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. 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 don't know a Kort nozzle, but if it's a protective cylinder
around the prop with a skeg and cross shaped blades supporting
it
in front, that's what I meant by 'shield'. Some old, single
engined tugs used to use these and I suspect the Greek
approach
is a half way house.

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.

Also on hydrofoils, where the prop is 2m deep with no hull
interference in sight. But that's a bit exotic, since most
hydrofoils are twin screw.

Not sure why this reference to depth of a prop in relation to a
hull. I may be
reading you wrong, but, for instance, take a single screw ship
where the prop
is generally set fairly deep on the hull..... they tend to
torque to a
fairtheewell.

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.
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.

JimB