USS Langley (CVL-27) and battleship in typhoon 1944

Ship motions are defined by the six

degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

that a ship, boat or any other

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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 balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

. 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. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...

''. 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 movements around them are 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

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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 exer ...

. 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 heel. 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)

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

Translational

Surge

The linear

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(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. Or ...

) 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 keels, or active mechanical devices like counterweights, antiroll tanks and

stabilizers Stabilizer, stabiliser, stabilisation or stabilization may refer to: Chemistry and food processing * Stabilizer (chemistry), a substance added to prevent unwanted change in state of another substance ** Polymer stabilizers are stabilizers used ...

References

* Society of Naval Architects and Marine Engineers (

SNAME The Society of Naval Architects and Marine Engineers (SNAME) is a global professional society that provides a forum for the advancement of the engineering profession as applied to the marine field. Although it particularly names the naval archit ...

), "Principles of Naval Architecture", 1989, Vol. III, Pg.41, Section 3 - Ship Responses to Regular Waves * Sukas, Omer Faruk; Kinaci, Omer Kemal; Bal, Sakir (2019)
Theoretical background and application of MANSIM for ship maneuvering simulationsMANSIM Lab
Ocean Engineering, 192, 106239. Nautical terminology