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From Boat Design Wiki
Stability Concepts (Slightly modified on a National Transportation Safety Board's (USA) Ethan Allen accident report)
A vessel that is floating upright in still water will heel when an off-center force or heeling moment is applied. Stability is the tendency of the vessel to return to its original upright position when the force is removed. In still water, a vessel's stability is a function of its underwater hull form and the distribution of mass of the vessel. The properties of stability are usually expressed in terms such as the magnitude of a heeling moment necessary to heel the vessel to a certain angle, the angle a vessel may heel to before capsizing, the amount of reserve energy available to return the vessel to its upright position, and other parameters that can be calculated.
National maritime authorities regulations and international standards specify the amount of stability a vessel must possess, depending on the type of vessel and its service. The requirements are usually expressed in terms that are easily calculated, such as GM, range of positive stability, righting energy, or other recognized technical measures. The specific stability characteristics of an individual vessel are determined based on the design drawings of its hull form (lines plan) and an inclining experiment of the vessel while afloat to determine its actual weight (displacement) and center of mass (center of gravity).
Although obtained under stationary conditions, the stability characteristics represent the vessel's ability to return to the upright position when the vessel is in service and is subject to external forces such as wind and waves, people moving about, and from inertial effects resulting from the accelerations and motions of the vessel as it moves around on the water's surface. Such dynamic influences on a vessel are random processes and can only be predicted statistically. Studies comparing the stability characteristics of vessels that capsized (inadequate stability) and vessels that did not capsize (adequate stability) form the basis of stability criteria that are in use today. To account for unknowns, uncertainties, and randomness of the physical environment and vessel response, large safety margins are built into the criteria. The stability criteria are generally recognized as providing an adequate level of safety for vessels that are operated prudently, which means not overloaded and not operating in dangerous conditions such as hurricanes.
The stability analyses of most vessels involve substantial calculations that generally require the services of a naval architect. The calculations are based on an inclining experiment, in which very precise measurements are taken on board the vessel in order to determine its displacement and center of gravity. The inclining experiment and associated calculations and analysis cost several thousand to tens of thousands of dollars. For large oceangoing ships, the stability assessment is minor compared to the total cost, and is necessary in order to determine the maximum amount and stowage of cargo. In small passenger or fishing vessels design, stability is not optimized and usually exceeds local maritime authorities stability criteria. Because of the relatively high cost of an inclining experiment and full stability assessment, such authorities use to permit variations of a Simplified Stability Test (SST) to be performed. Where permitted, those SST are quick, inexpensive, and more conservative than a full stability analysis. Being more conservative, the SST results in fewer passengers, or fishing gear/fish load, permitted, than would be allowed based on a full stability assessment.
Because the number of passengers permitted on a small passenger vessel is based on several criteria, such as deck area and seating capacity, stability is often not the governing criteria for such vessels. In those cases, the SST is sufficient. However, the owner of a small passenger vessel might have space to carry more passengers than would be allowed by the SST, and so the cost of performing a full stability assessment with an inclining experiment is justified to show that additional passengers may be carried in compliance with the detailed stability regulations. The margin of safety built into the stability criteria is what provides the safety of the vessel against capsizing. The margin of safety is intended to accommodate all the things that happen with a vessel, such as rolling in waves, heeling due to wind, or listing as passengers move from one side to the other. The margin of safety is reduced if the vessel is operated in extremely high winds, surf, or is overloaded. The stability criteria are not intended for such conditions. In such extreme operating conditions, the margin of safety may be still adequate to keep the vessel upright, or it may not be adequate, resulting in a capsizing. In any case, if the intended margin of safety is not maintained, the vessel should not be considered seaworthy (safe), whether or not it capsizes.
Because of the nature of stability, and the randomness and variabilities associated with it, safety is not absolute. Not meeting stability standards does not mean a vessel will capsize; it only means the margin of safety is lower than what the regulations require. A vessel can operate for years in an overloaded condition that does not meet the stability standards; because the margin of safety is less than it should be, the probability of capsizing is higher, but it could still be remote. It would take other forces, such as high winds or large waves, to cause a vessel to capsize. The more a vessel is overloaded, the less the margin of safety for stability and the higher the probability of capsizing. There is no obvious way to tell if a vessel fails to meet the stability criteria, other than through a stability test. This is why a stability test is usually required for all small passenger or fishing vessels. If something changes about the vessel, such as a structural modification that might affect the vessel's stability, another stability test and assessment should be conducted. After the stability assessment is completed, the results of the assessment and any limitations, such as number of passengers or limiting operating conditions, will be placed on the vessel's stability letter or equivalent document.