The Polar Diagram shows the speed potential of the yacht depending on the strength
of the wind and the angle of attack.
The speed potential means then possibility of the yacht to sail such speed with tanks half full, perfectly trimmed with maximum sails up, at calm seas, as predicted by the yacht designer.
Reading of the diagram:
Check the actual wind speed in kt and select one of the 4 colored lines, which fits closest. Let's call this the "wind force curve". Now check the "true angle of attack. This is the angle, between the centerline of the boat and the angle of attack of the wind if the yacht would stand still. This angle is shown in steps of ten degrees, surrounding the "wind force curve". The maximum potential speed is found at the crossing of the "wind force curve" and the angle of attack line.
MARY ANN reaches at 20 kt wind speed (about 5 Bft) a maximum height to the true wind of 35 degree, corresponding to a tacking angle of 70 degree. The optimum height to the wind would bee 38 degree at a speed of 7,6 kt through the water, corresponding to 6 kt towards windward. The maximum speed is 9,9 kt and will happen at an attacking angle of 115 to 120 degree. In reality higher speeds will be achieved, when the waves are taken into account and the yacht starts surfing down the waves. For week winds (6 kt = 1-2 Bft) the diagram predicts for courses between 55 and 115 degree still 6 kt plus.
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