Parametric roll is a phenomenon in which a ship in head or following seas develops a large, resonant roll even though the wave is not beam-on. It occurs because the ship's metacentric height (GM) and hence the roll natural frequency vary periodically as the wave passes along the hull: when a wave crest is amidships the waterplane area and GM are reduced, and when a trough is amidships the GM is increased. If the encounter frequency of the waves is about twice the natural roll frequency, the periodic variation of GM excites the roll in resonance, and the roll amplitude grows rapidly, even in moderate seas. Container ships are particularly susceptible because they have large, open hatch openings (reduced waterplane area and GM variation), a high centre of gravity (small GM), and a large bow flare and stern, which increase the variation of the waterplane area with the wave. Parametric roll can produce large roll angles (20-40 deg) that can cause cargo shift, container loss, and even capsize.

• Design: increase the GM (lower the centre of gravity, increase the beam), reduce the variation of the waterplane area with the wave (reduce the bow flare and the fullness of the ends), and provide adequate stability and damping (bilge keels, stabilisers). The hull form is designed to reduce the periodic variation of GM.
• Operation: avoid the resonant conditions by changing speed and course so that the encounter frequency is not about twice the natural roll frequency; use the roll damping devices; monitor the roll and the sea state; and follow the operational guidance (e.g. the parametric roll guidance in the loading manual and the weather routing). Reducing speed in head seas and changing heading can avoid the resonance.

• Lowering the centre of gravity by placing heavy weights (machinery, ballast) low and by limiting the height of the container stacks.
• Increasing the GM by increasing the beam and the waterplane area, and by lowering KG.
• Providing adequate freeboard and reserve buoyancy.
• Fitting bilge keels, anti-roll tanks and stabilisers to damp the roll.
• Using ballast to improve the stability and to reduce the GM variation.
• Designing the hull to reduce the susceptibility to parametric roll.
• Following the loading manual and the stability criteria, and monitoring the stability in all conditions.