Gould 0738 wrote in message
...
See my other post. If conditions are such that water is
ingested from one direction, and exits in another, there is
a change in lateral momentum. That's a lateral force,
usually behind the C of G. That's a yaw. That's turning a
boat.

I think you're making the assumption (without stating it)
that, engine in reverse, water will always be ingested from
dead astern; ie, the rudder is small and of such a distance
from the prop that it has no effect on the direction of
ingestion.

JimB

JimB: Are you attempting to make the case that a rudder
controls the direction
of a power driven vessel primarily by changing the side of the
prop on which
water is "ingested"?

No. I'm starting from the point that the rudder changes the
lateral momentum of water traveling past the stern, and this
creates a lateral force.

It's obvious that, engine ahead, the water comes from straight
ahead and can then be deflected laterally by the rudder to create
a lateral force.

With the engine in astern, I was saying that *if* the water
entering the prop is constrained to come from one side (big
rudder etc) then it's momentum is changed from having a lateral
component to having no lateral component, therefore a similar
force is developed.

However, you've made me reflect on that. Yes, there will be a
force on the rudder, since it is deflecting the water (and you
can feel this on the tiller). And I've witnessed (and had
explained to me) the effect on trawlers and tugs.
But I can't square this with the idea that, in astern with no
motion, the water ultimately has started with zero momentum (if
we're standing still) and is accelerated to have solely a fore
and aft element. That implies no net lateral force.

So something is missing here!

Perhaps there's a subtle change in prop walk in these types of
boats caused by the rudder angle in astern

Thanks for making me think . . .

JimB