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    Re: Revisiting hull pressure wave
    From: George Huxtable
    Date: 2005 Jan 26, 11:13 +0000

    Bill asked-
    >A while back the topic of not being able to achieve hull speed due to a
    >downward pressure wave interacting with a relatively shallow bottom.
    >My interest still runs high in finding a formula to relate
    >length-at-waterline and velocity to the depth that it comes into play, and
    >the affect as depth decreases past that threshold.
    >Any help would be appreciated.
    Response from George.
    Two of the books on my shelves touch touch this topic.
    Try "Sailing theory and practice", by C A Marchaj (Adlard Coles 1964),
    which is the more informative of the two.
    Or "The science of yachts, wind, and water", by H F Kay, (pub. G T Foulis,
    These authors agree that at a certain critical boat-speed, which depends on
    the depth, there is a considerable increase in drag due to shallow-water
    effects.However, if the speed can be considerably exceeded beyond that
    limit (possible by a planing craft, but not a displacement vessel) then the
    drag in shallow water can actually become slightly less than the
    corresponding drag in deep water would be.
    Marchaj puts the critical speed, in ft/sec, as 5.7 x (square root of depth
    in ft.). And he provides a graph which shows that around that speed,
    shallow water effects increase the drag by 220 % if the draught is 3 x
    depth, by 100% at 4 x depth, by 70 % at 5x, 50% at 6x, 25% for 8x.
    Marchaj gives no references for what work this graph is based on, and I
    would take the details with a pinch of salt. But it may well give a flavour
    of what to expect.
    For an authoritative naval architect's account of shallow-water effects,
    probably Kenneth C Barnaby's "Basic Naval Architecture" (Hutchinson, my
    edition is 1953) is as good as any, but like most naval architects, he is
    concerned with quite a different type of vessel (merchant and naval) so
    will not be very relevant to Bill's quest.
    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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