Principles of naval engineering - Historic Naval Ships Association

.Chapter 3-STABILITY AND BUOYANCYTrim by bow usually means a decreased KM.The center of buoyancy will rise slightly, butthis is usually counteracted by the decreasedBM caused by the lower moment of inertia ofthe trimmed waterplane.CAUSES OF IMPAIRED STABILITYThe stability of a ship may be impaired byseveral causes, resulting from mistakes or fromA summary of these causes andenemy action.their effects follows:ADDITION OF TOPSIDE WEIGHTThe addition of appreciable amounts of topsideweight may be occasioned by unauthorizedalterations; icing conditions; provisions, ammunition,or stores not struck down; deck cargo;and other conditions of load. Whenever a weightof considerable magnitude is added above theship's existing center of gravity the effects are:1. Reduction of reserve buoyancy.2. Reduction of GM and righting arms due toraising G.3. Reduction of GM and righting arms dueto loss of freeboard (change of waterplane).4. Reduction of righting arms if G is pulledaway from the centerline.5. Increase in righting moment due to increaseddisplacement.The net effect of added high weight is alwaysa reduction in stability. The reserve buoyancyloss is added weight in tons. The new metacentricheight can be obtained from:GjM^ =KM^ - KG^Stability is determined by selecting a newstability curve from the cross curves and correctingit for AG^ sine and G1G2 cos e.LOSS OF RESERVE BUOYANCYReserve buoyancy may be lost due to errors,such as poor maintenance, failure to close fittingsproperly, improper classification of fittings,and overloading the ship; or it may be lostas a result of enemy action such as fragment ormissile holes in boundaries, blast which carriesaway boundaries or blows open or warps fittings,and flooding which overloads the ship. When theabove-water body is holed, some reserve buoy-ancy is lost. The immersion of buoyant volumeis necessary to the development of a rightingarm as the ship rolls; if the hull is riddled itcan no longer do this on the damaged side,toward which it will roll. In effect, the riddlingof the above- water hull is analogous to losing apart of the freeboard, thus reducing stability.When this happens, if the ship takes wateraboard on the roll, the combined effects of highadded weight and free surface operate to cutdown the righting moment. Therefore, the underwaterhull and body should be plugged andpatched, and every effort should be made to restorethe watertightness of external and internalboundaries in the above- water body.FLOODINGFlooding may take place because of underwaterdamage, shell or bomb burst below decks,collision, topside hit near the waterline, firefightingwater, ruptured poping, sprinkling ofmagazines, counterflooding, or leakage. Regardlessof how it takes place it can be classifiedin three general categories, each of which canbe further broken down, as follows:1with respect to boundariesa. solid footingb. partial floodingc. partial flooding in free communicationwith the sea2. with respect to height in the shipa. center of gravity of the flooding wateris above Gb. center of gravity of the flooding wateris below G3. with respect to the ship's centerlinea. symmetrical floodingb. off-center floodingSolid FloodingThe term solid flooding designates the situationin which a compartment is completelyfilled from deck to overhead. In order for thisto occur the compartment must be vented as byan air escape, an open scuttle or vent fitting, orthrough fragment holes in the overhead. Solidflooding water behaves exactly like an added59