Boat speed difference between VORG and zezo.org
Post by iconPost by tlew | 2014-11-22 | 14:15:53

Previously it has always been accurate what boat speed zezo.org and VORG displays.
But the last hours I have noticed that the speed in VORG is lower compared to what zezo.org claims it should be. I have checked that the heading, sails, and position of the boat is the same. Also the wind and wind direction is the same between them two, but still, in VORG I get a lower boat speed compared to what zezo claims.

commenticon 5 Comments
Post by iconPost by tlew | 2014-11-22 | 15:29:52
Anyone that have an explation why it may differ?
As for now, Zezo recommends a heading of 56 degrees. Boatspeed would be 22 knots.
However, when I set the heading to 56 in Vorg the boatspeed is 21,1 knots. Sail, wind direction, wind speed, all are the same between them two, so how come this quite big difference? Don't think I have seen it before?
Post by iconPost by zezo | 2014-11-22 | 15:46:14
You have probably noticed the fact that sometimes (quite often, in fact) heading, twa and wind direction don't add exactly to 0/360

In your case the data is something like heading 55, wind 272 and twa 141

Heading 55 actually means twa 143, and because of the slope of the polars the corresponding speed drops about half knot in just 2 degrees.

The reason why heading and TWA don't match exactly is the wind interpolation. The heading is actually averaged direction to a point a bit further down the track.

I personally use the track tool to check if the green or the red line approximates the track better, and always check the polars to see if there is a crossover or optimum point nearby.

To recap: The indicated speed should match the indicated TWA, but the indicated heading may be off, and speed at indicated heading will differ. When sailing up/downwind you should generally consider the TWA, but there are cases when the router wants to get somewhere in straight line and then the heading is what matters.
Post by iconPost by tlew | 2014-11-22 | 16:24:12
Yes, the speed matches the TWA. Thanks for the explanation.

The following made me curious: "Check the polars to see if there is a crossover or optimum point". Not sure exactly what it means. Can you elaborate some regarding that?
Post by iconPost by zezo | 2014-11-22 | 16:31:51
Crossover points are where sail changes occur. At some wind speeds there are quite big dips between the sails. Take 24 knots as example - it would be faster to sail at TWA 100 and then 110 if you want to go in the direction of TWA 105 in straight line.

And then even single sails can have dips in the polars - like the jibs in the vvorg have a dips around 55, which might make sailing at 50 and then 60 faster than the straight line.
Post by iconPost by tlew | 2014-11-22 | 20:54:38
Thanks!
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