On Tue, 30 Mar 2004 18:09:21 -0800, "Rod McInnis"
wrote:


"JAXAshby" wrote in message
...


Q: same question, rudder centered, but tranny in reverse (bow of boat tied
to
dock)?

A: the stern will move to port due to prop walk


I would have said to starboard, reason being that I would expect it to be
the opposite of the prop walk direction when in forward gear.

I have heard several different theories for "prop walk".

"Asymmetrical thrust" is certainly one, and obviously the one being debated
the most right now.

Cavitation is another. Consider that both surfaces of the prop are being
used. The "push" side of the prop can generate unlimited PSI, constrained
only by what can be delivered down the shaft and the strength of the prop
itself. The "Pull" side of the prop can only develop the pressure of the
ambient.

Consider a prop that is spinning just under the surface of the water. As
the tip of the blade passes over the top, the total pressure that can be
developed on the "pull" side is 14.7 PSI. (at the tip). As the tip of the
blade passes down the bottom side it is in deeper water, and hence will have
more ambient pressure. If the prop is 12 inches in diameter then it would
have a foots worth of water more pressure, or approximately another 1/2 PSI
or so. This difference in pressure top to bottom gives the bottom of the
prop just a little more bite, and the prop wants to walk.

There can be other reasons why a given boat kicks one way or the other.
Some inboards I have seen have the rudder mounted just off center, which
allows removal of the prop shaft without having to remove the rudder (which
it would hit if the rudder was on center). I would expect an off center
rudder to have some effect on prop walk.

What is the dominant effect? I have no clue. I don't care. I do know that
my single engine inboard is impossible to steer reliably in reverse. It
usually (but not always) kicks to port in reverse. Which is why when you see
me going down a tight fairway in the marina I will be hugging the left side
as I can turn right much better than I can turn left.


Our boat exhibits no prop walk that I can detect. It also can't steer
from propwash in forward. It must be moving for any steering to occur.
The prop (18-inch Martec) is 12 feet forward of the spade rudder and
fairly close to the keel, so it has little lever arm to turn the boat.

A friend has a boat (Bristol 35.5) that walks horribly. It has a
Maxprop that is much further aft than ours in a boat about the same
length. I suspect the Maxprop is set for excessive pitch, because it
doesn't produce much thrust, but walks like crazy.



Rodney Myrvaagnes NYC J36 Gjo/a

"WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light