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Re: DR plotting techniques
From: George Huxtable
Date: 2003 Oct 18, 00:13 +0100

```Rodney Myrvaagnes wrote-

>On Fri, 17 Oct 2003 17:42:28 +0100, George Huxtable wrote:
>
>>
>>This is how GPS is used on my boat. I don't bother with waypoints, or with
>>tracks between them, because in a cross tideway such straight ground-tracks
>>are often highly inefficient. Steering is always done from the ship's
>>compass, not heeding any off-track error indication. At irregular intervals
>
>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.

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---.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.
================================================================

```
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