Metacentric height (GM) is the vertical distance between a ship’s centre of gravity (G) and its metacentre (M) — the point where the line of buoyant force crosses the ship’s centerline as it heels through small angles.
Why GM Is So Central to Stability Assessment
GM is calculated and checked for essentially every loading condition because it provides a fast, reliable indicator of a vessel’s initial stability without requiring the full GZ curve to be plotted:
- Positive GM (M above G) — the vessel is stable and will tend to return upright when heeled
- Zero or negative GM (M at or below G) — the vessel is unstable or neutrally stable, a genuinely dangerous condition where the ship may not return upright, or may adopt a permanent angle of loll
What Affects GM
- Cargo distribution — weight loaded high in the vessel raises G, reducing GM; weight loaded low lowers G, increasing GM
- Free surface effect — partially filled tanks reduce effective GM even without moving actual weight, since the liquid can shift as the vessel heels
- Ballast — adjusting ballast is a primary tool for correcting GM to within acceptable limits
Too Much GM Is Also a Problem
While negative GM is dangerous, excessively high GM isn’t automatically ideal either — very high GM produces a short, “stiff” rolling period, causing quick, jerky, uncomfortable rolling motion and higher stress on cargo securing and the ship’s structure. Stability planning aims for GM within an appropriate range, not simply “as high as possible.”
GM in Practice
Every voyage’s loading condition includes a GM calculation, checked against minimum (and sometimes maximum) values specified in the ship’s approved stability booklet — a real, mandatory pre-departure check, not a theoretical exercise.
Exam Relevance
GM calculation, its determining factors, and its role as the primary quick-reference stability measure are core, heavily-tested Ship Stability topics across all deck officer competency exam levels.