JAXAshby wrote in message
...
jim, asym thrust has been a known factor since before WW2.

Since Pontius was a pilot, Jax. So you got the idea from
aviation.

What I was querying (though I didn't make that very clear) was
your application of the theory to boat prop walk, where the boat
velocities are trivial, the lateral dimensions of the prop are
trivial, and the inertia of the vessel is large. Your argument
was a bit like talking about the effect on boat pitch due to yaw
through the gyroscopic effect of a running engine. It's there.
But it's trivial.

I was also querying your other theories of the causes of prop
walk, which I certainly don't understand.

JimB

Interesting theory. Can't think where you got that from.
Better check the trig though. 1 ft/sec astern, typical prop
tip speed about 50ft/sec, lets say one 1 degree.
Differential effect of 20deg shaft angle, 1-cos20 = 0.06deg.
Lets say 1/20 degree. Compared to a typical prop pitch of
20deg or so that means that 1/400 of your thrust (800 lb?
reduced to 2lb) is being exercised over a moment arm of 16
inches to turn your vessel.

OK, that's coarse maths from the back of an envelope, with a
margin of error of maybe an order. But I still don't think
that even 30 ft/lb is going to turn your vessel. That's
what
I use to tighten my nuts. And it reduces to zero when you
have zero stern way.

So your theory can only true when the boat is actually
travelling in reverse (your definition of backing up?).
It is utterly trivial compared to the paddle wheel effect.
You can test this statement by selecting reverse while
moving slowly forward. The vessel won't kick first
one way, then the other. It'll go the paddle wheel way.

It is also caused to some extent by the contrainment of
the prop wash against
the hull on the up side blade, compared to no
constrainment on the down side
blade.

Don't understand that.

All forces are the result of changes in momentum. The wash
spirals away from the prop. Read on.

On the upper side, the lateral speed of the spiral is slowed
by friction against the ship's hull. The lower side much
less so. So the lower lateral momentum added is greater
than the upper.

The result is a force as if paddled by the lower blades.

You could also think of it as the frictional force exerted
on the hull by slowing the lateral speed of the upper part
of the spiral.

Whichever, it's the opposite direction to your theory,
which, in turn, doesn't tie in with my experience.

JimB