Ship stability is the ability of a ship to float in an upright position and, if inclined under action of an external force, to return to this position after the external force has ceased acting.
Stability is not connected with a defined direction. However, ship inclination in transverse direction is most common and easiest to achieve and in practice transverse stability is the most critical to ship safety.
Stability of a loaded ship depends on her shape and dimensions and on the actual location of her centre of gravity. Small ships with low freeboard are more prone to stability accidents than other seagoing vessels.
After a ship is constructed, an operator has no influence on shape, dimensions, as well as mass and location of the centre of gravity of an empty vessel. Still, he has an influence on the final mass of the loaded vessel and the location of the centre of gravity defining amount of cargo, stores and ballast water as well as their locations.
A number of ship operations can adversely affectstability. Such effects must be understood and, where possible, mitigated. When liquid is consumed or removed from tanks than a free surface is created which decreases stability. 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 lost 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 VCG reducing the righting arm GZ.
- Damage stability – Stability of the ship in flooded condition. This stability is attained by installing a number of watertight compartments. If one of these compartments is breached, then the watertight bulkheads surrounding it will prevent the inflow of seawater from spreading to the rest of the ship. See also Damage stability calculations.
Further reading: “Ship Stability in Practice”