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    Re: DR plotting techniques
    From: Rodney Myrvaagnes
    Date: 2003 Oct 18, 09:24 -0500

    On Sat, 18 Oct 2003 00:13:24 +0100, George Huxtable wrote:
    >Rodney Myrvaagnes wrote-
    >>I can't see how an uptide or downtide arc could be more efficient than
    >>a continuously-corrected rhumbline. Could you explain which is more
    >>efficient, and why?
    >Rodney, I'm glad someone asked that question. I can explain it best by an
    >example, which not by chance happens to correspond rather closely with the
    >passage between my own home port of Poole and its opposite number on the
    >French coast, Cherbourg.
    >Consider a passage from port A to port B, which is due South of A, across a
    >channel which runs East-West, and is subject to a strong tide, running 6
    >hours each way. Say the distance A to B is such that in smooth water and at
    >the vessel's cruising speed, the passage would take just 12 hours. And say
    >the vessel departs from A, just when the East-going tide commences.
    Come on, George. That is an extremely special case.
    I am leaving to catch a tide in the East River, but I will try to
    pursue this further next week.
    >If the vessel just steers a Southerly course throughout, the tide will take
    >her, say 15 miles to the East of the direct straight A to B track, over the
    >first 6 hours. Then, the tide will turn Westerly, and over the next 6 hours
    >it will bring her back West by that same 15 miles, to deposit her right at
    >the doorstep of port B.
    >If the vessel was following a ground-track using GPS, waypoints would be
    >set at A and B, and a straight-line ground track drawn between them. The
    >helmsman would be commanded to steer West of South during the first 6 hours
    >to keep to that track and counteract the tide, and then, later, East of
    >South, to do the same. In the case of a slow vessel, and a hot tide, the
    >attempt to keep to the straight track may even become impossible. But in
    >any case, those Eastings and Westings are quite counterproductive,
    >cancelling each other out, and are made at the expense of the Southing,
    >which in this case is all that matters. Sceptics may find that a simple
    >vector diagram will convince them, but are welcome to argue back if it
    >contact George Huxtable by email at george---.u-net.com, by phone at
    >01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    >Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Rodney Myrvaagnes J36   Opinionated old geezer
    Eagles may soar, but weasels don't get sucked into jet engines.

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