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    Re: Speed and waterline length. was: Revisiting hull pressure wave
    From: George Huxtable
    Date: 2005 Jan 27, 20:17 +0000

    Jared wrote-
    >Bill, I would expect your "downward pressure wave" is the same as the
    >"ground effect" which gives aircraft extra apparent lift when they are about
    >to land. And for that matter, when two boats under way come alongside,
    >there's a point where the water between them also seems to help buffer them
    >In every instance you're seeing a point where the fluid (water or air)
    >simply isn't being displaced but is being forced back by another object (the
    >runway, the bottom, the other craft) and the flow is changing.
    Comment from George-
    Well, the problem about that is that when two vessels travelling together
    in parallel get too close, they tend to see an ATTRACTION that draws them
    closer; a dangerous state of affairs, that has in the past given rise to
    collisions, and about which our Maritime Safety Authority has issued a
    warning. It makes naval refuelling-under-way from ship to ship particularly
    Anyone in a small craft who has been overtaken by a large vessel in a
    narrow waterway will know what I mean. It takes some hard helming to keep
    So that's a force acting in the opposite sense to the apparent extra lift
    that Jared describes for an aircraft on landing.
    I wouldn't argue that these effects are unconnected: there may indeed be a
    common mechanism behind them both. But it can't be quite as simple as Jared
    makes out.
    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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