A planing boat's hull skims across the surface of the water rather than pushing through the water in the way a traditional displacement hull works. In the U.S. the term Hydroplaning is sometimes used instead of planing, but primarily to describe racing motorboats which plane. Hydroplaning is also an important concern in car safety (see Hydroplaning (car)).
Planing allows the boat to go faster by using its speed and hull shape to lift the front part of the hull out of the water. The boat travels on top of the water, greatly reducing the hydrodynamic drag on the vessel. The increase in aerodynamic drag is small by comparison, and can be compensated for by the increased power from the sails due to the faster speed of the craft, and by the crew trimming the sails.
Planing is usually a sailing term, but other watercraft are designed to plane to some extent including most small and some medium size powered boats. A rigid-hulled inflatable boat is an example of a powered hydroplaning craft. Hydroplane is sometimes used as a noun synonym for a hydrofoil, a motorized boat designed to hydroplane on special wings attached to the keel. The principle of a keel wing to lift heavier boats out of the water more efficiently has recently been translated into a surf board model whose wing-shaped skeg allows planing at very low speeds (about 4 knots according to one talk-show enthusiast).
1 External links
History
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, the 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.
Obviously this performance had an impact: other designers took on his ideas and developed them. Over the years, most dinghies have acquired some ability to plane, and there are now many high-performance dinghies (usually called skiffs, see these examples, or these in [1]), which will plane even in light winds, at all points of sail.
How planing works
Normally a non-planing, displacement, hull is restricted in its maximum speed by a formula related to its overall length 
, where HSPD is maximum hull speed, and LWL is the hull length at waterline). This speed is maximised when the boat sits between the bow and stern waves, with no intervening self-caused waves along its length.
At low speeds, a hydroplaning hull acts as a displacement hull. But, when the speed increases the hull begins acting as a planing hull. However, when the boat begins to plane the formula becomes irrelevant since the boat is climbing its own bow-wave. The bow rises slightly as it starts by mounting its own bow wave. When it reaches the speed where it overtakes the bow wave, the bow resumes its normal attitude. The boat can often be seen to leave its stern-wave some distance behind it. The hull is now planing.
Beginning to plane is the aquatic, and less dramatic, equivalent of an aeroplane breaking the sound barrier. The aeroplane at Mach 1 begins to pierce and go beyond its own 'bow wave', i.e. the compressed layers of air on its front surfaces and ahead of it.
A hydroplaning hull travels faster and more efficiently than a displacement hull of comparable size due to two factors:
- less area of the hull is in contact with the water. This reduces the friction on the hull caused by water.
- the hull is displacing less water from its path. Water is relatively heavy and the hull must displace its own weight of water.
The characteristics of a planing hull are that it is narrow at the prow, with a broader beam towards the rear. The shape of the underneath of the rear of a larger, planing, powerboat is often V shaped. To plane, the power to weight ratio must be high; sailing boats need a good sail area and powerboats need a highly powered engine.
Note that under some high wind conditions, very light craft (such as windsurfers and kitesurfers) can actually be pulled up onto the surface of the water, or into the air, by the upward lift of the sail alone. Although this certainly reduces water resistance, it is probably better described as flying, rather than hydroplaning, which requires forward velocity to exploit the shape of the hull.
How to plane in a sailing boat
Planing can happen in a suitably designed boat in moderate to strong winds if the crew do some or all of the following:
- Sail on a reach or broad reach to begin
- Slacken the jib
- Raise the centreboard
- Increase the speed
- Keep the hull level, trapeze if necessary
- Observe the wake until it is smooth and fast
- Move the crew weight increasingly towards the rear to begin and to sustain planing
- Sheet in as speed increases, and apparent wind correspondingly changes
- Keep the boat flat and level
- Bear away to maintain speed as necessary
- Flick or pump the sails
While planing, keep control of the waves and steer through them, avoiding to increase speed to collide with the wave in front. Also, in dinghies, keep good control of the sail power. A small change in wind direction can easily cause a capsize, watch also out for gybes. Boat control becomes easier as planing begins, but fast reactions are often needed to get there, to keep the speed up and to keep the boat level. Crew balance and trim are vital, as are sail trimming and minimal centreboard.
See also
Dinghy sailing, Windsurfing Planing
External links
Information on boat design
Some videos of planing sailboards
