Metacentric Height

Metacentric height, GM, is a quick indicator of a ship’s initial stability after a small heel. It is found by comparing two vertical positions measured from the keel: the metacentre (M), which represents how the buoyant force shifts for a slight tilt, and the centre of gravity (G), which is where the ship’s weight acts. If M lies above G, the buoyant and weight forces create a restoring couple, so the vessel tends to return upright; if G is above M, the couple can overturn it further.

Use this when checking the initial stability of a floating vessel, barge, pontoon, or loaded ship, especially after changes in cargo, ballast, fuel, or passenger distribution. It is also used in naval architecture and marine operations to assess whether a vessel is stiff, tender, or dangerously unstable.

• Subtracting in the wrong order and reporting a negative GM when it should be positive
• Using the waterline or deck height instead of the keel as the common reference datum
• Assuming a large GM always means complete safety, even though cargo shift, flooding, or large angles of heel can still make the vessel unsafe