"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.

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.

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.

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