On 30/09/2010 21:50, Rick Cavallaro wrote:
On Sep 30, 1:46 pm, Clive wrote:
On 30/09/2010 21:37, Rick Cavallaro wrote:

On Sep 30, 12:58 pm, Clive George wrote:
Ignoring Dennis, who delights in proving how wrong he can be on a
regular basis, I have to say I found the explanation of how it works a
bit opaque. I'm pretty sure I've got it, but the text and diagrams
didn't really help that.

Clive, there a a BUNCH of ways to describe what's happening here.
We've found that some people respond to one description and think all
others are nonsense, and others find one of the others useful. We'll
be happy to answer your questions and tailor the discussion to help
bring it across. Just let us know what's making sense and what's not
making sense to you. The energy analysis is very compelling to me at
one level, but it's also useful for me to think of each blade of the
propeller as being on a continuous broad reach - spiraling downwind
faster than the wind.

I prefer working with the forces rather energy - I think introducing
time into it unnecessarily complicates things.

Playing with the dynamics of the propellor also doesn't help me - see
how I've started my own explanation. The propellor comes later.

If you prefer the forces approach I can post a vector diagram that
shows how an ice boat can achieve a downwind VMG (velocity made good)
faster than wind speed on a broad reach. This works strictly on lift
and drag of the sail and keel/blades. From there you can imagine the
two ice-boats side-by-side connected by a telescoping pole, or you can
picture two ice boats circling their way down a cylindrical earth on a
continuous broad reach. In this case their sails are doing exactly
the same job as our prop - and in the same manner.

Go for it.