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