11.1 Ship Stability
Introduction
Ship Stability is an area of Naval Architecture that deals with how a ship behaves at sea both in still water and in waves whether the ship is in intact or damaged condition. Ship Stability calculations are based on the Centre of Gravity, Centre of Buoyancy, Metacentre and Metacentric height of the ships. It is the ability of a ship to float in an upright position and if inclined under action of the external forces to return to the upright position after the removal of external forces. The stability is not connected with a defined direction. However, the inclination of the ships in transverse direction is the most common and easiest to achieve and in practice, the transverse stability of a ship is the most critical to ship safety. Stability of a loaded ship is totally depending on her shape and dimensions and on the actual location of the Centre of gravity. Smaller ships with low freeboard are more prone to stability accidents than the seagoing vessels.
A number of ship operations can adversely affect the stability of the ships. Such effects must be understood and where possible they need to be mitigated. When liquid is consumed or removed from tanks onboard then a free surface is created which reduces the stability of ships. When a weight is lifted and suspended, its Centre of gravity rises to the point of suspension. When a quantity of loose dry bulk cargo moves transversely across the ship it will list to one side with some loss of stability. Phenomena such as absorption of moisture by timber or similar deck cargoes, ice accretion on decks and accumulation of shipped water will rise the vertical Centre of gravity thereby reducing the righting lever of the ship.
