The centre of gravity (G) is the single point at which a ship’s total weight can be considered to act, combining the ship’s structure, machinery, cargo, fuel, water, and everything else aboard into one theoretical point.
Why G’s Position Matters
A ship’s stability depends heavily on the vertical position of G relative to the centre of buoyancy (B) and the metacentre (M):
- Lower G (closer to the keel) generally increases stability, giving a larger righting moment when the ship heels
- Higher G reduces stability, and if G rises above M, the vessel becomes unstable (negative GM), risking capsizing or angle of loll
What Moves G
- Loading cargo high in the ship raises G, reducing stability
- Loading cargo low (in lower holds or double bottom tanks) lowers G, increasing stability
- Free surface effect from partially filled tanks effectively raises the ship’s apparent G, reducing effective stability even though actual cargo weight hasn’t moved
GM: The Key Stability Measure
The vertical distance between G and the metacentre (M) — called GM (metacentric height) — is the primary quick-reference measure of a ship’s initial stability, calculated and monitored for every voyage as part of standard stability assessment.
Exam Relevance
Centre of gravity calculations, GM determination, and the effect of loading/free surface on G’s position are core Ship Stability topics tested extensively in deck officer competency exams.