NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: DR plotting techniques
From: Rodney Myrvaagnes
Date: 2003 Oct 18, 09:24 -0500
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 >doesn't. > >George. > >================================================================ >contact George Huxtable by email at george@huxtable.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.