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As long as the load of a ship remains stable, G is fixed (relative to the ship). For small angles, M can also be considered to be fixed, while B moves as the ship heels. The metacentric height (GM) is a measurement of the initial static stability of a floating body. [1]
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 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.
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.
The metacentric height (GM), which dominates stability, can be estimated from the design, but an accurate value must be determined by an inclining test. The inclining test is usually done inshore in calm weather, in still water, and free of mooring restraints to achieve accuracy.
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.
Lightweight displacement – LWD – The weight or mass of the ship excluding cargo, fuel, ballast, stores, passengers, and crew, but with water in the boilers to steaming level. Loadline displacement – The weight or mass of the ship loaded to the load line or plimsoll mark. Deadweight tonnage (DWT) is a measure of how much weight a ship can ...
2b) "If a ship floods, the loss of stability is caused by the increase in KB, the centre of buoyancy, and the loss of waterplane area - thus a loss of the waterplane moment of inertia - which decreases the metacentric height" Interesting. in 2a) it was claimed that increase in KB improves stability and makes the hull stiff, now it's just the ...