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    Re: Prop walk
    From: George Huxtable
    Date: 2003 Apr 25, 08:05 +0100

    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
    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
    >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
    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.
    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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