JAXAshby wrote in message
...

it is caused by asymetrical thrust of an angled prop
shaft. When backing up the
blade coming up to the hull has a much greater "angle of
attack" than the blade
going down from the hull. Thus more thrust on one side
than the other.

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