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A number of pioneering thinkers and experimenters have written about the application of kite systems for the propulsion of boats, principally in the pages of the Amateur Yacht Research Society publications 29 30 32 and the Ancient Interface Conference Proceedings18 21 42. Hagadoorn and Roeseler29 8 suggested kites may improve sailing performance. A first reference of kite-sailing in a conference dedicated to commercial sail is given by Nance11. Schaefer and Allsop12 presented the first scientific paper on kite-sails for wind-assisted ship propulsion at a well-attended symposium on wind propulsion of commercial ships in London in 1980. Other more well-known propulsions systems were also discussed in considerable detail:

traditional square riggers, modern fore-and-aft rigs using automatic sail furling soft and hard wing sails, wind turbines magnus-effect rotors (Flettner, Thom, aspirated cylinder).

Already apparent then, with decreasing oil prices eroding the beneficial shock of the so-called oil crisis in 1973, was the importance of low cost sails. The AIAA/SNAME conferences in San Francisco in 198217 18 and in Long Beach, California, in 198421 indicated ship owners and shipping companies are not likely to embrace sails, especially kites, unless based on proven technology with a large potential for cost reduction, minimal investment and no extra crew. Another similar symposium, Windtech '85, was hosted in Southampton, England24 27. Here there were several papers on kite propulsion and Duckworth presented perhaps the only serious work14 26 by a major ship owner (British Petroleum) to investigate kite propulsion. The BP team tested and measured several large kite systems and installed some of the simpler ones on a small research vessel. These were of the parachute type with a L/D ratio of 1, rigged in chains and launchable one by one, the first from a compressed air cannon. These kites could be steered 40 degrees to either side by a remote-controlled weight shift system which has also been used successfully by Stewart25 and Schmidt10 13 15 19. Although the tests were successful, Duckworth concluded that scaling up to ship size would be so daunting as to be commercially non-viable. In particular, it was feared that any loss of control during a wind lull would mean irretrievably abandoning the entire deployed equipment for safety and operational reasons. High efficiency or even dynamic kites were thought to be even worse in this respect and so "far-out" to be entirely unacceptable to ship owners.

Roeseler in 1984 invested $50,000 in a 46 ft, 10 ton research vessel "Tonto Maria" and fitted her with fuel flow instruments, knot meter, and sails. She demonstrated 30% fuel saving in 10 kt favorable wind. Also during this time Air Commodore Nance bought BP's research Vessel "Assessor" and with the help of Schmidt installed a launching and retrieval system for stacks of Flexifoil Power kites7. This unique type of kite has a single flexible spar but is essentially a ram air inflated wing capable of speeds up to 100 kts in winds of only 20 - 30 kts and corresponding great forces. Its greatest disadvantage in this application is the required large amplitude and rate of control movement in the two lines, meaning either sophisticated and expensive line handling equipment, or in the case of simple winches, very fast acting ones. It was indeed found possible to deploy and retrieve 4 meter span Flexifoil kite stacks on Assessor, but impossible to fly them dynamically because of this problem. Another unresolved problem is the control system. While a human can control the Flexifoil kites for up to several hours, this is extremely fatiguing work once the initial sporting sensation has worn off. Efforts were made to investigate the properties of appropriate electronic control systems, but at this point the project was terminated as much more effort and funding would have been required to proceed further. The technical problems described were thought to be solvable. The Achilles heal of this and most heavier-than-air kite systems was thought to be the extreme difficulty of retrieving kites in sudden lulls. (Of course, the ship's speed and/or course may be altered in these infrequent lulls to keep the kite up, but this will not be popular, especially if there is traffic.) Only auto-rotating, powered and free-fly able, or lighter-than-air kites were thought to be more or less immune from this severe problem.

Also around this time, Englishman James Labouchere built a kite-boat using an entirely rigid kite. Although model tests had been successful, he did not succeed in getting the system to work at full scale. We suspect the weight of the kite was above one pound per square foot (psf), and that it wouldn't fly below 15 or 20 kts of wind. Hence the probability that the wind never exceeded minimums during the short time period he was able to devote to demonstration, so the possibility of a successful kite sail run was taken away by Mother Nature.

We have had many similar experiences since 1980. One of them was at Ancient Interface in 198523 when we actually launched our 200 lb kite with teenage pilot and our 400 lb Dynafoil (personal water craft) with two teenagers onboard. We failed to achieve stable flight in front of a dozen or so interested observers. Our goal that day was to tow the kite up behind the 40 hp powerboat in less than 10 kts of smooth air, then maneuver the kite off to the lee of the foiler and motor sail as we made our way from Seattle to Blake Island some 5 miles to the west in Puget Sound. A year later we did achieve stable flight with the same 400 square foot kite which measured 56 ft from tip to tip. We never did get the kite to tow the little 400 lb hydrofoil boat, but we learned a lot about how much it might cost to make it happen. We had invested $5000 in the commercial off the shelf (COTS) Dynafoil and $20,000 in the Hobie 18 rigs that we lashed together with pop rivets, titanium tubing from Boeing Surplus, and a few hundred feet of light guage stainless wire. Shop facilities were donated by the Flight Research Institute and a friend who lived on Lake Washington. We came at least half way to our goal with less than $100,000 in 1996 dollars and less than 1000 hrs of volunteer labor.

A few years later in 1991 we invested another $50,000 trying to get the same kite to tow a larger foiler made from a Capri 22 monohull. We were even less successful that time, never even achieving stable flight of the foiler under power, but we did learn that bigger boats cost a lot more, and older youth are more difficult to coax into these projects without near term financial reward. In 1995 we finally did realize Hagadoorn's dream by towing an Air Chair (a popular hydrofoil toy designed in 1985 by Mike Murphy and Bob Wooly for riding behind a ski boat) with a production Kiteski system.

During these same years, Ketterman was enjoying much more success with his Longshot Trifoiler (Fig 5) assaulting the unlimited sailing speed record above 40 kts. The sails on this boat were derived from Jim Drake's Windsurfer after 1,000,000 smart sailors had invested 10^8 hours and $10^9 improving on the basic concept. At the same time a dozen or more large companies invested $10^8 in gas powered garden tools to supply the demands of a $10^9 world market for chain saws and grass trimmers. A company in China invested $10^6 in development of a better hand truck for moving furniture, and now the price of a set of pneumatic tires, wheels, and ball bearings with 600 lb capacity has dropped by an order of magnitude from what was available 40 years ago when we were building go carts and motor scooters.

We could now build a kite based on this new technology. It would have a span less than 40 ft and a wing area less than 200 square feet, but it would weigh only 20 lbs, and it would fly very nicely at 10 kts. We could add a $50 30cc Weedeater motor, a $300 Israeli autopilot, and $20 pneumatic tires from China, and we could have ourselves a very capable little UAV kite for less than $3000. Such is the pace of COTS product development over just the last 10 years. That is a unit cost of just $100/lb or $15/ square foot of wing area. ($15 psf) This cost for a highly specialized UAV based on mass produced COTS components beats the best military systems by at least an order of magnitude. Fueled by demand from recreational sailors, gardeners, and people on the move, these COTS building blocks may now be used to create still more useful products and systems to address the transportation and environmental needs of our day.

If Labouchere had the motor, landing gear, and autopilot on his rigid wing kite at the speed trials in Portland Harbor ten years ago, he may have been able to put on quite a show for the assembled British royalty, the Grogonos, and other members of the AYRS. With a motor and wheels to get started, a rigid wing of more than 1 psf unit mass could indeed be flown, and could extract more than ten times the power of its engine from a 10 kt wind. We suspect Labouchere may indeed have been capable of sailing at speeds up to perhaps 30 kts in a relatively modest wind of less than 15 kts. The power required may have been of the order of 30 hp, or ten times what would be available from the $50 motor.

Inflated wings represent another unique category of semi-rigid kites. Using a buoyant gas, the problems associated with light winds disappear, to be partly replaced with operational difficulties in high winds. Englishman Keith Stewart developed a number of such kites and with Culp28 and Schmidt20 44 experimented with on-board launching systems on small boats. These systems were entirely successful in light or moderate winds and some were steerable on a single line using radio control. Besides boats, such kites were also used with hull-less hydrofoils, some submergible, also radio-controlled, making an extremely basic and low-cost sailing system useful for example for oceanographic data gathering.

The last ten years have seen little or no progress in kite system research for ships or even boats, but considerable activity in sporting applications, notably kite skiing on water and snow and kite-buggy racing on land. Several Arctic and Antarctic expeditions have kite sailed thousands of miles pulling heavy sleds, all using the German Beringer parawing system, soon to be covered by a book about kite traction systems. Another similar short-line system developed by German Strasilla allows the instantaneous switching from sailing to flying mode, allowing a skilled pilot to sail up and across mountains and take off at will. New Zealander Peter Lynn has shown his traction kites and buggies at hundreds of kite festivals all over the world.

Reviewing all the above, it is seen that the most successful kite-sailing systems are those which are entirely manageable using the strength of one person. Larger systems would only be feasible using automated handling and flying equipment and so far very little work has been done here. There is a good case that such systems could prove economical in spite of the cost of such specialized equipment. The available small-scale kite-sailing equipment is considerably cheaper than conventional sailing equipment of the same power, even though the former is manufactured in far smaller quantities than the latter. As an example, the custom-made Flexifoil stack and associated equipment of one-time C-class speed-sailing world record holder "Jacob's Ladder"16 was cheaper than the standard Tornado rig it replaced or the wing sails37 used by other C-class racers.

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