Ship motions are the

six degrees of freedom Six degrees of freedom (6DOF), or sometimes six degrees of movement, refers to the six mechanical degrees of freedom of movement of a rigid body in three-dimensional space. Specifically, the body is free to change position as forward/backw ...

that a ship, boat, or other watercraft can experience.

Reference axes

The '' vertical/Z axis'', or ''yaw axis'', is an imaginary line running vertically through the ship and through its

centre of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. For a ...

. A yaw motion is a side-to side movement of the bow and stern of the ship. The '' transverse/Y axis'', ''lateral axis'', or ''pitch axis'' is an imaginary line running horizontally across the ship and through the centre of mass. A pitch motion is an up-or-down movement of the bow and stern of the ship. The '' longitudinal/X axis'', or ''roll axis'', is an imaginary line running horizontally through the length of the ship, through its centre of mass, and parallel to the ''

waterline The waterline is the line where the hull of a ship meets the surface of the water. A waterline can also refer to any line on a ship's hull that is parallel to the water's surface when the ship is afloat in a level trimmed position. Hence, wate ...

''. A roll motion is a side-to-side or port-starboard tilting motion of the superstructure around this axis.

Rotational

There are three special axes in any ship, called longitudinal, transverse and vertical axes. The angular movements around them—affecting the ship’s moment of inertia, which sets the torque it requires to rotate in any direction—are the ship’s ''rotational motions'' (or ''rotary motions''), known as roll, pitch, and yaw respectively.

Roll

The tilting rotation of a vessel about its longitudinal/X (front-back or bow-stern) axis. An offset or deviation from normal on this axis is referred to as

list A list is a Set (mathematics), set of discrete items of information collected and set forth in some format for utility, entertainment, or other purposes. A list may be memorialized in any number of ways, including existing only in the mind of t ...

heel The heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg. Structure To distribute the compressive forces exerted ...

. 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 (or list) angle due to the ship's own weight distribution is referred in marine engineering as list. List normally refers to an unintentional or unexpected offset, as caused by flooding, battle damage, shifting cargo, etc.

Pitch

The up/down rotation of a vessel about its transverse/Y (side-to-side or port-starboard)

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

. An offset or deviation from normal on this axis is referred to as ''trim'' or ''out of trim''. A vessel that is pitching back and forth is usually termed to be hobby horsing.

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 A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...

(or true heading if referenced to the true north pole); it also affects the bearing.

Translational

Besides a ship’s own rotational motion, over which it has a measure of control, the water in which it moves can have motion independent of the ship, which displaces the ship by imposing forces that shift the ship’s entire mass linearly along any of its axes—longitudinal, transverse, and vertical. These ''translational motions'' (or ''translatory motions'') are known as surge, sway, and heave respectively.

Surge

The linear longitudinal (front/back or bow/

stern The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. O ...

) motion imparted by maritime conditions, usually head or following seas, or by accelerations imparted by the propulsion system.

Sway

The linear transverse (side-to-side or port-starboard) motion. This motion is generated directly either by the water and wind motion, particularly lateral wave motion, 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 lateral drift from its course.

Heave

The

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vertical (up/down) motion; excessive downward heave can swamp a ship.

Stabilization

There are methods for both passive and active motion stabilization used in some designs. They include static hull features such as

skeg A skeg (or skegg or skag) is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard."A small fin f ...

s and

bilge keel A bilge keel is a nautical device used to reduce a ship's tendency to roll. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase hydrodynamic ...

s, or active mechanical devices like counterweights,

antiroll tanks Antiroll tanks are tanks fitted onto ships in order to improve the ship's response to roll motion. Fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side and the reverse, the tanks are ...

and stabilizers.