Ship motions

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Ship Motions

A vessel in a waterway experiences dynamic motions in six degrees of motion. Stresses can also induce hull deformation.

Contents

Longitudinal Axis

The longitudinal axis is measured along the length of the ship (from fore to aft). A rotation about this axis is known as list or roll. Motions along the length of the the axis are known as surge.

Transverse Axis

The transverse axis is measured across the beam of the ship (from port to starboard). A rotation about the transverse axis are known as pitch. Motions along the length of the transverse axis are known as sway.

Vertical Axis

The vertical axis is measured from the top of the ship to the keel. A rotation about the vertical axis are known as yaw. Motions along the length of the vertical axis are known as heave.

Designing for Ship Motions

Roll

The tendency to roll is referred to as the stiffness of a boat - boats may be considered stiff, or tender. There are a number of ways to control roll such that the roll period is comfortable for passengers, or to limit roll for operational requirements. Systems can be active or passive, and include skegs, bilge keels, stabilizing fins, and/or anti-heel heel systems and anti-roll tanks.

Sway

Sway is a particular concern for sailing vessels, which can use keels, leeboards, bilgeboards, daggerboards, or centreboards to reduce this effect, often attempting to convert lateral motion (sway) into forward thrust (by producing lift, i.e., the use of foil sections).

Yaw

See main article, Yaw Yaw can be induced by control surfaces, such as rudders, by propulsors such as Voith Schneider drives or azimuthing propulsion, or by the environment. Alternatively, vessels can be designed to decrease susceptibility to yaw, such as skegs or larger L/B ratios.

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