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    Re: Prop-walk.
    From: George Huxtable
    Date: 2003 Apr 23, 11:15 +0100

    How satisfying! My innocent question about prop-walk has elicited a wide
    range of contradictory opinions. Somehow, I thought that might happen...
    Dave Weilacher has 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.  The
    top is more starved for free water flow at the top because of the close
    proximity of the hull."
    I think we have disposed of the density question, but some merit remains in
    the rest of Dave's "guess". One would expect the hull to interfere more
    with the necessary flow of water to the upper part of the prop (because the
    hull's wider up there, and the prop may be close under a counter) than to
    the lower. There would then be a greater reaction from the lower part of
    the prop, which would tend to push the vessel in the expected direction.
    There must be some contribution from this effect.
    Jared Sherman has said-
    "This conversation is akin to speculating on why an apple falls from a
    tree. Gravity--and prop walk--have both been extensively documented. Sorry,
    all, but speculating that they do not exist is an exercise in the absurd.
    All you will prove is that none of us have the engineering background to
    explain what is already documented."
    I'm not sure who, or what, Jared is arguing with here. Every contributor to
    this discussion has accepted prop-walk, in all or at least most real
    vessels, as a fact of life. Nobody has speculated that it doesn't exist.
    What I have done is to imagine special scenarios in which there can be no
    net prop-walk because simple symmetry would cancel it out.
    Is Jared suggesting that we should avoid THINKING about this question? The
    better we can understand what's going on, the better-equipped we will be to
    cope with its effects on our boats. If we can get an idea of the mechanisms
    at work, without a lot of equations, so much the better.
    Jared also responded to my question-
    < 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 find a Mixmaster. Not a hand mixer, but a mixer that is on a stand and
    comes with a bowl. Now fill the bowl with batter, turn on the mixer. As the
    shafts rotate...funny thing, the bowl begins to rotate as well. And always
    in the same ddirection."
    Well of course, with a single whisk there would be a rotational torque on
    the bowl, but what we are looking for is not that, but a sideways force on
    the bowl, as I said. In a symmetrical situation, there can be none: the
    force wouldn't know which way to point. It's the sideways force, at
    right-angles to the line of the prop-shaft, that causes prop-walk: all
    correspondents seem to agree about that.
    However, Jared has muddied the waters by introducing two contra-rotating
    whisks. These expel the batter in a particular direction, and the setup is
    no longer symmetrical. Not a good analogy.
    In another posting, Jared said-
    "Effectively a screw is just a like wheel (which also happens to produce
    axial thrust) and as the shaft turns, the wheel rolls sideways. That's all
    it takes to "walk" your stern sideways. The fact that water is being thrown
    sideways as it slips off the blades just enhances this prop walk."
    I think Jared has to justify this analogy better. A wheel needs a surface
    to roll on: a floor underneath it. With a propeller, there's no such floor.
    It's interacting with water above its axis, in just the same way as with
    water below its axis. One would push it sideways in one direction, the
    other opposite. So what's a propellor "rolling" on, in Jared's view?
    Peter Fogg said-
    "One of the problems with props is cavitation, they
    don't function well in the water nearer the surface that contains more foam
    than deeper water with less air bubbles."
    Well yes, there are three related effects possible here.
    1. Foam in the upper water. In ocean storm conditions, maybe. In a marina,
    forget it. Take a look in the water. Can you see foam?
    2. Cavitation. Formation of bubbles at the low-pressure forward-facing
    surface of propellor blades. It can happen if sufficient power is being
    applied. Manoevring in a marina, unlikely. Prop-walk happens even when very
    modest, "idling", power is applied.
    3. Entraining of air from the surface. This is the effect that a so-called
    "anti-cavitation plate" is intended to prevent, on the leg of an outboard.
    It's the formation of a vortex, like that above the plug-hole in your bath,
    allowing the propellor to suck air down if it's insufficiently immersed.
    Not normally a problem with an inboard, and never a problem at low power
    If any of these three effects applied, then they could explain some aspects
    of prop-walk. But manoeuvring in a marina, none of them do, I contend.
    It's generally accepted, I think, that some types of vessel are more
    susceptible to prop-walk than are others, but I have never seen any study
    of which characteristics worsen the effect. There's no "prop-walk index" to
    measure against: it's all subjective. Perhaps the experience of
    list-members can help here. I can't help much in this regard, having owned
    the same small boat for 30 years now. I think it's highly affected by
    prop-walk, but have little experience of other craft to compare it against.
    However, many boaters change their craft almost as frequently as they
    change their socks. Can anyone venture an opinion, based on the craft they
    have owned or known well, which were most and which least affected by
    prop-walk, and if this related to the detailed shape of the back-end, or
    indeed any other factor?
    If my suggestion is correct, much of the prop-walk derives from the
    interaction between the hull/rudder and the spiralling backwash from the
    propellor, and from differences in the geometry above the prop axis and
    below it. In that case,  prop-walk wouldn't be a problem in vessels
    propelled by an outboard. The water flow as it leaves the prop of an
    outboard is (in forward gear) very clean; it has no further interaction
    with the vessel. So if the pivoting of an outboard can be locked up,
    exactly in the fore-and-aft direction, then I suggest that with the vessel
    stationary, there would by little or no sideways kick when forward gear is
    engaged. Has anyone observed this (or the converse, even)?
    In my days as a youngster in Liverpool, sixty-odd years ago, the Mersey
    used to be crowded with merchant shipping. A common sight was to see an
    unladen freighter with her single screw way out of the water, perhaps only
    two-thirds covered, thrashing along the river, blades beating the surface.
    I always assumed (without really knowing) that she would not be going to
    sea in that state, but was simply passing between one dock and another. In
    that condition, prop-walk effects must have made her hard to manage.
    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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