Stability

Stability is the property that keeps a ship upright and lets it recover when something (a gust of wind, a wave, a turn, or a shift of cargo) heels it to one side. When a stable ship is pushed over, the shape of the immersed hull generates a righting moment that acts to bring it back upright; the strength of that tendency is what the crew must understand and control. Loss of adequate stability is among the most dangerous conditions a ship can face, since it can lead to a sudden and catastrophic capsize, so managing stability is a fundamental responsibility in loading and operating any vessel.

The cornerstone concept is the relationship between the ship's centre of gravity and a point called the metacentre. The centre of gravity, G, depends on how weights are distributed — a low centre of gravity, achieved by keeping heavy weights low in the ship, generally improves stability, while carrying weight high, or slack liquids that are free to move, raises G and reduces it. The metacentric height, or GM, the distance between the centre of gravity and the metacentre, is a practical measure of initial stability: a positive GM means the ship is stable, and a larger GM means a stiffer, more strongly righting ship, though an excessive GM produces uncomfortable, jerky rolling that has its own risks.

Stability must be assessed for the ship's actual loaded condition and kept within approved limits at every stage of a voyage. Officers use the ship's stability information and a loading computer to check that the centre of gravity, GM and the range of positive righting are satisfactory, taking account of free surface effects from partly filled tanks, the consumption of fuel and water during the passage, and any deck cargo or icing. Beyond this intact stability, ships are also designed for damage stability, so that they can survive specified flooding. A sound understanding of stability underlies safe loading, ballasting and the carriage of every kind of cargo.