Planing (boat)

Planing is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift (buoyancy).

The earliest documented planing sailboat was a proa built in 1898 by Commodore Ralph Munroe; it was capable of speeds of more than twice the hull speed.

Planing a sailing dinghy was first popularised by Uffa Fox in Britain. In 1928 Uffa Fox introduced planing to the racing world in his International 14 dinghy, Avenger. It had been designed with a hull shape which permitted planing. He gained 52 first places, two seconds and three third places out of 57 race starts that year.

This performance was noticed by other designers who took on his ideas and developed them. Over the years many dinghies have acquired the ability to plane. Advances in building materials have allowed for lighter boats which will plane faster and in lighter air. There are now many high-performance dinghies (sometimes called skiffs) which will plane to windward.

When it is at rest, a vessel's weight is borne entirely by the buoyant force. At low speeds every hull acts as a displacement hull, meaning that the buoyant force is mainly responsible for supporting the craft. As speed increases, hydrodynamic lift increases as well. In contrast, the buoyant force decreases as the hull lifts out of the water, decreasing the displaced volume. At some speed, lift becomes the predominant upward force on the hull and the vessel is planing.

A simple model of this effect is a solid slab of material which is heavier than water (like a steel plate) but is shaped and oriented to have a positive angle of attack. At rest, the slab will sink because it is heavier than water; the buoyant forces are overwhelmed by the force of gravity. However, if the slab is kept in the same orientation and pulled horizontally through the water, it will force the incoming water downward. This results in a reactionary force upward on the slab. At a high enough speed, this reactionary force (plus any small buoyant force) is larger than the force of gravity and the slab will stay afloat. In this way, the horizontal force (which may be supplied by a motor or a sail) is converted into a vertical force upwards. The concept of planing is often interpreted as analogous with aerodynamic lift (See lift on an airfoil), but in reality the acting forces are very different.

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