Ship motions


Ship motions are the six degrees of freedom that a ship, boat, or other watercraft can experience. These can be affected by the ship’s own propulsion as well as by the water and air through which it moves, which move independently of the ship.

Rotational

[Image:Rotations.png|thumb|upright=1.35|Axes of a ship and rotations around them]
There are three special axes in any ship, called longitudinal, transverse and vertical axes. The angular movements around them—affecting the ship's moment of inertia, which sets the torque it requires to rotate in any direction—are the ship's rotational motions, known as roll, pitch, and yaw respectively.

Roll

The tilting rotation of a vessel about its longitudinal/X 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. The rolling motion towards a steady state angle due to the ship's own weight distribution is referred in marine engineering as list. List normally refers to an unintentional or unexpected offset, as caused by flooding, battle damage, shifting cargo, etc.

Pitch

The up/down rotation of a vessel about its transverse/Y axis. An offset or deviation from normal on this axis is referred to as trim or out of trim. A vessel that is pitching back and forth is usually termed to be hobby horsing.

Yaw

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. This is referred to as the heading of the boat relative to a magnetic compass ; it also affects the bearing.

Translational

[Image:Translations.PNG|thumb|upright=1.35|Translations]
A ship can also move linearly along any of its axes—longitudinal, transverse, and vertical. These translational motions are known as surge, sway, and heave respectively.

Surge

The linear longitudinal motion imparted by maritime conditions, usually head or following seas, or by accelerations imparted by the propulsion system.

Sway

The linear transverse motion. This motion is generated directly either by the water and wind motion, particularly lateral wave motion, 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 compared to the vessel's lateral drift from its course.

Heave

The linear vertical motion; excessive downward heave can swamp a ship.

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.