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Why Kites?

While kite powered sailing is obviously an unusual, exciting new approach to sailing, what's the advantage? Why put up with launching hassles, light wind problems, and traffic nightmares to fly these odd prime-movers? Put simply, it is because they are capable of far more power than conventional rigs. Kitesail boats are potentially the fastest form of soft water sailing known to man.

There are three major advantages to kite rigs as compared to conventional sailing rigs. First, since a kite flies some 50-150 feet above the water, it works above the turbulent boundary layer of wind over water that conventional rigs must deal with. This provides cleaner, less turbulent air flow at significantly higher velocity than on the surface, as much as 15-30% higher. As power derived from the wind varies with the square of the wind speed, 25-70% more energy is available to the kite, all other variables being equal.

Second, as any boat and its rig increases speed, the apparent wind both draws forward and increases. Very efficient boats use this effect to "make their own wind." As actual airflow over the rig increases, power derived from the rig increases dramatically, allowing further acceleration, again increasing apparent wind, and continuing the cycle. Efficient boats under optimum conditions are able to attain boat speeds 2--2 1/2 times the true wind speed. However, a conventionally rigged boat must accelerate both the hull and rig together to gain this additional power. Unless the boat is very efficient at low speeds and optimized for high speed as well, the limited power available will be insufficient to accelerate the boat into high speed regimes.

A kite rig, which is independent of the hull, can accelerate to several times the wind speed before the hull begins to move (and thus the characteristic zig-zag or figure eight course of the kite stack). The kite rig is often capable of generating 4-8 times the force of a conventional rig of the same size at zero hull speed. This effect is in addition to the increase in power due to altitude. While this advantage decreases as hull speed increases, it is very useful at slow speeds to, for instance, bring a planing hull or hydrofoil supported hull up onto its feet. Also, at practical boat speeds (up to 2x windspeed), an efficient kite is still capable of exceeding boat speed and thus the effect is still very beneficial.

Third and most dramatic, the tensile force from the kite rig can be applied to the hull at any location on its surface. By moving the attachment point to the leeward rail or even the leeward waterline, a boat can be built which does not heel. This theoretically allows a designer to put a very large rig on a tiny hull platform with minimal regard to stability. In practice, it is possible to precisely balance rig forces and hull sideforces to result in no residual pitch, roll, or yaw moments; only pure forward drive!

This means that the boat can be made self-steering without rudders. It needs no fixed or live ballast, no transverse displacement of buoyancy, and no reserve buoyancy at all. As speed and thus rig/hull sideforce magnitudes increase, the effects of gravity and wind waves become relatively trivial, and the boat becomes more and more stable. It feels like it's "on rails" even in extreme wind and sea states.

The kitesailing story isn't all milk and honey, of course. There are very real practical considerations. The extremely large overall size of the boat--the rig is as much as 200 feet away from the hull--leads to handling difficulties even in relatively uncrowded waters, not to mention bridge clearance. Currently available kites, while marvelously efficient and strong, are mostly incapable of launching or landing on water. They require either a beach launch or tender assisted launch and relaunch. Actual sailing is tricky and arduous if shorthanded, as the skipper's attention is divided between flying the rig and sailing the hull.

Kitesailing is truly an emerging technology sport. There will be huge advances in the near future in both technology and technique in this exciting new field.

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dave@dcss.org