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    Allowing for current. was: Re: Noon sun fix example
    From: George Huxtable
    Date: 2009 Sep 30, 22:39 +0100

    This was triggered by Jeremy's reference to the effect of current, but it's
    about a rather different situation, of tidal currents.
    
    It's arisen here before, a long time ago. I can illustrate it best by a
    little problem that faces many small-boat navigators crossing the English
    Channel, between Anvil Point, a headland South of Poole, and Cherbourg, to
    its South by about 50 miles. I will simplify the details, but for many small
    craft that passage will take about 12 hours, or just two tides' worth. Say
    the tide flows at about 2.5 knots, a total displacement of about 11 miles
    East, over the first 6-hour period; then 11 miles West, over the next 6
    hours..
    
    Many navigators will set the destination of Cherbourg as an intended
    waypoint, then religiously adjust their heading to keep their ground-track
    along that intended line, angling against the current to keep it so. They
    are, of course, wasting time and energy. Because, over 12 hours, the net
    tidal displacement, East then West, will sum up to be close to zero, there
    will be no overall effect of tidal current. In which case, the correct and
    easy procedure is to head due South, allowing your craft to be swept
    up-channel first, then back again later. By ignoring the instantaneous tide,
    and the resulting cross-track error, the vessel is heading exactly
    Southwards all the time; the best that can be done.
    
    Indeed, set out like that, it seems pretty obvious, but it's hard to
    convince many navigators that they can, and should, ignore those warnings of
    cross-track error.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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