Ship motions

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

Translation

"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

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

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 (front/back) axis

Roll

is the rotation of a vessel about its longitudinal (side-to-side) axis

Yaw

is the rotation of a vessel about its vertical axis

Stabilization

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

• 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