Ship motions


Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.

Rotational motions

There are three special axes in any ship, called longitudinal, transverse and vertical axes. The movements around them are known as roll, pitch, and yaw respectively.
; Pitch: The up/down rotation of a vessel about its transverse/Y axis. An offset or deviation from normal on this axis is referred to as trim or out of trim.
; Roll: The tilting rotation of a vessel about its longitudinal/X axis. An offset or deviation from normal on this axis is referred to as list or heel. Heel refers to an offset that is intentional or expected, as caused by wind pressure on sails, turning, or other crew actions. The rolling motion towards a steady state angle due to the ship's own weight distribution is referred in marine engineering as heel. List normally refers to an unintentional or unexpected offset, as caused by flooding, battle damage, shifting cargo, etc.
; Yaw: The turning rotation of a vessel about its vertical/Z axis. An offset or deviation from normal on this axis is referred to as deviation or set. This is referred to as the heading of the boat relative to a magnetic compass ; it also affects the bearing.

Translational motion

; Heave: The linear vertical motion; excessive downward heave can swamp a ship.
; Sway: The linear transverse motion. This motion is generated directly either by the water and wind currents exerting forces against the hull or by the ship's own propulsion; or indirectly by the inertia of the ship while turning. This movement can be compared to the vessel's drift from its course.
; Surge: The linear longitudinal motion imparted by maritime conditions.

Stabilization

There are methods for both passive and active motion stabilization used in some designs. They include static hull features such as skegs and bilge keels, or active mechanical devices like counterweights, antiroll tanks and stabilizers.