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    Re: Cross-tide strategy (was: DR plotting techniques)
    From: Rodney Myrvaagnes
    Date: 2003 Oct 25, 18:48 -0500

    On Sat, 25 Oct 2003 21:06:55 +0100, George Huxtable wrote:
    
    >
    >>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.
    >
    >I don't see the logic in that statement above. In semidiurnal tide areas, a
    >"tide", between reversals, takes a bit over 6 hours, so 38 hours would be
    >about 6 such tides and should indeed cancel out. But, by that same
    >reasoning, a passage time of 32 hours would be 5 whole tides, and
    >(depending on the phase at departure) you easily could end up displaced
    >sideways from your goal by one whole tide's-worth, to port or to starboard.
    >If the passage time is as loosely specified as "30 to 40 hours", then
    >cancellation can not be relied on.
    >
    George is correct here, and I apologize for not correcting myself
    sooner. I did realize that the --
    
    integral {sin(x) dx} is cos(x) over any span, and the error at the end
    would be just as great or small no matter how many whole cycles had
    passed.
    
    --shortly after I sent the last but I was on my way to a play and
    forgot about it by the time I got home.
    
    I will try to get back to the remainder ASAP.
    
    
    
    
    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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