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.

Translation[edit]

"Translations"
Heave
is the linear vertical (up/down) motion
Sway
is the linear lateral (side-to-side) motion
Surge
is the linear longitudinal (front/back) motion

Rotation axes[edit]

The vertical 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 of the ship.

The lateral 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 of the ship.

The longitudinal 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 motion of the superstructure around this axis.

Rotation motions[edit]

For other uses, see Euler angles#Tait-Bryan angles.
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 rotation of a vessel about its transverse (side-to-side) axis. An offset or deviation from normal on this axis is referred to as 'trim' or 'out of trim'.
Roll
is the rotation of a vessel about its longitudinal (front/back) axis. An offest 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 rotation of a vessel about its vertical 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 can 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