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Subject: Re: Prop walk
From: George Huxtable (george@XXX.XXX)
Date: Fri Apr 25 2003 - 03:05:52 EDT
I'm not sure whether I have fully understood the geometry described by
Stan, or its effects. But his conclusion-
>a blade at
>the top would have more of a sideways effect than one at the bottom, causing
>prop walk.
is unlikely to be correct, because prop-walk as observed is in the opposite
direction. The stern is pushed the opposite way to the direction of travel
of the lower blade. There appears to be a consensus about that, if nothing
else.
====================
Clive Sutherland said-
>I agree that the effect of water density on the thrust of a ship
>propeller is negligible but the effect of pressure is not. I do not
>believe that the shape of the hull per se is important.
>My understanding is this:
>When from astern of a boat, looking forwards, towards a clockwise
>propeller, driving ahead,
>1. The extreme upper and lower portions of the propeller rotation
>produce transverse forces which cancel out and therefore have no effect
>on swing.
>2. The upwards moving propeller blade produces a wash of water with an
>upwards component of motion. This in turn produces a 'Hump' of water on
>the port side of the stern, adjacent and aft of the propeller. This
>extra height of water creates pressure against the hull, pressing the
>stern to move to starboard. Likewise,
>3. The downwards moving blade sucks water down from the surface
>producing a 'Hollow' on the starboard side. This creates a reduction in
>pressure on the starboard side of the hull sucking the stern to
>starboard.
>Consequently the boat yaws to port.
There is little separating Clive and me on this matter. His description of
the ensuing motion of water around the boat is equivalent to saying that
the outflow from the prop picks up some of the propellor's rotation. Where
we differ is in the involvement of the surface. Clive refers to a hump in
the water on one side, and a dip on the other. I have never noticed such an
effect. I'm not saying it doesn't occur, to some small extent: just that
because it isn't apparent, then it doesn't provide evidence to back his
reasoning.
However, there is value in the picture he paints, but he may be confusing
cause and effect. He says that the upward flow of water gives rise to a
hump at the surface, and the hump at the surface creates extra pressure to
push the stern. But really, if there's a hump in the water, it's the
surface RESPONDING to the additioal local pressure from below caused by the
upward flow of water from the prop. That additional pressure will also
press on that side of the hull, and that side of the rudder, which would
create the prop-walk forces. There's no need to invoke the surface at all.
The prop-walk would occur, just the same, no matter how deeply the vessel
was immersed. That's my own opinion, anyway. I consider that it's the
rotating water leaving the prop which causes these pressure differences as
this flow impinges on the hull and rudder at the stern.
======================
We now have a thoroughly satisfactory state of affairs, one that I
predicted to myself might happen. There seem to be at least as many
explanations for prop-walk as there are correspondents, many mutually
contradictory. There could be room for more, waiting out there. More than
one mechanism may be contributing to the observed effects.
I have found much enlightenment and interest from the ensuing
correspondence, but have not yet been convinced by any of it that my own
explanation to myself is wrong. Everyone else can say the same, no doubt.
George.
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contact George Huxtable by email at george@XXX.XXX 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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