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WHY KITES

One of the big advantages of kites over conventional rigs, rotating cylinders, and wind turbines is the relative freedom from heeling moment. This will allow us to attach kites to most commercial ships without significant modifications. Another advantage is dynamic sheeting, or the ability to fly patterns in the sky to maintain relative winds at the kite that are several times stronger than the wind on the deck. For example, a Kiteskier running downwind in 10 kts of true wind will outperform a sailboard of the same sail area. Both sailors will choose a broad reaching course to maintain 10 kts of relative wind on the deck, but the kiteskier will fly patterns in the sky to maintain a relative wind at the kite of more than 20 kts. This mechanism is similar to what a wind mill ship will do. The wind energy extracted is not so much a function of the blade area, but the entire disc area swept by the blades.

A related phenomenon is the Flettner or Thom rotor. As the lift coeficienty based on area of the rotor can be increased from 1 to 10 by dragging the air column using its viscosity. Frenchman Jacque Cousteau has done much to popularize this type of sail rig. These devices are of interest to the shipping community due to the ease of de powering them when entering port or in high winds. The theoretical advantage of kites relative to cylinders and wind turbines is covered by Wellicome27.

Fig 6 shows the power extracted by various sail types at various course angles. This was originally published by Loyd Bergessen in support of the design of Mini Lace in 1981, then adapted for kites by Schmidt in 1985, and finally by Roeseler in 1996 for more efficient kites. The advantage of efficient kites over conventional sails in assisting slow moving cargo ships will approach 10:1. Mini Lace was a 220 ft Greek freighter outfitted in 1981 with a 3000 square foot Dacron sail by Windship Development Corporation of Norwell, Massachusetts. This sail was hydraulically furled from the bridge. The mast rose 116 ft from the deck. No extra crew members were needed to operate the $500,000 sail on this 3,000 ton cargo ship because its 53 ft boom was sheeted automatically to optimize fuel saved. This and the tugantine "Norfolk Rebel" were discussed at some length at the National Conference/Workshop on Applications of Sail-Assisted Power Technology at Virginia Institute of Marine Science, College of William and Mary in May of 1982.

Fig 6 Power From the Wind

Fig 6 shows why kites may replace prior commercial rigs once the problems of launch, retrieval, and control are worked out.

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