View source for Engineering:Ship motions

{{short description|Terms connected to the six degrees of freedom of motion}}

[[File:USS Langley (CVL-27) and battleship in typhoon 1944.jpeg|thumb|upright=1.35|Aircraft carrier rolling during a typhoon]]

'''Ship motions''' are defined by the six [[Physics:Degrees of freedom (mechanics)|degrees of freedom]] that a ship, boat, or other watercraft, or indeed any conveyance, can experience.

== Reference axes ==
{{seealso|Aircraft principal axes}}

The ''vertical/Z axis'', or ''yaw axis'', is an imaginary line running vertically through the ship and through its [[Physics:Centre of mass|centre of mass]] . 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 ''[[Engineering:Waterline|waterline]]''. A roll motion is a side-to-side or port-starboard tilting motion of the superstructure around this axis.

== Rotational ==
[[Image:Rotations.png|thumb|upright=1.35|Axes of a ship and rotations around them]]
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 (side-to-side or port side to starboard) axis. An offset or deviation from normal on this axis is referred to as [[Angle of list|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 (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 (bow to staern or front to back) axis. 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 [[Earth:Heading (navigation)|heading]] of the boat relative to a magnetic compass (or true heading if referenced to the true north pole); it also affects the [[Earth:Bearing (navigation)|bearing]].

== Translational ==
[[Image:Translations.PNG|thumb|upright=1.35|Translations]]
=== Surge ===
The linear longitudinal (front/back or bow/[[Engineering:Stern|stern]]) 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 and starboard|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 linear 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 [[Engineering:Skeg|skeg]]s and bilge keels, or active mechanical devices like counterweights, [[Engineering:Antiroll tanks|antiroll tanks]] and stabilizers.

== See also ==
*{{annotated link|Physics:Rotation}}
*{{annotated link|Engineering:Naval architecture}}
*{{annotated link|Engineering:Seakeeping}}
*{{annotated link|Engineering:Ship stability}}
*{{annotated link|Engineering:Ship motion test}}
*{{annotated link|Physics:Six degrees of freedom}}
*{{annotated link|Physics:Flight dynamics}}

== References ==
* Society of Naval Architects and Marine Engineers (SNAME), "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). [https://doi.org/10.1016/j.oceaneng.2019.106239 Theoretical background and application of MANSIM for ship maneuvering simulations] [https://www.mansim.org MANSIM Lab]. Ocean Engineering, 192, 106239. 
[[Category:Nautical terminology]]

{{Sourceattribution|Ship motions}}