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My thanks to George for shattering a long cherished illusion re prop walk and
forcing me to think this effect thru more carefully.

Consider a helicopter rotor with fixed pitch. The advancing blade experiences
higher air speed than the retreating blade due to the net forward motion of the
helicopter. This results in more lift on the advancing half of the disc than on
the retreating half and introduces a rolling moment. This is why helicopters do
not employ fixed pitch rotors but use cyclic variation of the blade pitch to
conteract the resulting roll.

Similarly, a component of incident water velocity perpendicular to the shaft of
a boat propellor causes a differential thrust on the left and right halves of
the prop. This component of flow in the plane of the propellor is due to the
shaft angle relative to the horizontal and to effects of the flow over the
hull.

A Korts nozzle would seem to minimize any non-axial flow and thus not be
suceptible to prop walk.




--- George Huxtable  wrote:
> Walter Guinon said-
>
>
> >Let me first say that I like prop walk, it makes landing port side to much
> >easier.
>
> George comments- I agree. I berth port side to also. When I engage reverse,
> the stern shifts sideways to port and snuggles nicely into the berth. My
> propellor is righthanded, turning clockwise (seen from aft) when going
> forward.
>
> >I beleive the primary cause is the deviation of the shaft angle from the
> >horizontal.
>
> My prop shaft goes downhill from engine to propellor, and I think every
> other craft, including Walter's, will be the same.
>
> >In the extreme case with a vertical prop shaft, the reaction to the
> >shaft torque tends to YAW the vessel.
>
> Yes, but which way would it yaw the vessel? Seen from below, a propellor
> driving in the ejecting-water direction (which would be "forward" if we
> hadn't turned it to point vertically down) will, looked at from below the
> vessel, be turning clockwise. So the reaction of the torsion on the prop
> shaft would be to yaw the vessel, as seen from below, anticlockwise. This
> would push the stern to port: the opposite way to what we find with a
> righthanded prop driving forward.
>
> So that can't be the mechanism involved.
>
> George Huxtable
>
>
> ================================================================
> contact George Huxtable by email at george@huxtable.u-net.com, by phone at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ================================================================


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