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William Roeseler & David Culp
The catamaran's performance is similar to the skier's with three exceptions. 1) The hydro-dynamic sideforce generating devices (daggerboards rather than inclined planing surfaces) are more efficient at low and moderate speeds, resulting in higher efficiencies and thus higher Vb/Vt ratios. This advantage seems to be lost at higher speeds as the daggerboards' roots reach the surface, lose the hulls' end plate effect and begin to ventilate. Also the catamaran's slightly higher wetted surface/weight ratio at speed results in significantly lower power to drag ratios. 2) The skier's inclusion of his own body into the sailing structure facilitates very quick and fine adjustments to the the craft's aero-hydrodynamic force balance as the skier literally "feels" for the fastest course through wind and waves. This, we believe, is a major component of boardsailing's current hold on most sailing speed records. 3) The catamaran can be made truly non-heeling by taking the kite line attachment point to the leeward rail and raising the leeward centerboard. The aero- and hydro- forces can be made to exactly align.
The skier cannot precisely align these forces as he is limited by 1) the vertical distance between the kite attachment point (his waist harness), and the skis' bottoms; and 2) by the skis' inability to generate any downward lift. When overpowered, Roeseler must fly, sometimes dramatically, to leeward at a dead loss to his windward ability. The catamaran's sail-carrying ability is limited only by the strength of the structure. Parenthetically, as the absolute value of the aero- and hydrodynamic sideforces increase to very large numbers, parasitic drag sources such as wetted surface, strut and appendage drag and air drag of the pilot and kitelines become relatively trivial. The overall efficiency of the sailcraft approaches that of the theoretical rig and theoretical daggerboard efficiencies alone.
It is possible to "use" some of the power from the rig to provide vertical lift to the hull(s), unweighting them and reducing hydrodynamic drag. This is done through a slight and careful misalignment of the sideforce couple, giving a vertical resultant. It requires near perfect control of both kite and hulls as a slight perturbation in such large forces at speed will result in either capsize or flying the craft, neither of which is typically survivable. To our knowledge, this has only been successfully attempted with an inclined-rig "conventional" sailcraft. (Ref. 6)
In comparing the catamaran and skier, it seems that the boat is more efficient and thus faster at low and moderate wind speeds, while the skier is faster and more survivable (given an accomplished athlete) at higher windspeeds, particularly off the wind. The catamaran currently goes to windward better than the kiteskier. Whether these relationships will hold true as both projects continue is a point of contention between the authors. Time will tell.
There are two schools of thought on kitesailing development. Ian Day, Dan Eisaman, Lee Sedgewick, and others are working within existing kite, hull and controls technologies to maximize actual kiteboat efficiencies and boat speeds, both on and off the wind and on both closed courses and blue water. Costs are reasonable and progress is rapid, but performance limitations, particularly with commercially available kites, are likely to curtail real breakthroughs.
Giles Durand, Bruno and Dominique Legaignoux, James LaBouchere, and others are taking a basic research and development approach, designing and building purpose-built, efficient kites from scratch and using them to pull sometimes very innovative hull platforms. Progress is slow but impressive. We believe that the sport worldwide will soon see dramatic breakthroughs in both technology and performance.
We have traced the development of kitesailing through history, discussed the kitesailing adventures of the authors over the past ten years, and hinted at what the future might hold. The Ancient Interface of wind and waves has been tamed by many varied creations, none stranger than the Big Boat from New Zealand, the winged catamaran that beat her at sea, the modern sailboard that beat the catamaran in the Canary Islands in 1986, or the flying kiteskier that beat the boards at "The Gorge" in 1989. If we have stimulated your curiosity, then we have been successful.
Wm. G. (Billy) Roeseler spent 22 years in the Structures Staff at Boeing after receiving his Master's degree in aeronautics and astronautics at MIT in 1966. His sailing research began in 1975, towing his young son, Cory, behind his Hobie 18 on water skis. This led to his first kitesailing paper in 1979 at Ancient Interface and to a very large kite in 1979 consisting of a pair of Hobie 18 rigs and a small hydrofoil boat. The first stunt kite came in 1987, and the father/son team has been sailing faster and faster since then.
Billy has 6 children and two grandchildren and lives with his wife, Brook Burnell, on San Diego bay, commuting by motorcycle to work at United Technologies Advanced Systems division, where he is Section Head for Structural Analysis.
Dave Culp attended Stanford University, the University of California at Davis and the NAEBM/Westlawn School of Yacht design.
Mr. Culp's design interests include all aspects of high speed sailcraft design; also commercial sail, both retrofit and new construction; and fast cruising yacht design. He has specialized in kite rig and free flying kites since 1977, when he was commissioned to design a kite propelled speedsailboat for the 1978 Schmirnoff World Speedsailing Trials at Weymouth, England. Culp-designed kiteboats were entered at Weymouth in 1978-82, '86 & '87. Mr. Culp publishes Kitesailing International, a bi-monthly newsletter devoted strictly to kitesailing and other forms of traction kiting.
Mr. Culp makes his home near San Francisco Bay in California with his wife Shelley
and three children.
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