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

See also: Aircraft principal axes

Vertical axis

Vertical axis, or yaw axis — an axis drawn from top to bottom, and perpendicular to the other two axes. Parallel to the fuselage station.

Yaw axis is a vertical axis through an aircraft, rocket, or similar body, about which the body yaws; it may be a body, wind, or stability axis. Also known as yawing axis.

The yaw axis is defined to be perpendicular to the body of the wings with its origin at the center of gravity and directed towards the bottom of the aircraft. A yaw motion is a movement of the nose of the aircraft from side to side. The pitch axis is perpendicular to the yaw axis and is parallel to the body of the wings with its origin at the center of gravity and directed towards the right wing tip. A pitch motion is an up or down movement of the nose of the aircraft. The roll axis is perpendicular to the other two axes with its origin at the center of gravity.

Lateral axis

Lateral axis, transverse axis, or pitch axis — an axis running from the pilot's left to right The lateral axis passes through the plane from side to side. Rotation about this axis is called pitch. Pitch changes the vertical direction the aircraft's nose is pointing. The elevators are the primary control of pitch. Also called Transverse axis.

Longitudinal axis

Longitudinal axis, or roll axis — an axis drawn through the body of the vehicle from tail to nose in the normal direction of movement, or the direction the pilot faces. Parallel to the waterline.

The longitudinal axis passes through the plane from nose to tail. Rotation about this axis is called bank or roll. Bank changes the orientation of the aircraft's wings with respect to the downward force of gravity. The pilot changes bank angle by increasing the lift on one wing and decreasing it on the other. This differential lift causes bank rotation around the longitudinal axis. The ailerons are the primary control of bank. The rudder also has a secondary effect on bank.

Rotation motions

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.

Roll

is the rotation of a vessel about its longitudinal (front/back) axis

Pitch

is the rotation of a vessel about its transverse (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, anti-roll 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