On Sep 30, 1:55*pm, Clive George wrote:

Go for it.

Here's a simple vector analysis:

https://rcpt.yousendit.com/958723733...d7d26acf72285e

And here's the explanation that goes with it:

Here is the vector diagram for an ice-boat that’s maintaining a course
such that its downwind velocity component is faster than the wind.
I've assumed a boat going 45 degrees downwind at a speed of twice the
wind speed. The ticket is to then compute the necessary L/D of the
sail and keel to make this possible (this would have the boat making a
downwind velocity component of 1.414 times the wind-speed).

Given the wind velocity and the boat velocity, we can easily compute
the apparent wind over the sail. From this we see the direction of
lift and drag on the sail. What we need to do to make sure this
sailing configuration is possible is to insure that "alpha" is small
enough so that the resultant force has a forward pointing component
(relative to the boat). In this case alpha would have to be 45 - 16.3
degrees or 28.7 degrees (or less). That relates to a sail whose L/D is
1.83. Obviously this is easily achievable. However, I've assumed a
keel with infinite L/D in this case. The drawing gets a little more
cluttered if we include the keel's L/D. So I'll try to describe it in
words. I'm going to assume an L/D of 10:1 for the keel (easily done
with an ice-boat). This will trim 5.7 degrees off of my 28.7 degree
budget. That leaves me with an allowable 23 degrees(L/D = 2.35:1) to
achieve this configuration. So with a keel that has a 10:1 L/D and a
sail/boat that has an overall 2.35:1 or better L/D this configuration
can be achieved (we can achieve a downwind component about 40 percent
faster than the wind speed).