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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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DR Errors and usable LOPs
From: Bob Goethe
Date: 2016 Aug 13, 08:56 -0700

Bernard Moitessier set the "record for the longest nonstop passage by a yacht, with a total of 37,455 nautical miles in 10 months" (Wikipedia, "Bernard Moitessier").  He wrote a book about this adventure, the English translation of which is called "The Long Way".  He had, in a footnote, an interesting comment on the role of the DR/EP postion in celestial navigation:

The log turns steadily. Before I left, I could not see the point of continually towing a log in the middle of the ocean, wearing out the mechanism for nothing. I find a log most useful when nearing the capes or in coastal sailing, where accurate dead reckoning is essential.1

Nonetheless, I promised the Vion company to tow the log during the whole trip, to test out their equipment. I do not regret it, because the log helps me trim the sails to their optimum. A variation of a quarter of a knot is hard to feel; the log picks it up. And a quarter of a knot means six extra miles in 24 hours.

[Footnote 1:  In working a celestial sight, one can base the calculation on an estimated position that is completely absurd; the position line will bring the boat back to her true location. If the intercept is too long one need only redo the calculation using the new estimated position, which will be more accurate than that shown by the log, especially after a meridian sight. Just for fun, I have sometimes deliberately picked a position 600 miles off. In two calculations (easily done with the HO 249 tables) the boat took her true position on the chart.]

I have in the past, for curiosity's sake, started with an assumed position that I knew was 120 miles too far east, and then worked out my LOP.  I am intrigued with the extent to which Moitessier experimented with this...and with the usable results he got.

Bob

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