"Wilbur Hubbard" wrote in message
anews.com...

"Charles Momsen" wrote in message
...
What is the primary cause of propeller walk - torque or thrust?


Propeller walk is a term used to describe the results of the bottom of the
propeller operating in a more dense environment than the top of the
propeller. The primary factor is neither of the above. Drag, due to the
propeller's interaction with a denser medium, is increased at the bottom
half of the propeller. A prop turning clockwise as the vessel proceeds
forward as seen from astern will cause the vessel's stern to move to
starboard due to increased drag. Whereas thrust in a vessel is a
description of fore and aft forces, prop walk is a description of
transverse forces caused by drag differential.

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.