Planing (sailing)

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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.

The term 'planing' refers to a craft which is predominantly supported by hydrodynamic lift, rather than hydrostatic lift (buoyancy). The extent to which a boat is said to be planing is usually derived from the dimensionless Froude number.

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, [http://www.18footer.org/ see these examples] , or these in [http://www.skiff.org.nz/] ), 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 $HSPD\; =\; sqrt\{LWL\}*1.34$, where HSPD (in knots) is maximum hull speed, and LWL is the hull length in feet 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. When the speed increases the hull then begins acting as a planing hull. 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 nautical equivalent to an aeroplane breaking the sound barrier. The aeroplane at Mach 1 essentially catches up to the compression waves ahead of it, which coalesce into a shock wave.

A hydroplaning hull travels faster and more efficiently than a displacement hull of comparable size due to two factors:
# The wave drag is diminished. When a hull is displacing, the imbalance of high pressure at the bow and low pressure at the stern, or on the transom, produce drag. The imbalance is caused by the bow wave that stands above the ambient level of the water and stern wave that forms a hollow in the water.
# Less wetted area. This reduces the skin friction on the hull caused by 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. It is also not a sustainable state, as sailing (or kite flying) involves the extraction of energy from the shear force between the wind and the water. If the entire hull leaves the water, the craft will quickly come to rest relative to the wind, and lose its lifting/driving force.

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 moves forward
*Keep the boat flat and level
*Bear away to maintain speed as necessary
*Flick or pump the sails (although there are restrictions on doing this in a race)

While planing, it is important to steer through the waves, avoiding any collision 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 or 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.

ee also

* Dinghy sailing
* Windsurfing
* Planing

External links

* [http://www.boatbuilding.com/content/ratios.html Information on boat design]

* [http://www.ukwindsurfing.com/pics_n_vids/ Some videos of planing sailboards]

* [http://naca.larc.nasa.gov/reports/1958/naca-report-1355/ Seminal 1958 NACA technical report on hydroplaning]