toad wrote:

On 14 Oct, 14:17, Andy Champ wrote:

An apparent wind from dead ahead can add nothing but a force directly
astern.

The case where a true wind from ahead can be used to drive a windmill
that can drive a propeller to propel the vessel is different; but this
requires a true wind.

Care to explain why a windmill which is capable of powering itself
forward against it's own drag can only do it with a true wind? How
does it know if the wind it is 'feeling' is true or not, it has no
concept of true wind which is merely the wind speed and direction at
an arbitary stationary point.

As far as the windmill is concerned it has a 20kt headwind and
(alledgedly) it can take that energy, use some of it to hold itself
stationary against the wind and _still_ have surplus energy to drive
forwards. If it can do that you could gear it to the engine of the
20kt powerboat and save petrol equivalent to the surplus power that is
left over once you subtract the energy required to overcome the
windmill's own drag from the total energy harnessed by the windmill.

MY recollection of this is that with a windmill it's simply not possible
to reduce the drag sufficiently to get a sufficient energy to make it
useful. Wingsails are much better at it or even even proeprly trimmed
sails.

ASCII news isn't the best medium to get the point across, but I'll try.

If one is motoring in a calm on a flat millpond then there is an
apparent wind equal to the speed of the boat from dead ahead. Hoist a
sail and you can make no use of that wind, agreed. However that only
applies if you maintain the same course. Now do what any sensible bloke
would do and adjust your course to make use of the wind as well as the
motor. You now have wind in your sails and you still have an apparent
wind. If you look at the force triangle there is still a component from
the apparent wind.

No doubt the craptain also doesn't beleive in back EMF or any of the
other phenomena which appear to produce "something from nothing" however
it's not the case that something is being produced from nothing and in
this case the extra energy is achieved at the usual expense of not being
able to sail directly into the apparent wind.