"Edgar" wrote in message
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"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
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"Capt. JG" wrote in message
easolutions...
"Edgar" wrote in message
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troll sh*t removed
That is the easy answer that seems to satisfy most people but if you
think about it do you really believe that water gets measurably denser
because it is about 18" (say) deeper at the bottom of an 18"(say)
propeller than the water at the top? Of course it does not. Water is
hardly compressible at all except at very high pressures.
. Density will only be a factor if air is being entrained from the
surface.
You will find boats displaying propeller walk even if the propeller is
too deep for air entrainment to occur. My boat does.
So the question is 'why does the opposite direction of the blade at
the top not cancel the walk created by the blade at the bottom'?
I suggest that this is because as the blades rise upward towards the
top the water impinges on the hull and creates a reaction force which
acts in the same direction as the walk force created at the bottom.
This is because, in yachts especially, the blades pass very close to
the hull which usually is somewhat vee shaped and deflects the water
sideways. In your example water would be deflected to port but the
reaction on the hull would be a starboard force.



Think hydrostatic pressure. Also, Neal is wrong.

Yes, he certainly is.
I have tried your suggestion about thinking 'hydrostatic pressure' but
it is the 'static' bit that is somewhat puzzling me because propeller
walk is a dynamic phenomenon. Can you expand on this a bit?


Not an engineer. However, there is a difference in the "pressure" between
the high and low of the prop. I have a hydrostatic PFD (vs. the old pill
melting) autoinflate. It's a good choice if you go offshore or sail
dinghies, and you don't want the spray to trigger the PFD. I believe it's
activated in 2' of water, but don't quote me. I had the pill melt on me
once while sailing a dinghy, and I don't want to wear the bulky PFDs.

Yes, OK that is hydrostatic pressure but nothing to do with prop walk.

Actually, I think it does, although prop walk is also related to Newton's
3rd law, the angle of the shaft and the pitch of the prop. There *is* a
hydrostatic pressure difference between the top and the bottom. Not being an
engineer, I don't know how to calculate the diff or the effect.

Another way to look it is, it seems to me, is to look at a limiting case.
Take the paddle wheel. The paddle wheel pushes the water to cause the boat
to go forward (or reverse, whatever) by resistance against the water (N's
3rd), but there is pressure against the paddles above the surface of the
water... minimal though it is. I wonder what the effect would be if the
paddle wheel were tilted? It seems to me that that would be similar to the
situation approaching a normal sailboat prop. I wonder if this has ever been
tried.

My understanding is that it's only a small part of the effect. As most of
us know, the prop wash can be observed on the opposite side from the
direction the prop walk forces the stern (usually, the prop wash is
observed most strongly on the starboard side, thus the stern moves to
port). This is much like a fire hose that forces water out the business
end and pushes against the person holding the hose. In the case of the
boat, the stern moves in the opposite because there's nothing but some
water to oppose it.

A hose stays in one position and certainly produces a reaction force that
you have to resist in order to hold it still.
But the prop is rotating and consequently the sideways force it generates
is also rotating.
So, as I said, one has to explain why the force to one side produced while
the blade is at the bottom is not balanced by the opposite force produced
while it is at the top. This is what I tried to explain in my first post.
Neal's explanation was quite wrong and I am quite sure he knows it..

See previous. My continued understanding it that's a combination of several
things.

I think Donal said it best, that it really doesn't matter, since the
issue we're all concerned with is how to either use it or deal with it. I
find pulsing the engine works vs. actually putting the engine in neutral.
I get 90% of the benefit with 1/2 the hassle.

I fully agree with that. Pulsing is the best way to make use of prop walk.
Now Joe has just said that to really walk a boat you need twin screws and
that is not right either. With twin screws you are turning the boat by
regulating the thrust by going ahead on one and astern on the other as
necessary.

I believe twin screws turning in opposite directions is one solution.
Another is restricting the prop blades inside a tube. Again, the limiting
case is a long, narrow tube with a prop inside. You don't get prop walk with
this arrangement, but it's expensive to make/use.

However, I have seen fishermen in heavy deep draught traditional boats
with big single screws that could park them perfectly alongside in a
narrow gap using prop walk while carrying on a conversation with the guy
in the next boat!
I have handled such a boat and can vouch for the usefulness of the effect
once you are used to your boat's characteristics.

I've seen such but have never been on one, at least not in a position to
observe the helm maneauvers.

My own boat is 38' and has a short(ish) central keel, no skeg and a spade
rudder. You can spin her on the proverbial dime ahead or astern once you
have steerage way but backing out of my marina slot needs great care
because prop walk when you first engage reverse from rest is a real
problem especially if a cross wind makes it worse, and I always have to be
careful that my pulpit does not gouge the side of the power boat in the
next slot before I have steerage way.



Yes, I have the same issue. Fortunately, I'm able to basically let her go in
reverse and use the prop walk to make a big U turn in reverse to get out. I
have my students keep the wheel hard to starboard until the bow is clear,
then gently turn back to port. The starboard wheel will keep her straight
long enough to get moving in a straight line, and I could keep backing
straight with the pulsing technique, but I'd run out of room in the fairway.

--
"j" ganz @@
www.sailnow.com