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    Re: Allowing for current
    From: Peter Fogg
    Date: 2009 Oct 2, 13:01 +1000

    George you wrote:
    "I will simplify the details, but for many small
    craft that passage will take about 12 hours, or just two tides' worth."
     
    Your assumption relies on that coincidence of time and tide.  If the passage is slower or quicker the navigator is.. necessarily off track. 
     
    Another assumption is that a direct course can be steered, which is rarely enough the case for a sailing boat.  And this has to be a slow sailing boat, to take the requisite 12-hours.  If the same distance can be made in 6-hours or less, thus possibly on the same tide, then presumably some account needs to be made for the offset.
     
    Its not just simply when, as you put it: "beating was called for".  Given a choice for a cruising boat of sailing uncomfortably and slowly up-wind, paticularly if heading into some swell, or easing the sheets and making better speed more comfortably along some sort of reach, most sane folk (we'll exclude racing crews) will go for the latter.  Making better speed will often tend to largely counter the longer distance sailed. 
     
    George you claim that DD has provided "No such explanation" of why he rejects your assertion.  This is what he said:
     
    "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."
     
    That sounds like an explanation to me.  Pretending it didn't get put is somewhat different to either acknowleging or refuting it.
     
    Then you have abandoned your own real-world example, and proposed replacing it with some hypothetical scenario.  This, it seems clear, is an implicit acknowledgement of the strength of DD's argument, and the weakness of your own. 
     
     
     
     
    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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