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polar diagram Used for predicting and evaluating sailing vessel performance. They are graphical displays of the vessel's theoretical or measured speed as a function of true wind angle (TWA)for a given true wind speed (TWS)—they can also be presented in terms of apparent wind, but this is less common. A yacht racing handicap certificate in the International Measurement System (IMS) includes this data in tabular form. The graphical display shows directly the optimum true wind angles for maximum up wind or downwind speed. Samples are shown in Starpath Weather Trainer graphic G313 for a 33-foot LOA and for a 55 footer. We have more information in our article Introduction to Polar Diagrams and Optimum VMC. The navigation and weather program called qtVlm has an excellent presentation of polar diagrams with convenient digital interpolation for any TWA and TWS.

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