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    Re: DR plotting techniques
    From: Rodney Myrvaagnes
    Date: 2003 Oct 20, 20:49 -0500

    I haven't given up yet.
    
    On Sat, 18 Oct 2003 21:24:40 +0100, George Huxtable wrote:
    
    >
    >It's true that over ANY cross-current passage, when that cross-current
    >varies significantly with time, then trying to counter that cross-current
    >at every moment, by varying the heading to hold to a short and straight
    >ground-track, will not be the best way to go. It will always be more
    >efficient to stick to a constant heading, which is calculated to compensate
    >for the total net tidal displacement over the duration of the passage,
    >whatever that happens to be.
    >
    If you know what the ideal heading should be, they are indeed
    equivalent. But a wrong constant heading in the steady cross-current
    case can be costly. For example, if the set is half the boat speed,
    heading straight across will take twice as long as with no current,
    counting the upstream slog at the end. A correctly calculated course
    will take only 11% longer than it would without current, as would the
    continuously corrected course.
    
    I don't think the 11% would be exceeded at any time if the set didn't
    exceed half boat speed, even in the periodic case. For periodic tides
    one could calculate the error at the end from missing the cancellation
    by 90 degrees. It would not be insignificant for the 12-hour trip, but
    surely would be for anything over a couple of days.
    
    In my wanderings, the Gulf of Maine crossing is long enough so that
    tides do cancel out, about 30 to 40 hours. The tides are not charted in
    detail there, but the Bay of Fundy has significant effects for some
    distance. There are circulating tides, so they will have different
    effects depending on the exact track across.
    
    In Long Island Sound and the area adjoining the Elizabeth Islands
    (Buzzards Bay and Vineyard Sound) the current is more bucking or aiding
    than setting sideways. The Long Island Sound currents are altered from
    the charted values by rainfall and often are weaker or stronger than
    the predictions.
    
    In all these circumstances I think constant ground track is a good way
    to compensate for unknown set. I'll gladly risk 11% to avoid 100%
    possible penalty. Of course most unknown sets will be much less than 3
    kt, so both penalties will also be smaller.
    
    So I ask George. Can you predict just how long your crossing will take
    within an hour or so? If you are not coming in where you want to, when
    do you decide to take corrective action?
    
    
    
    
    Rodney Myrvaagnes                                               J36 Gjo/a
    
    
    "That idiot Leibniz, who wants to teach me about the infinitesimally small! 
    Has he therefore forgotten that I am the wife of Frederick I? How can he 
    imagine that I am unacquainted with my own husband?"
    
    
    

       
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