The diagram predicts a boat spped of 6 knots at 90 degrees true wind and a 3 knots wind. Is it so or I am wrong in reading polar diagrams? I got 2.5 knots on my 409 Performance in these conditions last Sunday. Juan Luis
I believe the polar diagram only goes down to a true wind speed of 6 knots, which is the inner red curve. That red curve crosses the 90 degree true wind direction line at the dotted line that corresponds to 6 knots of boat speed.
The same red curve crosses the 180 degree true wind direction line at the dotted line that corresponds to 3 knots. I suspect that's where you made your error.