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    Re: Prop-walk.
    From: Aubrey O?Callaghan
    Date: 2003 Apr 23, 13:37 +0100

    There was an excellent article on this very topic in Practical Boat Owner
    about 15 years ago.
    As George has so succinctly debunked the prop efficiency based on the
    density difference, so did the article in PBO.
    
    Unfortunately, I cannot remember the reason for it in detail, but I do
    remember that a diagram was drawn showing what I think was pressure on the
    prop blades with a mirror image below and above, the prop. Now as the prop
    is close to the surface, the mirror image on the top side enters a
    different medium (air) and so "imbalances" the system.
    
    This would imply that if the prop is sufficiently deep then there would be
    no prop walk, the question is how deep ?
    As an extreme test of this, do submarines suffer from prop walk when
    operating below the surface ?
    
    This would also explain why George does not get prop walk when stirring his
    paint - the system will be symmetrical with respect to his cylinderical
    tin. Perhaps as a test, George would pour the paint into a baking tin, deep
    enough just to cover the propeller of the paint stirrer when held
    horizontally, and switch it on. If my theory is right then he should
    experience prop walk. However, he must take great care not to let the
    propeller reach the surface !
    
    By the way the article I referred to in PBO above was sitting in an old
    bookcase in my Aunt's house in Zimbabwe, I'm pretty sure it's still there,
    but not easy to get at.
    
    Aubrey.
    
    At 01:16 23-04-03, you wrote:
    >Some comments on prop-walk from George.
    >
    >I'm not convinced by any of the explanations so far.
    >
    >Dave Weilacher was certainly wrong when he said-
    >
    > >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.
    >
    >That simply just ain't so. Water is virtually incompressible. Any
    >difference in density is infinitesimal.
    >
    >Perhaps list members may like to ponder on the following hypothetical
    >situation.
    >
    >Take an immersed cylindrical submarine, ballasted so as to be neutrally
    >buoyant (and so as not to be rotated by the prop-shaft torque). Will that
    >show prop-walk? To me, simple symmetry implies that there will be none.
    >Turn the sub through 90 degrees, so that its stern is vertically down. Now,
    >will there be a force, at right angles to the prop-shaft? No, there's no
    >way of choosing one direction over another: it's symmetrical. It's like
    >using a propeller-type paint-stirrer, driven from an electric drill: the
    >type which always ends up flinging paint around the garage. But before that
    >happens, is there any sideways force on it, or on the paint-tin? Symmetry
    >says no. Does anyone disagree so far?
    >
    >Go back to the horizontal cylindrical submarine. Attach a vertical fin to
    >its tail, behind the prop (just where a rudder would be), extending exactly
    >as much below the prop as above it. Again, this is symmetrical, so there's
    >no overall sideways force. Now, remove the bottom half of this fin,
    >destroying the symmetry. The water-flow from the prop is a backward-facing
    >jet of water, but it also picks up a spiralling motion from the rotating
    >propellor. If the propellor is turning clockwise (seen from aft) then the
    >upper fin will be pushing the stern to starboard, because of the water-flow
    >impinging on it from the propellor. That would have been exactly balanced
    >by a corresponding pressure on the lower fin, pushing the stern to port, if
    >the lower fin was in place. But now we have removed the lower fin, so that
    >balance has been lost, and the result is prop-walk.
    >
    >That thought-experiment was designed to remove any influence of the nearby
    >water-surface, or any ship-shape of the hull. Prop-walk can arise from
    >surfaces of the vessel which intercept some of the spiralling outflow of
    >water from the propellor.
    >
    >Now, back to surface vessels. For a vessel with a thin vertical sternpost,
    >and a vertical rudder which extends well above and below the outflow from
    >the propeller, I suggest that propwalk would be minimal. For a boat such as
    >mine, with a sternpost and rudder-pivot at 45 degrees, I would expect to
    >see a lot, because there's a much more hull-and-rudder-area behind and
    >above the prop than there is behind and below it. If any vessel had its
    >sternpost angled the other direction (going further aft as you descend), I
    >suggest propwalk would be in the opposite direction: but I know of no such
    >vessel.
    >
    >In reverse, a corresponding effect must occur. The spiralling outflow goes
    >forward from the prop, embracing the hull, divided into two parts by the
    >keel. With the propellor turning anticlockwise, water leaving the prop on
    >the port side of the keel is free (to a large extent) to pass downwards
    >underneath the hull without being intercepted. However, water leaving the
    >prop on the starboard side of the keel, with a partly upward motion,
    >becomes trapped between the hull and the surface, and has to be deflected
    >by the hull before it can flow away.
    >
    >Try fixing your rudder in the straight ahead position, and with the boat
    >lashed to its berth, put the engine into reverse. Give a few seconds for
    >the resulting current flow around the boat to stabilise. If your boat
    >behaves like mine does, and your prop is a clockwise-forward (right-handed)
    >one like mine is, you will see a strong surface current leaving the
    >starboard quarter, in a direction about 45 degrees clockwise from the bow:
    >but on the port side, little or nothing. That strong flow direction must
    >imply a corresponding reaction on the stern of the boat in the opposite
    >direction, pushing the stern to port.
    >
    >I'm sorry not to be able to put numbers or experimental results into the
    >arguments above. I just hope to have persuaded some of you that the
    >detailed shape of boat and hull may have a large part to play in the effect
    >of prop-walk.
    >
    >George.
    >
    >================================================================
    >contact George Huxtable by email at george---.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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