For the first time in history, a kite powered water skier will be an official entry among the world's fastest sailors and sailcraft at the Johnnie Walker International Speed Sailing Trials at Portland Harbor, Dorset, England this year.
Cory Roeseler, 18 year old expert division competition water skier and student from Kirkland, Washington State, USA, only began kiteskiing little over a year ago under the tutelage of his father, aerospace engineer Bill Roeseler, also of Kirkland. However, the father-son team have worked for a number of years both conceptually and experimentally in the field of kitesailing.
For his speedsailing efforts at Portland, Roeseler will eschew specialized custom gear in favor of proven, off-the-shelf components. His kites will be Flexifoils®, either stock Super Tens or special order 12'6" kites, flying with lightweight carbon fiber Skyrods® spars. His flying lines will be 500 pound woven Spectra® fiber leading to a 6' aluminum control bar. Cory hooks into this bar using standard boardsailing harness lines and a waist hook. His skis are Jobe® competition jumping skis with high-wrap water ski bindings. Roeseler chose double skis over single skis for their superior recovery ability under extreme conditions, their easy planing, and superior windward ability.
Roeseler brings impressive credentials to Portland Harbor. He finished second at Washington State's Green Lake Open waterskiing competition this summer, and while kiteskiing placed 47th of 187 top boardsailors at the 20 mile Gorge Cities Blowout in Oregon, USA. This grueling one and a half hour event is a severe test of both ability and endurance.
While the Gorge Blowout is a downwind race, Roeseler knows that it is high speed windward ability and thus high overall aero-hydrodynamic efficiency of a sailcraft which produces the very high speeds necessary to break world records. "I prefer jumpers over slalom skis, and have sailed fast on true courses as close as 85° to wind," asserts Roeseler. (Competition speedsailing courses are set at 110-125° to the wind.)
For sailing neophytes, a course of 90° true when sailed at the same speed as the wind (Vboat = Vwind) yields an apparent wind angle of 45°, an impressive feat for a craft with no centerboards and only vestigial skegs.
See "Kitesailing Upwind: Fact or Fiction?" elsewhere in this issue. Roeseler has reportedly hit maximum speeds of 40 nautical miles per hour, in 30-35kts. of wind, and has been reliably radar timed above 25kts. in much lighter winds. Whether he will have the winning combination of fair winds, skill, and luck at Portland Harbor remains to be seen. Watch Kitesailing International for results!
"It can't be done, right? I mean, like, the kite's on the end of the string, right? And the whole thing's downwind of the boat, right? Sail upwind? It can't be done!" Right? Wrong!
The same laws of physics that allow an America's Cup yacht to sail upwind govern a kite powered boat's performance as well. And just as with conventional sailboats, it is the over all aero- and hydro-dynamic efficiency of the rig and hull that determines how close the boat sails to the wind and how fast.
Let's walk through it a step at a time, conceptually. First, we'll assume that we're flying a simple single line kite. It always flies dead down wind of the hull. Let's assume the hull also moves dead down wind, following the kite (see fig. 1). For any given wind speed, the boat and kite will sail, in equilibrium, at a fixed speed somewhat slower than the true wind speed. The kite "sees" an apparent wind which is equal to the true wind speed minus the boat's speed. (Apparent wind is the wind that a boat's rig "feels." It is affected by the boat's speed and also its direction of travel relative to the true wind.) If the hull is draggier, it will slow down. The apparent wind seen by the kite will increase, making the kite pull harder and finding a new equilibrium. If the hull is slipperier, it will speed up. The kite sees less apparent wind and the pull drops off, sometimes to the point where the kite drops into the water.
Now, let's keep the same kite, but put a reasonably efficient centerboard and rudder on the hull and turn the boat to an angle to the wind of say, 60° from downwind (fig. 2). Logic tells us that this is still an acceptable course (the boat will "sideslip" a bit, referred to as its "leeway angle," but let's not muddy the drawing up). In fact, we might even pinch the course a bit closer to the wind, maybe to 70° or so, just as long as the kitelines' angle of attachment to the hull is less than 90° off its bow, which is the theoretical limit. Let's leave it at 70° as a practical limit. (Sailboat courses are usually referenced from the wind's origin, so this course would be referred to as "120° off the wind.")
Notice what our apparent wind is doing now (fig. 2a). If the boat speed is fairly fast, perhaps half the wind speed or a bit more, the change in apparent wind is seen more in its direction rather than in its velocity as with the example sailing downwind (vector sums are a little harder to conceptualize, but aren't absolutely vital to our understanding of windward sailing. Try tying a piece of yarn to your car's radio aerial and driving across the wind at varying speeds). As the boat accelerates to a respectable fraction of the wind's speed, the apparent wind's velocity doesn't drop much, so pull from the kite is little affected. However, the apparent wind's direction "draws forward," forcing the boat to sail a course further from the true wind. (It is a fundamental rule of high speed sailing that, to sail close to the wind, the boat must slow down.)
OK, so far we're not sailing upwind, but we've established that an efficient hull can move forward as long as the kitelines' angle is no more than about 70° off the boat's bow. Now let's introduce stunt kites.
While standing still, it is possible to fly a two, three, or four line stunt kite anywhere from dead downwind, to nearly overhead, to about 80° out to either side. As the kite approaches its limit to either side, the pull drops off, but it will pull hard quite near this limit. In our example, let's say the kite pulls acceptably hard out to 65° from downwind, or using the sailor's jargon, "115° from the wind." (fig. 3Ñwe'll simplify things again by only considering the horizontal componentÑa top viewÑof the kite). The kite can be "parked" at this angle. Its pull is constant and for the sake of argument it can be considered a "skyhook" with a fixed angle relative to apparent wind and a pull which varies with the speed of the apparent wind.
Now let's put it all together (fig. 4). Put the stunt kite flier aboard the boat and "park" the kite at a 115° angle from the wind. We now orient the hull 70° higher that the kitelines' pull and "viola!" we see that we are sailing at an angle of 45° from the wind, upwind.
For those who didn't get lost in the earlier apparent wind explanation, we'll note that if the boat accelerates on this course until its speed is high in relation to the true wind speed (fig. 4aÑwe'll assume that boat speed equals true wind speed), then the course will remain at 45° to apparent wind, the the apparent wind velocity will be 1.41 x boat speed and the true course will be 90°, or at right angles to the wind. This is a condition which approximates speedsailing courses and shows why, with very high speed sailboats, a true course across the wind results in an apparent course upwind. This is why high efficiency to windward is so important for very fast sailing.
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 warterline, 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 boatÑthe rig is as much as 200 feet away from the hullÑleads 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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