Parametric roll

Parametric roll is a resonant build-up of roll in head or following seas, driven not by a direct roll moment but by the periodic change of waterplane (and so $GM$) as wave crests and troughs pass along the hull. When the wave encounter period is about half the roll natural period and roll damping is low, small disturbances amplify into rolls of 30 degrees or more. Fine-bowed, flared container ships are most exposed; the IMO second-generation intact stability criteria screen for it.