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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.
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.
Simpson's rules are used to calculate the volume of lifeboats, [6] and by surveyors to calculate the volume of sludge in a ship's oil tanks. For instance, in the latter, Simpson's 3rd rule is used to find the volume between two co-ordinates. To calculate the entire area / volume, Simpson's first rule is used. [7]
The stability conditions of watercraft are the various standard loading configurations to which a ship, boat, or offshore platform may be subjected. They are recognized by classification societies such as Det Norske Veritas , Lloyd's Register and American Bureau of Shipping (ABS).
The Code for Intact Stability was first issued in 1993 under IMO resolution A.749(18)). [1] In 2008, the Code was updated by the IMO. [1] In December 2019, amendments to the Code were adopted that entered into force on 1 January 2020. [5]
The GM position is determined by moving weights transversely to produce a known overturning moment in the range of 1-4 degrees if possible. [2] Knowing the restoring properties (buoyancy) of the vessel from its dimensions and floating position and measuring the equilibrium angle of the weighted vessel, the GM can be calculated.
Initial stability or primary stability is the resistance of a boat to small changes in the difference between the vertical forces applied on its two sides. [1] The study of initial stability and secondary stability are part of naval architecture as applied to small watercraft (as distinct from the study of ship stability concerning large ships ).