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    Re: Allowing for current
    From: George Huxtable
    Date: 2009 Oct 1, 16:26 +0100

    Well, this becomes interesting.
    
    I had pointed out the disadvantages of following a ground-track from
    departure to destination, by continually adjusting heading to counter a
    cross-tide, in circumstances when the net amount of such tidal displacement
    over the voyage cancelled out. And took, as an example, a well-frequented
    passage between Anvil Point and Cherbourg, across the English Channel,
    adding- "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."
    
    And indeed, it appears that we might have the good fortune to find a
    proponent of that practice within our midst, who has written, in [10005]-
    
    "Even when allowing for tidal streams every half hour with the DR plot as I
    used to do,...one thing is certain - this gives a much better end result
    than a 'straight ahead' strategy over the tidal cycle." I have often
    wondered why this view is so widely held. Perhaps we now have the chance to
    discover why.
    
    What arguments were advanced to back that assertion?
    
    1. That the English Channel currents are more complex than I described.
    True, and I had acknowledged simplifying the details, but so what?
    
    2. That the current can be greater in one direction than another.
    Conceivable, but then you would just have to apply an offset correction that
    is not exactly zero.
    
    3. That different considerations apply to sailboats, for some unspecified
    reason. That could certainly be the case, if there wasn't a free wind, and
    beating was called for. And the overall passage time is more unpredictable
    under sail, which might well call for a mid-passage reassessment. However, I
    didn't specify sailing craft or exclude them; just referred to a vessel with
    known speed.
    
    So now I ask Douglas Denny to confirm the procedure that he uses, when
    "allowing for tidal streams every half hour with the DR plot", which "gives
    a much better end result". Is he really saying that at each half-hourly
    point of the passage he heads to offset the cross-tide so as to keep to a
    straight ground track? I want us to be certain that we are not at
    cross-purposes here.
    
    If that really is his procedure, then I ask him whether he would apply it,
    not in the complex English Channel, but in a hypothetically ideal
    environment in which such complications have been swept away. Take a vessel
    travelling to a destination to its South across an E-W going tideway, which
    in the absence of any tide would make the crossing in just 12 hours. And a
    tide that will sweep 11 miles Eastward over six hours, then back 11 miles
    Westward over the next six, In such a simple, predictable world, in which
    the net tide over that crossing-time is known to cancel to zero, would he
    then steer to counter the instantaneous tide at each point to maintain a
    straight ground-track, or would he adopt a constant Southerly heading? We
    can assume a sailing craft with a fair wind, which under the conditions can
    maintain a speed of four knots, and can start an auxiliary if the wind
    fails.
    
    And if he chooses the straight ground-track, I await his explanation of what
    makes him so certain that it "gives a better end result", if that is indeed
    his claim.
    
    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.
    
    
    ----- Original Message -----
    From: 
    To: 
    Sent: Thursday, October 01, 2009 12:54 AM
    Subject: [NavList 10005] Re: Allowing for current. was: [NavList 9997] Re:
    Noon sun fix example
    
    
    
    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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