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Here are some guesses.  The bottom end of the prop is more efficient than the 
top half.  The water is more dense by 3% of an atmosphere at the bottom than 
at the top.  The top is more starved for free water flow at the top because 
of the close proximity of the hull.  This might make it 'rob' back some of 
the water from aft the prop.

So my right hand screw tends to walk the stern to starboard in forward.  To 
compensate for this, the shaft is set just off-center to nuetralize the 
effect.

So then I put it in reverse.  The nuetralizing offset in forward is now an agravattor in reverse.

Yet another guess, torque.  The prop revolving right attempts to make the 
whole boat revolve left.  It can't because of bouyancy of the hull.  So it 
tries to pivot around the hull pulling the prop and stern to the right with 
it.  I would like this one best but for Jarad's notion that if you encase the 
prop in a tube, the effect goes away.

Dave W

-------Original Message-------
From: George Huxtable 
Sent: 04/22/03 03:05 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Prop-walk.

>
> Doug Royer said-

>This subject of harbor or slip manouvering is a good subject for further
>discussion both in theory and practical applications.What about
discussions
>on propeller charactoristics,hull shapes, fluid dynamics or just boat
>handling in tight spots or in adverse conditions?Do you guys consider the
>above as on subject or off?Maybe someone out there never operated a
single
>wheel vessel and might wonder about it or gain some insight from someone
on
>list that has the experiance etc.

====================

George rises to the bait-

Well, the causes and effects of prop-walk are, I think, often
misunderstood, and that could make a fruitful thread.

Why does the stern kick sideways? Does yours, like mine, do so much more
strongly when the vessel has no way on, or is in reverse, or with the
propellor back-pedalling to slow the vessel down, than it does when
driving
forward? If so, why? Why does it kick in the direction it does? Do
single-screw merchant vessels show the same behaviour?

I have heard all sorts of explanations, some quite ludicrous, on this
topic, but never anything that seems fully satisfactory. I have developed
my own ideas, which satisfy me if nobody else, and will be happy to debate
them if any interest is shown.

Has anyone found a satisfactory publication about prop-walk?

George Huxtable.

PS. I sent this posting a few minutes ago under a different thread-name,
but have now renamed the thread "prop-walk", just in case it takes off.

PPS No, I didn't, dammit! But I really have retitled the thread this time.

================================================================
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.
================================================================
>

Dave Weilacher
.US Coast Guard licensed captain
.    #889968
.ASA instructor evaluator and celestial
.    navigation instructor #990800
.IBM AS400 RPG contract programmer

   

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