On 15 Oct, 08:09, toad wrote:
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.

Well, you went completely silent the last time the presence of a
propellor was pointed out to you. I think it was pretty obvious that
you didn't realize its significance.

But here - yet a-bloody-gain - is what you need to know.

1) There is a downwind force on the windmill.

2) There is a change of wind speed across the disk of the windmill.

3) The shaft power produced by the windmill is the downwind force
times the change in wind speed.

4) The power required to move the windmill disk relative to a fixed
point is the force multiplied by the upwind velocity component of the
disk

4a) So to move the disk directly upwind requires power,

4b) To move the disk downwind produces power (in addition to any shaft
power) and

4c) To move the disk across wind requires no power

5) The shaft power produced by the windmill may be fed into a device
capable of moving the windmill - gears, propellor, paddles, generator
and linear electric motor.

6) If, after efficiency has been taken into account, the shaft power
produced by the windmill is greater than the power needed to move it,
it will move.

6a) So a windmill craft will always be able to go down wind

6b) Will be able to go crosswind if the shaft power equals the
propellor losses and

6c) Will be able to go directly upwind as long as the shaft power
exceeds the propellor losses, the maximum speed attainable upwind
being determined by the size of those losses.

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

Good. Since the energy required to push it into the wind is
proportional to the absolute forward speed of the disk, it can be as
small as you like. To - again bearing in mind inefficiences in the
propulsion system - any available shaft power can produce forward
motion. QED.

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:

Can I assume that you have a rasonable base in fluid dynamics and
Kelvin-Froude actuator disk theory?

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.

You have given insufficient information.

But the important thing to remember is ... IT HAS BEEN DONE. By lots
of people. I've seen one of the vessels capable of doing it myself -
it used to live at Tighnabruiach and is now in the Scottish Maritime
Museum Collection.

Arguing about theoretical possibilities is one thing. Arguing that,
because you don't understand the theory, working machines are mass
delusions is a little bit silly.

Ian