Routing limitation/bug or my misunderstanding?
Post by iconPost by Lasse | 2018-01-11 | 16:24:29

In the Clipper race sezo wanted me to do some tacks (red arrows) which I couldn't see the meaning with. Instead I used a fixed TWA of 45 degrees because I would then reach the point with the blue arrow 10-20 minutes faster. Is it a bug or limitation in the routing algorithm? Or have I misinterpreted the green dotted line?


commenticon 8 Comments
Post by iconPost by toxcct | 2018-01-11 | 16:31:59
Pro Winches option checked maybe ?
Post by iconPost by Lasse | 2018-01-11 | 16:51:10
Yes. Which is also the only option my boat has.

The green dots and circles on the green line lies further away than the isochrones. How is that possible?? I thought that the isochrones represents every reachable point (at whole hours i.e. 12.00, 13.00) with perfect routing. But the green line with fixed TWA can easily take you further in the pic above. I don't understand...
Post by iconPost by zezo | 2018-01-11 | 16:51:36
It does that kind of stupid things sometimes, but it's hard to tell exactly why after the fact.
Post by iconPost by zezo | 2018-01-11 | 17:10:50
One possibility is that the short segments on port tack at your red arrows are "too backwards" to the destination, so fall outside of the router scope. Looks unlikely in this particular case, but hard to tell without the numbers over few of the black dots.
Post by iconPost by Lasse | 2018-01-11 | 17:27:03
Is this what you need?

Post by iconPost by zezo | 2018-01-11 | 17:59:30
Post by iconPost by zezo | 2018-01-12 | 00:23:09
Hmm. Saw something simililar myself right now. It looks like the router skips sailing at TWA 50 for certain winds speeds in the Clipper. Will have to take a better look.
Post by iconPost by zezo | 2018-01-12 | 07:56:24
I found the culprit. Off-by one error that would skip sailing at the TWA next to the optimum in certain conditions, like skipping TWA 50 with VMG at 45 above.

Actually the list of possible angles read 35,45,50 ... and they are indexed, so the 4th was 55 instead of 50.

That explains why the isochrones fall back behind the green line between 45 and 55.

The problem was present in all races, but most visible in the Clipper because of the sharp peak in the polars at TWA 50.

Thanks again for the note.

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