GM stability estimator

The metacentric height (GM) is the primary indicator of initial transverse stability. A positive GM means the vessel will right herself when heeled; a negative GM means she is unstable and will loll or capsize. This tool calculates the key stability parameters using a rectangular box approximation.

The calculation

• BM = B² ÷ (12 × d) — distance from centre of buoyancy to the metacentre (transverse). Derived from second moment of the waterplane area divided by the displaced volume.
• KB = d ÷ 2 — height of centre of buoyancy above keel (rectangular box: centroid of displaced volume).
• KM = KB + BM — height of the metacentre above the keel.
• GM = KM − KG — metacentric height. Positive = stable; negative = unstable.

The block coefficient (C_b) for a real ship is typically 0.55–0.85 and the waterplane coefficient (C_w) is typically 0.70–0.90. Using a box (C_b = C_w = 1.0) overestimates BM and therefore KM, which means estimated GM will be higher than the actual value.

See also

• · Vessel stability reference — GM, GZ, inclining experiment, loading conditions.
• · Load lines — freeboard, zones, and seasonal marks.
• · Tonnage — GT, NT, displacement, and deadweight.

Last updated 2026-04-27