loadlines; or a modified tonnage when they are loaded less deeply. Such ships are marked with a special "Tonnage Mark" to indicate which tonnage is to be used. Grain and Bale Measurement.- These terms are often found on the plans of ships and refer to the volume of the holds, etc. Grain Measurement is the space in a compartment taken right out to the ship's side. In other words, it is the amount of space which would be available for a bulk cargo such as grain. Bale Measurement is the space in a compartment measured to the inside of the spar ceiling, or, if this is not fitted, to the inside of the frames. It is the space which would be available for bales and similar cargoes. Displacement.-Is the actual weight of the ship and all aboard her at any particular time. Since a floating body displaces its own weight of water, this means that displacement is equal to the weight of water displaced by the ship. Light Displacement is that of the ship when she is at her designed light draft. It consists of the weight of the hull, machinery, spare parts and water in the boilers. Loaded Displacement is that of a ship when she is floating at her summer draft. Deadweight.-This is the weight of cargo, stores, bunkers, etc., on board a ship. In other words, it is the difference between the light displacement and the displacement at any particular draft. When We say that a ship is of so many tonnes deadweight, we usually mean that the difference between her light and loaded displacements is so many tonnes. {) MERCHANT SHIP STABILITY Draft.-This is the depth of the bottom of the ship's keel below the -surface of the water. It is measured forward and aft at the ends of the ship. When the drafts at each end are the same, the ship is said to be on an even keel. When they differ, the ship is said to be trimmed by the head, or by the -stem, according to which is the greater of the two drafts. Mean Draft is the mean of the drafts forward and aft. Freeboard.-Statutory Freeboard is the distance from the deck-line to the centre of the plimsoll mark. The term "Freeboard" is often taken to mean the distance from the deck-line to the water. Ordinary Load-lines.-The load-lines and deck line must be painted in white or yellow on a dark background, or in black on a light background. The deck-line is placed amidships and is 300 millimetres long and 25 milli- metres wide. Its upper edge marks the level at which the top of the freeboard deck, if continued outward, would cut the outside of the shell plating.

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