Bug on router ?
Post by iconPost by GeGaX | 2018-12-11 | 22:47:32

Hi Cvetan,
I just realized that the router offers me a TWA above 160 ° and Best VMG is 160 ° ...
There would not be a little bug?

commenticon 4 Comments
Post by iconPost by DonJuan21 | 2018-12-13 | 18:06:59
You best route is not simply following the best VMG at a particular time.
Post by iconPost by zezo | 2018-12-13 | 20:49:16
But it's always within the optimum VMG limits (unless it' for a very short time and the tack penalty takes more time than sailing in sub-optimal conditions)

The problem here is that the directions are somehow averaged, and in shifting wind conditions TWA 160 with fixed heading in one step becomes 163 in the next one.

Also, at 41 knots TWS the VMG falloff above the maximum is very slow.

But the VMG table for Multi 50, full options looks like this (index is speeed in km/h for historical reasons):

static unsigned best_downwind[] = { 140,140,140,140,140,140,140,140,140,140,140,140,140,140,140,140,140,140,
140,142,142,143,143,143,142,142,141,140,140,140,140,141,142,147,147,148,
148,148,150,150,150,150,150,150,150,150,150,150,150,150,150,150,150,150,
150,150,150,150,151,154,156,158,160,160,160,160,160,160,160,160,160,160,
160,160,160,160,160,160,159,160,160,160,160,160,160,160,160,160,160,160,
160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,
160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,160,
160,160,160,160}

So the router won't even attempt sailing above 160.

Post by iconPost by DonJuan21 | 2018-12-14 | 06:37:02
Interesting. How are the polars actually set? From concept, theory, or field testing?

DonJuan21
Post by iconPost by zezo | 2018-12-14 | 07:38:06
From theory, before the boat is built.

https://en.wikipedia.org/wiki/Velocity_prediction_program

Then real wold performance data is collected, of course. Large modern boats are highly instrumentated with hundreds of sensors. But that does not get in the game ;-)

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