schlackoff, three day ago you said a prop in reverse would affect a rudder,
then you read my explanation of the phenomenon and flip-flopped. Now, you have
forgotten what your read, or remember what you understood then, and have
flip-flopped back.

tell us again how you are going to win the Nobel prize in physics as you prove
Feynman wrong.

Shen44 wrote in message

Subject: Thrust vectoring

There's very little prop walk on traditional wood trawlers,
when
the prop is set almost half way down a 3m deep keel in a
cutaway.

Please explain what you mean by "trawler" (fishing or yacht)
and "cutaway" and
what the depth of the prop has to do with it.

To be exact, a Brixham Trawler, a 75ft gaff rigged ketch,
designed with a powerful rig for sail trawling. The keel is
straight from stem to stern, about 1m draft at the stem, 3m draft
at the stern. The rudder is mounted 20degrees off vertical. The
aft hull is flat, wide beamed, and lifts above waterline at the
rudder post. The prop is mounted 1.5m deep, in a 60cm (2ft!) arc
cut into the keel just forward of the rudder hinge. The effect is
that the reverse wash of the prop runs down each side of a
vertically straight keel - which I assume kills the spiralling of
the prop wash on both sides equally.

OK, think I see what you're saying.
First off, I used to run a 65 foot boat with a similar configuration and
remember a beautiful old 45' ketch with the same thing ... both had large
degrees of "propwalk".
Secondly, I think we may have different views of what is the major cause of
prop walk (I attribute it to the pitch of the blades pulling to a greater
degree on the "down stroke" side of the revolution, than the "upstroke
side").

Whereas most modern vessels
have the upper part of their prop wash losing energy to the hull
bottom through friction, and no other interference.

I think we're on the same track but drawing different conclusions.


I can see where this stuff together would cut down propwalk. A
Kort nozzle can
be found on fishing boats, but your main application will be
tugs. It's a
shaped cylinder, generally attached to the hull, which
surrounds the prop,
entirely.
It's main function is to increase "bollard pull", but because
it surrounds the
prop, the prop can not "pull" to one side (propwalk) since all
thrust is
directed out the nozzle, either ahead or astern.

I think the key factor here is the cruciform blades supporting
the cylinder in front of the prop, which act as flow
straighteners. No spiral flow along the hull = no prop walk.

In the case of a Kort nozzle, there are no cruciform blades supporting the
nozzle in front of the prop. The nozzle is "hung" beneath the hull and left
as
clear as possible in front to aid a clear flow to the prop (kinda like the
nacelle surrounding a jet engine).


If the prop's deep, but close to a horizontal surface (the hull)
you'll get good prop walk. Because the hull, through friction,
kills the lateral velocity of the spiralling wash on top, but the
lower spiral rushes off, giving you asymmetric reverse thrust.

Here I have to disagree that the prop has to be close to the hull when deep
to
get a good degree of walk .... take for instance any twin screw vessel where
the props are away ... you still get good prop walk (Before some one says
this
is due to off center thrust .... shift to inboard turning wheels and you'll
see
a marked difference).

Paddle wheel effect if you like. If the prop is well clear of the
hull, just look at the shaft supports to design prop walk out. If
you've got a vertical blade supporting the shaft, stick a
balancing skeg below it. Keeps the ropes clear too.

BG Why would I want to design "propwalk" out? "Propwalk" is my friend!

Shen