On 15 Oct, 07:41, Ian wrote:

As I recall, your main failure to understand came from thinking that
there was only a windmill involved, so it case you have forgotten,
please remember that all these designs use a PROPELLOR IN THE WATER
COUPLED TO THE WINDMILL.

Errr, no. My failure to understand is I don't know how much energy a
windmill can harness, and I don't know how much energy is required to
push a windmill into the wind. What mechanism turns that energy into
forward motion is irrelevant. If the surplus energy is there it can be
used to drive the boat with any mechanism you choose.

If the energy a windmill can harness is greater than the energy
required to push it onto the wind it must go forward.

Claiming I don't understand is rather futile. Of course, _I_ don't
understand. The point is you claim you do. So post the figures that
you base your understanding on and then I will share the same
understanding and this issue will be put to bed for good:

20kts of wind on the nose. Assume no friction or drag anywhere in the
system apart from the push backwards on the windmill. How fast does it
go. Show your workings.