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Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on centers of gravity , centers of buoyancy , the metacenters of vessels, and on how these interact.
The metacentric height is an approximation for the vessel stability at a small angle (0-15 degrees) of heel. Beyond that range, the stability of the vessel is dominated by what is known as a righting moment. Depending on the geometry of the hull, naval architects must iteratively calculate the center of buoyancy at increasing angles of heel.
The vessel is in normal operational configuration. The hull is not breached in any compartment. The vessel will be expected to meet various stability criteria such as GMt (metacentric height), area under the GZ (righting lever) curve, range of stability, trim, etc.
Angle of loll is the state of a ship that is unstable when upright (i.e. has a negative metacentric height) and therefore takes on an angle of heel to either port or starboard. When a vessel has negative metacentric height (GM) i.e. , is in unstable equilibrium, any external force applied to the vessel will cause it to start heeling.
A heavily listing ship. The angle of list is the degree to which a vessel heels (leans or tilts) to either port or starboard at equilibrium—with no external forces acting upon it. [1] If a listing ship goes beyond the point where a righting moment will keep it afloat, it will capsize and potentially sink. [2]
Simpson's rules are a set of rules used in ship stability and naval architecture, to calculate the areas and volumes of irregular figures. [1] This is an application of Simpson's rule for finding the values of an integral, here interpreted as the area under a curve. Simpson's First Rule
The Code contains both mandatory regulations and recommended provisions, setting out the minimum stability standards for ships. [6] This includes information on precautions against capsizing, metacentric heights (GM), righting levers (GZ), rolling criteria, Free surface effect and watertight integrity.
Hogging is the stress a ship's hull or keel experiences that causes the center or the keel to bend upward. Sagging is the stress a ship's hull or keel is placed under when a wave is the same length as the ship and the ship is in the trough of two waves.