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This Installation Plays an Important Part in Minimizing a Ship's Chances of Disaster After Collision with an Iceberg or Another Vessel HAT a watertight deck con- | tributes to the safety of a ship is generally recognized. The purpose of this paper is to indicate to what extent, and under what circum- stances, additional safety may be ob- tained by this method of subdivision. It is necessary, therefore, to consider — some of the more serious types of damage met with at sea, and to apply to each a suitable criterion of’ safety. Here it ‘is proposed to consider first, the long ripping blow, such as may be occasioned by collision with ice, and second, the deep-cutting blow caused by the stem of another vessel. The first type of damage, being longitudinal, is naturally most effectively met by longi- tudinal subdivision, and it is when con- tact of this nature occurs with a sub- merged, or partially submerged object, that the greatest increase of safety re- sulting from the fitting of a watertight deck is to be looked for. For such cases the most satisfactory criterion of safety. appears to be the extent of damage, which under the least. favor- able circumstances may cause disaster. Serious Result From Slight Dafage The smallest amount of damage necessary to cause foundering is that just sufficient to cause the floodable length to be exceeded. Thus a_ ship subdivided only by transverse bulk- heads, and designed to float with any two compartments flooded, will sink if she chances to be ripped open under water from the fore side of one bulk- head to the after side of the first bulk- head abaft it.. In other: words, ‘her damaged length requires merely to ex- ceed, however slightly, one-half of her floodable length. The additional security against longi- tudinal damage, obtained by intricate transverse subdivision, is apparently by no means so great as the disparity in the number of floodable compartments would seem at first sight to suggest. Under a watertight deck floodable and damageable length may be considerably increased, the extent of such. increase depending mainly on the ratio which the space below the watertight’ deck bears to the space between it and the bulkhead deck. Under such a deck too, From a paper read at the spring meeting of the fifty-sixth session of the Institution of Naval Architects. damageable length increases by steps from the point of its minimum value. In contrasting the lengths of, damage permitted by the different systems of subdivision, it should be remembered that any reduction in the spacing of bulkheads, which does not actually add to the number of floodable compart- ments, must entail a corresponding re- duction in the ordinates of the dam- ageable length curves. The placing of a watertight deck at some distance be- low the waterline having recently been advocated, the writer has examined the effect produced on initial stability when flooding takes place above a watertight deck. It is assumed that flooding oc- curs about a point one-quarter of the ship’s length from the fore perpendicu- lar. This position was selected in preference to amidships, on account not only of the practical difficulty of get- ting a watertight deck in way of ma- chinery space, ‘but also because it is in connection with damage forward that there is most likelihood of the water- tight deck being brought into play. In obtaining these results the method of regarding the internal water as_ lost buoyancy was adopted, the ship’s dis- placement and position of center of gravity thus remaining unaltered. It appears that when flooding takes place over a watertight deck, the rise of the center of buoyancy is relatively slight, and is not sufficient to counter- balance the reduction in . waterplane moment of inertia, even though the ratio of initial draught to breadth is favorable to stability in the damaged condition. The result of flooding the same per- centages of the lengths of similar ves- sels having the same draught’ but greater breadth has been calculated for ships with breadths of 79 feet, 8&9 feet 6 inches, and 100 feet, respectively. This investigation demonstrates that although a submerged. watertight deck will probably limit the loss of buoyancy resulting from a given length of dam- age more effectively than one placed above the waterline, danger exists that stability may suffer, the loss -of. initial stability varying approximately as the length flooded. The risk on ‘this ac- count becomes greater as the ratio of breadth to draught increases. The chief danger of the ripping blow lies in the fact that its length cannot 134 MSS MS ) Y By K. G. Finlay be predicted; in addition, no assistance from outside, such as may reasonably be looked for by a vessel damaged in collision, is to be expected, conse- quently the greatest value of the water-- tight deck will be realized on occas— ions when the necessity is greatest. Provided that the designer’s assump- tions regarding the permeability of flooded spaces and the watertightness. both of the bulkheads and of the ship’s sides up to the bulkhead deck are sound, the effect of being cut into by another vessel can be determined, and transverse bulkheads can be spaced so: as to ensure flotation, even though in- jury be sustained by one of them. In accidents of this nature, the chief source of danger would probably be the resultant loss of dynamical stability. A watertight deck fitted in a ship which has transverse bulkheads of the usual type extending from the double bot- tom straight up to the bulkhead deck, will be of no service in such an event. It is bound to suffer damage; the water will be free to rise above it, and the result will be the same as if there were no watertight deck. Spacing of Bulkheads is Important By spacing bulkheads more closely above the watertight deck than below it, however, it is possible to effect some improvement in stability. As the length of ship increases, the International Con- vention provides for a gradual reduc- tion in the ratio which the length of compartment bears to the floodable length, and the writer has attempted to ascertain whether the greater margin of stability in the damaged condition thus. ensured, could not be obtained by apply- ing the “factor of subdivision” solely to the bulkheads above the watertight deck, and leaving those below that deck at a spacing of not more than one-half of the floodable length. The greatest loss of freeboard must occur in the event of a bulkhead both above and below the watertight deck being damaged, four compartments: being opened thereby. In this case the whole floodable length below the water- tight deck and two-thirds of that length above it will be flooded. The greatest loss of metacentric height will be sustained when the only bulkhead damaged is one above the watertight deck.