roulis

Roulis, or roulis, is the rolling motion of a ship or boat about its longitudinal axis, resulting in a side-to-side tilt. It is one of the primary seakeeping motions, alongside pitch and yaw, and arises from the interaction of the hull with waves, wind, and changes in ballast or loading.

Physically, roll is described by the roll angle phi and the roll rate p. The stability and behavior of rolling are influenced by the metacentric height GM; a positive GM yields a restoring moment for small angles, while insufficient GM can allow large-amplitude rolling. The waterplane area, hull form, and damping from water and structure affect how quickly the motion decays. In practice, seakeeping analysis models rolling under various sea states to estimate comfort, stability, and structural loads.

Natural roll characteristics depend on vessel design, mass distribution, and loading. A larger GM generally increases initial stiffness and reduces roll amplitude but can raise dynamic loads; a smaller GM tends to produce slower, softer rolling. External factors such as wave direction, wind, and vessel speed can sustain or amplify roll, sometimes leading to persistent motions in certain seas.

Mitigation and control strategies include ballast management to maintain safe GM, bilge keels to passively damp roll, and antiroll devices such as fin stabilizers (passive or active) or gyroscopic systems. Proper trimming and loading, hull designs that promote favorable rolling characteristics, and active stabilization technologies are used to reduce excessive roullis and improve safety and comfort.

See also: Roll, Seakeeping, Ship stability.