to plane

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A term used to describe the action of a boat which attains sufficient speed to cause the forward part of the hull to rise and for the boat then to run along the surface of the water. In order to start planing the hull must have a suitable form and be very light in weight in relation to its sail area or power available. Power boats with a V-sectioned bow and a broad flat hull are noted for their ability to start to plane above a certain speed, and to skim along the surface with only the after part of the hull and the propeller and rudder in the water.

Lightweight, high-performance racing dinghies, given suitable wind and sea conditions, can get up and plane for shorter or longer periods depending on the continuing strength of the wind and the skill of the helmsman. While planing, a boat's speed can rise to twice or even two and a half times the theoretical maximum sailing speed of a displacement (normally heavy) boat of the same length obtained from the speed formula 1.4 times the square root of the waterline length in feet (). Thus a dinghy with a waterline length of 4.9 metres (16 ft) has a theoretical maximum speed under sail of 5–6 knots if it cannot plane, but if it is of the planing type with a sufficiently high power–weight ratio, under the right wind and sea conditions its speed may rise to 8 or 9 knots, when it will surge along the surface with its speed rising in bursts to 12 or even 14 knots. While the boat is poised in this way on the surface, the tiller feels almost rigid in the helmsman's hands, and great skill is needed to prevent a violent sheer to one side or the other, and a sudden capsize. A powerboat (see yachting, power), on the other hand, is usually quite stable as the thrust driving the boat is beneath the water surface, and the flat form of the underwater body enables the boat to be steered in sharp turns without much chance of a capsize.

See also hydrodynamics.

See also hydrodynamics.

Subjects: Maritime History.

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