The points and levers
| Symbol | Meaning |
|---|---|
| G — centre of gravity | Where the total weight acts; rises when weight is added high |
| B — centre of buoyancy | Centre of the underwater volume; moves as the hull heels |
| M — metacentre | The point the ship appears to roll about at small angles |
| GM — metacentric height | Distance G to M; the measure of initial (small-angle) stability |
| GZ — righting lever | The lever of the righting moment at a given angle of heel |
| The points and levers that define stability. |
Working from the stability book
A positive GM means the ship returns upright when heeled; a large GM makes her stiff (a quick, hard roll), a small GM tender (a slow, sluggish, lolling roll that signals marginal stability). At management level the Chief Mate works from the ship's approved stability book and any loading computer rather than from feel, checking each loading condition against the stability criteria — the required minimum GM, the range of positive stability and the area under the GZ curve — for the intended voyage.
NOTE — Work from the approved information Stability at this level is managed from the ship's approved stability book and loading computer, not from a rule of thumb. Command means keeping the ship within the conditions a naval architect has approved.
WARNING — A very small GM is a warning, not a comfort A tender ship with a very small GM rolls slowly and lolls, and her stability is marginal — a dangerous state, not a gentle one. If loading has left her tender, correct it before proceeding.