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Re: Relative plotting vs Geographical plotting
From: George Huxtable
Date: 2002 Jan 12, 1:38 AM

Peter Smith wrote-

>George Huxtable [mailto:george@HUXTABLE.U-NET.COM] said:
>>
>> I don't think Peter Smith's solution can possibly be correct: commonsense
>> tells me so. We have two ships travelling in nearly the same direction,
>and
>> the second ship is slowly overtaking the first. In that case, the speed of
>> the overtaking ship must be greater than that of the ship being overtaken.
>
>Hmmm. I'm not sure from the above who's the "second ship" and who's
>the "first ship". The target is ahead of us and to Port (bearing 322d).
>The bearing is constant, so we are converging. Since the target is
>ahead and the range is decreasing, we must be overtaking the target.
>
>> ...
>> If you add the vectors 12 knots at 150 and 1.45 knots at 142, using trig
>or
>> drawing, you end up with 13.44 knots at 149.1.
>
>But that gives the target a greater speed than our own ship, even
>though we are overtaking it.
>
>Moreover, the target is on our Port bow (bearing 322d). For our
>courses to converge (constant bearing), the target's course must
>be a little to starboard (i.e., numerically GREATER) of ours. Thus,
>if we're making 150d, he should be making a little greater then 150d.
>If he's making 149.1d, we would be diverging.
>
>Can you read an Excel spreadsheet? I have both the geographic and
>relative vectors problems worked out trigonometrically. I'll send
>it to you and see what you think.
>
> -- Peter

=================

George Huxtable replies-

Aha! The penny drops... Now I see why we disagree. I didn't read Pierre
Boucher's problem fully. Here is what he said-

===================

>Own course and speed:
>150d   12.0 kt
>
>On the radar screen (relative bearing mode):
>at 0448 a "spot" at 7.0 nm  322d   on the screen
>at 0554 a "spot" at 5.4 nm  322d   on the screen
>
>Request :
>Other ship's course
>Other ship's speed

===================

Pierre states quite clearly "relative bearing mode". For some reason, I
took it as North-up mode instead. It makes all the difference, of course. I
was quite wrong. Sorry about that.

When I repeat the calculation, summing vectors, I now get exactly the same
result as Peter Smith, 10.9 knots at 154.7 degrees.

Thanks for putting me right, Peter.

George Huxtable.

------------------------------

george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
------------------------------

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