A capsizing moment is a heeling moment large enough to exceed a vessel’s maximum available righting moment, driving her heel past the point where she can recover.
The Balance a Ship Normally Holds
At any angle of heel, a vessel’s righting arm (GZ) generates a righting moment — displacement multiplied by GZ — that opposes whatever is pushing her over: wind pressure, a turn, cargo shift, beam or quartering seas, or free surface effect in a slack tank. Plotted against heel angle, GZ rises from zero, typically peaks somewhere around 30–40° for most merchant ships, then falls back toward zero at the angle of vanishing stability — beyond that angle GZ is negative and there is no righting moment left at all.
Under normal conditions, the heeling moment and righting moment reach equilibrium at some steady angle well before the peak of the GZ curve, and the ship rides at that heel until the heeling force changes.
When It Becomes a Capsizing Moment
If the heeling moment exceeds the maximum righting moment the vessel can generate — the peak of her GZ curve — no equilibrium angle exists. The ship keeps heeling past the point of maximum GZ, and because GZ is now falling as heel increases, the righting moment keeps shrinking exactly when more of it is needed. If the heeling moment stays dominant as the vessel passes the angle of vanishing stability, GZ goes negative, the righting moment reverses sign, and the vessel capsizes rather than settling at any stable angle.
Common Real-World Causes
- Negative or marginal GM — a vessel already lolling or close to it has a compressed positive-GZ range, so a comparatively small additional heeling moment can exceed what’s left of her righting moment.
- Free surface effect — slack tanks reduce effective GM and flatten the GZ curve, lowering the maximum righting moment available.
- Cargo or ballast shift — a sudden, large heeling moment from shifted deck cargo, grain, or liquid cargo in a partially-filled tank.
- Beam or quartering seas and synchronous/parametric rolling — wave-induced heeling moments that build with each roll cycle rather than acting once.
- High deck cargo or icing — raises the centre of gravity, reducing GM and the peak righting moment together.
Exam Relevance
Reading a GZ curve — identifying maximum GZ, the angle it occurs at, and the angle of vanishing stability — and explaining how a given heeling moment compares to the available righting moment at each stage, is standard material in MEO and Mate/Master stability orals.