Rod,

Actually, the asymmetric response to forward and reverse can be used
to do some neat tricks (which I use all the time). Try this, next
time you're out in your boat with some time to spend:

Motor out to a bouy (navigation, crab pot, whatever) in open water,
where there's no current. The bouy doesn't have to be very close --
it's only to gauge forward motion.

Stop the boat, put the rudder hard over to starboard, put the motor in
forward, and gun it. The first reaction will be that the stern kicks
to port. The next will be that the boat starts to move forward.

Immediately put the motor in reverse, LEAVING THE RUDDER HARD TO
STARBOARD. Apply moderate juice. The stern will continue to move to
port, and eventually the boat will come to a stop. Keep reversing,
and the stern will continue to move to port, due to the effects that
have been described already in this thread. Note where the boat is
with respect to the buoy, and continue backing until you've canceled
any forward motion and she's back where she started.

Now put the motor in forward and repeat the whole operation. With a
little practice, you can almost eliminate all the forward and aft
motions, and simply turn the boat inside her own length, by simply
pushing the stick forward and back, all the while holding the rudder
hard over. When you need to do this in close quarters (say, in a
marina with only a foot or two of room between your bow and the boat
ahead of you, and between your stern and the boat astern), you will
draw applause from onlookers.

Now, suppose some day you need to turn the boat in the opposite
direction, say, 90 degrees. Is there a similar way to turn her inside
her length, but in the opposite direction? No. So, simply turn her
the 270 degrees the way she wants to go (clockwise) until you've
accomplished the turn you need! Big ships do this all the time, and
it works great.

So no need ever again to hug the port side of your channel!

"Rod McInnis" wrote in message ...
"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.

Rod McInnis