Recognize the signs of wind sheer and gradient and adjust your sailing technique accordingly. Choose the best course to win a long distance race by using a polar plot. What follows is a condensed excerpt from the Ockam U manual covering true wind, polars and the Wally. True wind is the wind that passes over a boat as if it were not moving. Sails are trimmed to the Apparent wind, but boat performance is best characterized with reference to the true wind.
Wind direction is the magnetic bearing of the true wind.
This number is the combination of True Wind Angle plus Heading plus Leeway, and is the most important output of true-wind instruments as we will see shortly. Polars Polars describe a boat's performance under given true wind conditions. In the figure above, the curved lines represent how fast a boat can sail at a given true wind speed for any true wind angle. The squares mark the fastest point of sail upwind and downwind. An arrow has been drawn at the port tack upwind sailing point, showing that that boat can be expected to make 8.3 knots at 35 degrees off the true wind.