Ship motions

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

Linear motion[edit]

"Translations"
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. 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 comapred to the vessel's drift from its course.
Surge
is the linear longitudinal (front/back or bow/stern) motion imparted by maritime conditions.

Rotation axes[edit]

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[edit]

Axes of a ship and rotations around them

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. 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.
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[edit]

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[edit]

References[edit]

  • Society of Naval Architects and Marine Engineers (SNAME), "Principles of Naval Architecture", 1989, Vol. III, Pg.41, Section 3 - Ship Responses to Regular Waves