GM (metacentric height)

Metacentric height $GM = KM - KG = KB + BM - KG$ is the vertical separation of the center of gravity G and the transverse metacenter M, the first-order measure of initial stability. A positive $GM$ gives a stable upright ship; the IS Code 2008 sets a corrected minimum of 0.15 m for most cargo vessels. $GM$ fixes the natural roll period through $T = 2\pi k / \sqrt{g \cdot GM}$, so a stiff ship (large $GM$) rolls fast and uncomfortably.