On 27/09/2010 21:33, Gib Bogle wrote:
On 28/09/2010 8:40 a.m., Andy Champ wrote:
On 27/09/2010 10:12, The Natural Philosopher wrote:

There was one interesting idea that was mooted, that on analysis I
couldn't find a flaw with: A wind powered boat or vehicle that could
sail or drive directly into the wind using a turbine facing the wind to
drive a water prop or wheels..also said to be capable of going faster
than the wind..directly downwind..less sure about that, though.



You can go as fast as you like in any direction, provided you have one
thing
moving against another that you can extract energy from.

Typically this is the wind blowing over the water, or the land.

There is no theoretical speed limit; it's just the faster you go the
more the
efficiency of your aerofoils / hydrofoils / wheels matters.

Andy

OK, Einstein, when you are travelling at the same speed as the wind, in
the direction of the wind, what "thing is moving against another",
except for the vessel moving against the water, which creates drag?

Odd, my post from last night seems to have gone missing. Oh well, I
kept a copy...

I ain't Einstein. Luckily this is far easier.

As you so delicately told Harry, you need a turbine. Or some such.

The one I've seen uses wheels and an airscrew - the principle is the
same for a prop. in water and an airscrew.

In order to generate a force against the wind the airscrew, which is not
moving relative to the wind, requires only enough energy to overcome
friction losses and inefficiencies. Polish it enough, and that won't be
much.

Put that force down to the bottom of the vehicle, where it's moving
against the substrate, and you have a force moving through a distance.
From that you can extract substantial amounts of power. Certainly
enough to keep the airscrew spinning in what it sees as still air.

For the numbers, imagine this: Airscrew is generating 1kN of force, and
as the air is still relative to it it requires minor amounts of power only.

The vessel is moving downwind at 1m/s. Half of that kilonewton is used
to shove it through the water; the other half is pushing a prop through
the water. 500 newtons at 1 m/s is half a kilowatt. Not enormous, but
as much as a racing cyclist... OK you won't get it all out of the prop,
and you won't get it all up to the airscrew, but certainly some!

Andy