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Kitesailing Progress

William Roeseler & David Culp

4.2 Overview: Hulls

Current hull platforms worldwide run from pure displacement catamarans and proas such as Ian Day's Tornado conversion or Keith Stewart's Culp-designed proas; to hydrofoil supported platforms such as Gilles Durand's O-PAF system or to Keith Stewart's Mayfly conversion, to purely planing hulls. Our own as well as others experience in high speed watercraft, both powered and sail, lead us to conclude that displacement hull drag due to skin friction becomes unacceptable at speeds above 30 kts. Ventilation and cavitation tend to limit practical hydrofoil performance to 40-50 kts. Only planing hulls show a flat enough drag curve to consider at very high speeds. While current record speeds are certainly within the upper limits of all hull types, we believe that the effects of these limits are being felt by displacement and hydrofoil platforms.

While Culp has raced both displacement and planing proas, both authors' current boats are planing catamarans. Culp's is a 14' x 7' fiberglass cat with single concave bottoms similar to high speed sailboard hull shapes; Roeseler's is simply a pair of competition jumping water skis. Culp's boat utilizes conventional daggerboards and rudders for sideforce and steering. Roeseler's is controlled entirely through inclining the skis' planing surface. While inefficient at speeds under 20 kts, this generates more efficient and predictable sideforce at 40+ kts in a chop than ventilation-prone conventional fins and hydrofoils in the turbulent air/water interface encountered at these speeds. The best evidence of this is the fact that fast sailboards often "spin out" when loading their fins in a chop, while the kiteskier, using a fully ventilated, one-side-only foil (the bottom of his skis), tends to leave the fallen board sailors behind.

4.3 Current Skysail Progress (Flexifoils, Spiderline, and Jobe brand skis)

A sailcraft will seek a speed where thrust equals drag. Thrust is the component of sail force along the flight path. Drag includes kite drag, line drag, pilot aerodynamic drag, and hull drag. The course angle to the apparant wind (beta) can be used to quantify the various sources of drag (as described by Marchaj in Ref 4). For instance, sailing directly into the wind gives a (beta) of zero, and only the most sophisticated arrangement of fins and wings will extract wind energy. Drifting downwind gives a (beta) of 180°, and any drag producing device such as a hull or bare sticks will extract wind energy.

Fast sailboats, including kiteboats, tend to operate within 45° of a beam reach. The initial (beta) for these courses is between 45° and 135°. Given (alpha) course sailed at 90° to true wind, with boat speed at zero; initial (beta) is 90°. As boat speed increases to equal true wind speed, (beta) must be reduced to 45° as the relative wind speed increases from Vt to 1.41 Vt.

Drag on the kite lines is significant, especially when line length exceeds 100 ft. We use Spiderline® from Catch the Wind, a special braid of Spectra fiber from Allied Chemical. It floats on water and is stronger for its weight than steel, providing minimum windage and maximum ease of handling.

In our 1979 "Ancient Interface" paper we suggested that a kiteboat could sail at 40 kts in a 10 kt wind. Beam reaching in those conditions yielded a relative wind speed of 41.2 kts and a required (beta) of only Tan-1 .25 = 14°. By using a wing and fin, each with L/D = 40, we proposed to balance forces, with wing drag yielding e(sub)A of just Tan-1 .025 = 1.43° and e(hull) the same. That gave us (14 - 2) x 1.43 = 11° for all parasitic aero- and hydrodynamic drag, hull drag, etc.

In the ensuing ten years, we have not applied an efficient wing or fin to the kiteboat, primarily due to lack of resources. We have contented ourselves with commercially available stunt kites and ordinary water skis. The L/D has been measured at 5 for the kite and 2-3 for the skier. Kite L/D is pretty flat from 20 to 80 kts, and skier L/D is pretty flat from 10 to 40 kts. We determined kite L/D with a radar gun and anemometer, recording 92 kts kite speed in an 18 kt wind. We measured skier drag with a spring scale towing a skier behind a power boat. By measuring the towline's angle relative to the towboat's course, we were able to determine the skier's L/D.

Referring to fig. 4), let's look at the approximate force balance on the kiteskier at the design point of 40 kts at 130¡ to a true wind of 30 kts. This is an actual point of sail that we have achieved with a 10 M^2 stack of Flexifoil kites and a pair of Jobe water skis. The apparent wind has components 40 cos 40° = 31 kts normal to the true wind and 30 - 40 sin 40° = 30 - 26 = 4 kts parallel to the true wind.

That gives an apparent wind of (sq root of)(312 + 42) = 31 kts at an angle of cot-1 4/31 = 83° to the true wind or 47° to our course. We know from spring scale drag measurements behind and to the side of the power boat at 40 kts that the resistance R is 80 lbs, and we know from speed measurements on the kites that L/D = 5. This allows us to estimate e(sub)A = cot L/D = 11°, and gives e(sub)H = (beta) - e(sub)A = 47 - 11 = 36°. The line force is R / sin e(sub)H = 80 / sin 36 = 136 lbs, which also agrees with actual forces estimated by the skier under these conditions.

This system demonstrates how it is possible to sail 40 kts with rather poor hull performance (80 lbs resistance for 200 lbs all-up weight and 110 lbs side force). The crosswind force coefficient on the kite is L/qS where:

L = crosswind force = 136 cos 11°¡ = 134 lbs.
q = dynamic pressure = V2/295 = 312/295 = 3.26 psf.
S = kite area = 100 sq ft. CL = 134/3.26 x 100 = .41.

Total hydrodynamic force normal to course is (sq root of)(2002 + 1102) = 228. Hydrodynamic L/D = 228/80 = 2.85. In this case the lines are assumed to be horizontal (kites near surface), and the kites are weightless. The skier and skis weigh 200 lbs. Roeseler sailed the Blowout in winds of 20 kts gusting above 40 kts, with over 100 square feet of sail aloft and less than 200 lbs total mass. This gave a Sa/Displacement ratio of 100/(disp) = 100 x 2240 / 200 = 1120, or roughly twice that of the Austrailian 18 or the 18 M^2 catamarans. Stars and Stripes, in her successful Cup defense in '88 carried 1900 square ft on 7000 lbs, for Sa/Displacement = 1900 x 2240 / 7000 = 608. Roeseler's displacement/length ratio was 200 / 2240 x 63/1003 = 413. His speed/length ratio was 40 / (sq root of)6 = 16. Both these are about one order of magnitude higher than conventional fast sailboats.

A reasonable goal for next year is to sail 50 kts in the same 30 kt wind (see fig. 5) by bearing up into the wind at 110° rather than 130°. This will increase the apparent wind to 50 cos 20° = 47 kts normal to the true wind and 30 - 50 sin 20° = 13 kts parallel to the true wind.

That gives apparent wind = (sq root of)(472 = 132) = 49 kts at cot-1 13/47 = 75° to the true wind or 35° to our course. With kite L/D = 5 as before, e(sub)H must equal 35 - 11 = 24°, and we will need to increase line force from 136 lbs to 230 lbs (feasible for the relatively short duration of a high speed run), with only a modest increase in resistance to 94 lbs (FS = 230 cos 24 = 210 lbs). The normal hydrodynamic force will be (sq root of)(2002 + 2102) = 290 lbs, so the hydrodynamic L/D must increase from 2.85 to 290/94 = 3.09. The kite crosswind force drops 30%, and line force increase is due to dynamic pressure increase. CL = 226 x 295/(100 x 492) = .28. Increasing the hydrodynamic L/D from 2.85 to 3.09 will require little more that switching from a pair of jumping skis to a slalom ski. We already have lots of experience on a slalom at 50 kts (behind power boats) and the line forces are not excessive. We also have lots of experience with line forces of 200-400 lbs, which are typical for slalom ski competition. The trick will be to get official times with 30 kts of wind and with wave heights below a foot so that the slalom ski can work efficiently. Thus we anticipate converting in one step from the world's fastest catamaran to the world's fastest monohull. Major obstacles to date are lack of radar signature at the Gorge Tech Speed Check and lack of judge's visibility in fog at Weymouth. The kiteskier returns only a small signal to the radar gun, which is designed for picking up automobiles in the freeway. It can pick up a 5 square meter sail at 100 yds., but it didn't pick up the kiteskier in Oregon in early August, 1989 until he was in the middle of his jibe away from shore at about 30 yds. The sailboards were getting radar speeds of 30 to 40 mph, and Roeseler, who was passing them easily out away from shore, recorded a best speed of only 27 mph. In frustration, he extended one speed run too close to shore, lost control, and had to jump over a rock the size of a house using kite lift to avoid injury.

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