Because the Greenbird style wing doesn't use a traveler to control the angle of attack, it uses the stabilizer and trim tab, it can't sail directly downwind no matter what area it has, because the lift is perpendicular to the wings sides.
There is a ~60˚ dead zone directly down wind, much like there is a ~60˚ upwind zone that the boat can't sail into or from. If the boat is very efficient however, those zones can be greatly reduced and/or eliminated because of the apparent wind.
The big advantage of the Greenbird wing apart from its automatic trimming is its lack of drag. If there are no stays and if the boat is aerodynamically shaped, the total drag on the boat is reduced by half at displacement hull speeds and a lot more than that at higher speeds. Also a Greenbird wing averages about twice the lift of a sail, with much less drag, so half the surface area of a sail, will produce about the same lift as a sail. The Greenbird wing should weigh and cost a lot less than a comparable wingsail, sail, mast, lines, winches, hardware, etc. and be easier to build.
The speed limiting factor is always drag, drag always increases faster than lift, (induced drag anyway).
Talador
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Posted by: taladorwood@yahoo.com.au
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