Metacentric Height

The metacentric height (GM) is the vertical distance between the centre of gravity (G) and the transverse metacentre (M) of a floating vessel, and is the primary indicator of initial transverse stability. A positive GM (M above G) means the vessel is initially stable and will produce a righting moment when heeled: the greater the GM, the stiffer the ship and the shorter its roll period. A negative GM (G above M) means the vessel is initially unstable and will loll to one side or capsize. The Admiralty Manual of Navigation and SOLAS Chapter II-1 both impose minimum GM requirements for different vessel types, and the IMO Intact Stability (IS) Code sets criteria for the full righting lever curve. The chief mate calculates GM throughout the loading and discharging process using the ship's loading instrument or computer program, verifying that it remains above the minimum required value at all stages. Free surface effect—the virtual reduction in GM from liquid in partially filled tanks—must be deducted from the solid GM to give the effective GM. A vessel with very high GM rolls quickly and uncomfortably but is safe; a vessel with GM close to zero rolls slowly but is vulnerable to cargo shift. Cargo loading plans must achieve a GM within acceptable limits throughout the voyage, accounting for the progressive consumption of fuel and fresh water which changes the displacement and the position of G.