Post by
zezo | 2015-01-05 | 18:35:11
Copying from another post, because the information is buried a bit.
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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.
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