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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Cross-tide strategy (was: DR plotting techniques)
From: Rodney Myrvaagnes
Date: 2003 Oct 25, 18:48 -0500
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?"