Ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
Linear motion
- Heave
- is the linear vertical (up/down) motion; excessive downward heave can swamp a ship.
- Sway
- is the linear lateral (side-to-side or port-starboard) motion, which does not present much of a challenge for most modern ships.
- Surge
- is the linear longitudinal (front/back or bow/stern) motion imparted by maritime conditions.
Rotation axes
The vertical/Z axis, or yaw axis, is an imaginary line running vertically through the ship and through its centre of gravity. A yaw motion is a side-to side movement of the bow and stern of the ship.
The lateral/Y axis, transverse axis, or pitch axis is an imaginary line running horizontally across the ship and through the centre of gravity. 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 gravity, and parallel to the waterline. A roll motion is a side-to-side or port-starboard tilting motion of the superstructure around this axis.
Rotation motions
There are three special axes in any ship, called vertical, lateral and longitudinal axes. The movements around them are known as roll, pitch and yaw.
- Pitch
- is the up/down rotation of a vessel about its lateral/Y (side-to-side or port-starboard) axis. An offset or deviation from normal on this axis is referred to as 'trim' or 'out of trim'.
- Roll
- is 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 or heel. Heel refers to an offset that is intentional or expected, as caused by wind pressure on sails, turning, or other crew actions. List normally refers to an unintentional or unexpected offset, as caused by flooding, battle damage, shifting cargo, etc.
- Yaw
- is 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.
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.
See also
- Translation (physics)
- Rotation
- Naval architecture
- Seakeeping
- Stern suction
- Ship stability
- Ship motion test
- 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