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    Re: Allowing for current. was: Re: Noon sun fix example
    From: Douglas Denny
    Date: 2009 Sep 30, 16:54 -0700

    Now I don't want you to get paranoid George and think I am 'getting at you' as 
    I assure you when I disagree with you it is not a personal attack! (which I 
    am beginning to feel you might think);  however, I have heard this chestnut a 
    number of times and it simply does not work with a sailing vessel. (i.e. a 
    'stick and flag' type of vessel).
    The 'stink and rag' boys get over there so fast it doesn't matter anyway.
      
    The differences actually experienced with the tidal streams in the English 
    Channel as you cross for a variety of reasons are not symetrical and you get 
    shoved one way more than the other. I know I have done it.
    Probably such things as the topology of the Channel sea-bed; swirls of current 
    around bays; the Alderney race flushing out across the Channel; faster stream 
    near the centre but more one side than the other ..who knows? I guess a 
    thousand and one reasons for it.  Another fine theory in principle, that does 
    not work out practically.
    
    Even when allowing for tidal streams every half hour with the DR plot as I 
    used to do,  there can still be differences appearing which make the passage 
    lop-sided as you cross, but one thing is certain - this gives a much better 
    end result than a 'straight ahead' strategy over the tidal cycle.  There are 
    too many  variables for such a simplistic approach.
    
    I have to admit I have not done it for some years so would be interested to 
    know of anyone who has done it recently with GPS to guide them. I only had LF 
    beacons such as St Catherine's Point on the IOW, and Nab Tower... or best of 
    all:  Mk I eyeball when there wasn't fog or rain or cloud.
    
    Douglas Denny.
    Chichester. England.
    ==========================
    
    Original Post:
    
    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,
    
    
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