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This paper is about the potential for economical transport sail craft, building on the technology developed in Europe, Japan, Isreal, China, New Zealand, and the US in the last 20 years. Kites have already towed small craft across oceans and continents, and the addition of electronic flight controls from the modern UAV industry may allow us to scale up this technology to a point where it becomes commercially viable for large ships.
Prior to the invention of the steam engine, transport sail craft were the main means of intercontinental transportation. Then for over 100 years, we have relied on fossil fuel for running engines that turned propellers that pushed ships and planes over the sea and air. Now with the predictable increase in the real cost of fossil fuel over time, and the shift of focus back to renewable energy, we may once again see transport sail craft on some of oceans of the world. This shift began in response to hotly debated environmental issues, but now the motivation to use wind assist is to increase a ship's overall efficiency. The Windstar cruise ships and the Shin Aitoku Maru Coastal freighters17 may be the first $100,000,000 examples of a resurgence of wind power for commercial shipping.
The relative cost of technology and labor will make the modern transport sail craft quite different from those of past centuries. Major advances in aerodynamics, structures, and flight control will make it possible to extract wind energy for a small fraction of the man hours per horsepower hour that it took on the 19th century windjammers. One example is the unballasted sailboard invented in the late 1960's which extracts 10 hp from a 20 kt wind. This invention6 by James Drake, an aeronautical engineer from southern California, has in 20 years become the fastest and most popular type of sailboat in the history of the world. The advanced sensors and adaptive control system here were supplied by the magnificent human organism, and we now know how to supply these functions electronically at low cost.
The second example of enabling technology is the Kiteski, Fig 2,45 which extracts 20 hp from a 20 kt wind and has the potential for sailing even faster than the sailboard. Fig 3 shows a speed polar for our production Kiteski. In terms of potential application to commercial sailing, the Kiteski takes the lifting wing of the sailboard one step farther by detaching the wing from the hull.
Fig. 3 Kiteski Speed Polar32
The next step beyond the Kiteski may be the incorporation of automatic flight controls such as those described by McGeer and Holland on the aerosonde type of unmanned air vehicle,39 such that the man is taken out of the loop altogether, and we are able to eliminate one of the two drag producing tow lines. Wallace has already started testing this type of kite sail48, and the results are encouraging.
It is not yet clear whether soft sails derived from modern parafoil technology or rigid wings more like those found on modern sail planes and Global Hawk (Fig 4) will provide the most economical wind energy extraction. However, the enabling technology continues to build internationally. The 1996 UAV survey article by Steven Zagala33 gives data on 100 UAV's from 40 companies in 12 countries.
Fig 4 Global Hawk
A hybrid wing of rigid spar and soft sail based on multihull and land yacht technology may also be competitive. More research and development will be required. We are concentrating on the application of kite sails because they have the best potential for application to a wide variety of commercial ships. They also present some of the greatest control challenges. It is therefore likely more conventional sail arrangements38 will be used prior to kites in commerce in the next century. In spite of much publicity, kite sailing is still not generally accepted. Much of the reason for this is explained by Francis Reynolds, Crackpot or Genius41.
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