Jax,

I accept that 'suction' will not create a force. Any forces come
from the new exit momentum of a fluid (point the hosepipe where
you will . . . .) presumably this was Feynman's case. I haven't
met the guy, Fine. The effect is easy to observe on VTOL
aircraft. They suck from forward, but don't have to lean their
jet output forward to cancel any 'sucking' force when hovering
static. Here we agree.

I'm looking for an explanation of the phenomenon I thought I had
seen on a very old tug, also on an old trawler, neither of which
had any significant prop walk in astern, though both had big
props. The phenomenon was that rudder deflection with engine in
reverse (boat static) could be used to create a little yaw. The
explanation given to me was that 'flow over the rudder' created
this effect. I rationalised this explanation (perhaps wrongly) by
assuming the rudder changed the momentum of the water ingested by
the prop. ie, water speed along the inside of the rudder is
faster than water speed over the backside of the rudder; a very
simple 'hydrofoil in free stream' effect.

As a result of this thread I am re-examining this assumption and
my observations.

Now, it could be that my observations were wrong, and the
phenomenon did not occur. I was, perhaps, seeing yaw caused by
another effect - inertia due to a previous action maybe. And
perhaps my observations were clouded by the pre-conception
planted in my mind that it worked. But I'm afraid your
explanation (paraphrased very crudely) 'you're wrong because
Feynman says so' doesn't help me. Also, Derek Rowell's experiment
shows that there is some effect which needs explanation - rather
than dismissal.

If you could demonstrate, prove or explain why water speed should
be identical along each side of the rudder (which I assume would
porve that pressure on each side is identical), irrespective of
rudder deflection, when the boat is static with engine in
reverse, I'd happily accept your thesis that rudder has no
effect. As in many of these cases, it may help to explain this
for an extreme case; a balanced spade rudder at 70degrees
deflection close to the prop?

If the rudder suffers some net pressure, then I'd like to
understand what mechanism cancels it.

Can you help without appealing to higher authorities?

JimB