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    Re: Revisiting hull pressure wave
    From: George Huxtable
    Date: 2005 Jan 26, 17:27 +0000

    Earlier today I posted about message on this thread, in which I wrote-
    
    "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."
    
    That was wrong. Where I wrote "draught is 3 x depth" (and so on), it should
    of course have been "Depth is 3 x draught". Otherwise, the vessel's speed
    would certainly have been zero! Sorry about that.
    
    So that paragraph should have read-
    
    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 depth is 3 x
    draught, by 100% at 4 x draught, by 70 % at 5x, 50% at 6x, 25% for 8x.
    
    Presumably, with a thin-keel or centreboard, the relevant draught would be
    that of the hull only, without appendages. As for the relevant draught of a
    planing craft, who knows?
    
    ===================
    
    To save some arithmetic, Marchaj gives the critical speed for a depth of 10
    ft as 10.6 knots, that speed being proportional to the square-root of
    depth.
    
    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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