The tendency of a floating vessel to return to its original upright position of equilibrium after being tipped by the forces of wind and sea.
In order to maintain its stable upright position a vessel's Center of Gravity must lie vertically below its Metacenter on the Center of Buoyancy line.
- Center of Gravity - The central point around which the total mass or weight of a vessel and its cargo is evenly distributed and balanced. The point at which all of the downward forces of a vesselâ€™s weight can be considered to act. If the Center of Gravity is low the vessel is "stiff." If the Center of Gravity is high the vessel is "tender" and tends to roll more in the waves. If the Center of Gravity is too high the stability decreases and it could capsize. The Center of Gravity is "C" on the diagram below.
- Center of Buoyancy Line - The vertical line at which all of the upward forces of the waterâ€™s support (buoyancy) can be considered to act. The center plane of a vessel; "A" on the diagram below when it is floating upright, and "B" when slightly tipped.
- Metacenter - The highest point to which the Center of Gravity may rise and still permit a vessel to have positive stability; "M" on the diagram below. The Metacenter must be above the Center of Gravity, or the vessel would be top-heavy and capsize.