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On 07 Jun 2004 18:46:33 GMT, (Shen44) wrote:

Steve wrote:

That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang. This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve

Not true, in my opinion. During the lower rotation of the blade it is pushing
sideways in "solid" water .
Looking at the blade from astern (RH) and using a 360 deg circle and watching
the pich angle for angle of push.
As the blade approaches 45* it is pushing back/down, as it approaches 90* it
starts pushing back/sideways against "solid" water..... pulling the stern to
stbd.
As it rotates to 180* it is still pushing against the water, back/sideways
against "solid" water, but as it starts coming up to 270* it begins to push
back/up and loses efficiency, as it starts to lift the water up/back. This
continues through 360* where it's pushing back/sideways (and to a degree, up),
but at that reduced efficiency because it can lift that water column up and
back into the air.
I realize this is not the easiest visualization and my writing skills may not
be explaining the point at it's best.

That was a good explanation and I got what you're trying to say. The
only problem with it is the overhang. My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull. Not
only is there no noticable bulge of water behind the boat from the prop
but there isn't even a sign on the surface that there's a prop turning
at all. I just don't get that stream of bubbles behind me or a bulge in
the water that powerboats get or anything other than the same sort of
wake I get when sailing. And yet there's pronounced prop walk when
reversing.

Steve

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