Ship stability

Ship stability is a vessel’s ability to resist capsizing and return toward an upright position after disturbance. It depends on hull geometry, loading, buoyancy, and the movement of water and cargo.

Center of buoyancy: The center of the volume of water displaced by a floating body, through which the buoyant force acts. As a vessel heels, its underwater shape shifts this point and creates the righting effect.
Buoyancy: The upward force exerted by a fluid on an immersed or floating object. It supplies the upward force that balances a ship’s weight and supports its righting response.
Intact stability: A vessel's ability to remain stable with its hull and watertight boundaries undamaged. It covers the ordinary operating condition before flooding or structural damage occurs.
Static stability: A system's tendency to return toward equilibrium after a small displacement, considered without motion over time. It describes the vessel’s immediate righting tendency at a given heel angle.
Metacenter: The point where the vertical line through a displaced body's center of buoyancy intersects its original upright centerline. Its position relative to the center of gravity indicates initial stability in an upright vessel.
Archimedes' principle: A floating or immersed body experiences an upward force equal to the weight of the fluid it displaces. It links vessel displacement to the buoyant force underlying stability calculations.
Damage stability: A vessel's ability to remain afloat and stable after specified flooding or damage. It tests whether flooding compromises the vessel’s reserve of buoyancy and righting ability.
Dynamic stability: A system's capacity to resist disturbances when its motion and energy over time matter. A vessel may have favorable static righting force yet still be vulnerable to waves and rolling.
Metacentric height: The vertical distance between a floating body's center of gravity and metacenter. A positive value indicates initial righting tendency; a small value can produce sluggish, excessive rolling.
Displacement (ship): The weight of water displaced by a vessel, equal to the vessel's weight when afloat. Changes in cargo, fuel, or stores alter displacement and can change stability.

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