The kite sail will help reduce annual fuel costs by 10-35%. Reduced fuel also means fewer harmful carbon emissions ...
news.bbc.co.uk/2/hi/europe/7205217.stm

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Note added at 12 mins (2008-10-06 14:07:14 GMT)
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...multiply the size of these kites by ten or even a hundred times, add some sophisticated deployment and control mechanisms, and you have something else entirely: a kite sail, which can pull a large yacht or even a cargo ship.
http://itotd.com/articles/603/kite-sails/

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Note added at 16 mins (2008-10-06 14:10:26 GMT)
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Launching a flexible airfoil such as a kite, whose span can exceed
15 meters, requires a particular technique. Firstly, it is necessary to
launch the auxiliary kite: this operation is easy, since it is very easy
to launch such a kite manually from the craft. The self-stabilizing capacity of the kite is used to "pilot" deployment of the main kite .
www.patentstorm.us/patents/6918346-description.html

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Note added at 1 hr (2008-10-06 15:28:39 GMT)
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Yes those patents can be dodgy but I couldn't find "self-stabilizing" and "kite sail" together in any other convincing document.

I don't think "aerofoil" is quite the same same register as "cerf-volant", and it could apply to any kind of sail couldn't it?

I also came across "skysail" but that might be open to confusion as may "kite" on its own...

I suppose the "autostable" bit means it doesn't collapse like a spinnaker.

Being picky, they seem to me to be more akin to paragliders than kites... I wonder what happened to my bright blue Peter Powell stunt kite?

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Note added at 2 hrs (2008-10-06 16:23:05 GMT)
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2.972 How A Sail Boat Sails Into The Wind
The SAIL in the wind acts as an AIRFOIL and the HULL in the water acts as a HYDROFOIL, so there are two sets of forces acting on a sailboat: AERODYNAMIC and ...
web.mit.edu/2.972/www/reports/sail_boat/sail_boat.html