Center of Gravity and Center of Buoyancy

Center of gravity, center of buoyancy and stability

The Centre of Gravity is the point where the gravitational force acting on a body may be taken to act.

The Centre of Buoyancy is the center of the gravity of the volume of water which the hull displaces.

When the hull is upright the center of gravity and center of buoyancy is on the same vertical line. The hull is stable. For most hulls the center of buoyancy is below the center of gravity and the hull is said to be metastable.

When the hull tilts the center of gravity remains in the same position related to the hull (the hull is not changed and/or the cargo is not moved). The center of buoyancy moves to fit the new center of gravity of the volume of water replaced by the hull. At first the gravity force and the buoyancy force creates a righting torque that tries to move the hull back to upright position. If the hull is tilted to much the center of buoyancy moves to a position where the buoyancy and gravitation force starts to create a moment that will capsize the hull.

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