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In order to be commercially viable, the sail must be built and operated for less cost than the cost of the oil saved by its use. For the 1000 hp sail described in the abstract, the cost must be less than $100/hr. Current rigid wing UAV's cost more than that, especially those that are large enough to extract 1000 hp from a 20 kt wind. Perhaps the technology used in the marine industry would be more appropriate. Consider, for example, the Cogito catamaran of Fig 11 which recently won back the Little America's Cup in Australia>

Cogito catamaran This 30 x 8 ft wing was built of carbon and Mylar at a cost of less than 4000 man hours. A kite sail based on this technology would have a span of 60 ft, an area of 600 square feet, and would weigh less than 400 lbs. It would be capable of operating at a lift coef of 1 in relative winds up to 50 kts at an L/D greater than 10. That would yield a force of 6000 lbs, perhaps 3000 lbs of thrust, which would extract 300 hp from a true wind of 20 kts. The fuel saved would be 150 lbs/hr = 20 gal = $20/hr. The cost of the first unit was more than $400,000. That included the cost of the design and tools. If more than 100 units were built, the cost per unit should be less than 400 lbs x $100/lb = $40,000. Assuming a useful life of 5,000 hrs, that works out to $8/hr, so it looks like we may have a profitable system if we can keep the cost of the systems below the cost of the structure. The systems in this case would include the electronic flight controls needed to keep the wing up there doing useful work and the cost of the winch and retrieval mechanism on the deck of the ship. The large fleet of parafoil tow boats operating in tourist areas of the world convince us the winch and pylon should cost less than $10,000. In this example, the tail of the bird would fold parallel to the wing so the entire sail could be stowed on the deck of the ship when not in use. There would be a 1 hp winch with 1000 ft of line to tether the bird, and a small ram air turbine would supply 100 watts of electrical power for the flight. Thomas Jeltsch in 1995 helped Kiteski perfect a manual winch and brake for this purpose with a retail cost below $1000.

The following table summarizes some of the wings discussed in the text. The Aerosonde, Kiteski, and Predator are operational, providing actual cost and performance data to the industry. Cogito, Trifoiler, Parafoil, and Condor have flown extensively in related modes and represent technology that could be readily adapted to kite sailing. Global Hawk will fly this year, at a unit cost of $10m including engine and electronic payload. Our $100k cost estimate would cover only a simplified wing and kite sail controls. The two versions of the Acre Bird, one based on America's Cup technology and the other an inflated wing, are awaiting major capital investment to get into the hardware stage.

Table 1 Relative Cost of Various Kite Sail Systems:

Power (HP)
($)    ($/hp-hr)
Aerosonde 10 10 0.5 2 10,000 10k 0.5
Kiteski 20 70 0.1 20 100 0.8k 0.4
Trifoiler 40 200 0.2 30 1,000 3k 0.1
Predator 48 150 1.3 40 10,000 80k 0.2
Cogito 60 600 0.7 200 5,000 80k 0.08
Parafoil 150 7,000 0.01 1,000 100 70k 0.07
Global Hawk 116 250 2.0 300 10,000 100k 0.03
Condor 200 1,200 1.0 1000 10,000 200k 0.02
Acre Bird 360 30,000 1.0 10,000 30,000 3,000k 0.01

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