Forces on sails: Difference between revisions

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The two sails cause an overall larger displacement of air perpendicular to the direction of flow when compared to one sail. They act to form a larger wing, or airfoil, around which the wind must pass. The total length around the outside has also increased and the difference in air speed between windward and leeward sides of the two sails is greater, resulting in more lift. The jib experiences a greater increase in lift with the two sail combination.<ref>{{Cite book|last=Anderson|first=Bryon D.|url=https://www.worldcat.org/oclc/52542601|title=The physics of sailing explained|date=2003|publisher=Sheridan House|isbn=1-57409-170-0|___location=Dobbs Ferry, NY|oclc=52542601}}</ref>

==Sail performance design variables==

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*For light air (less than 8 knots), the sail is at its fullest with the depth of draft between 13 and 16% of the cord and maximum fullness 50% aft from the luff.

*For medium air (8–15 knots), the mainsail has minimal twist with a depth of draft set between 11 and 13% of the cord and maximum fullness 45% aft from the luff.

*For heavy (greater than15 knots), the sail is flattened and allowed to twist in a manner that dumps lift with a depth of draft set between 9–129 and 12% of cord and maximum fullness 45% aft of the luff.

Plots by Larsson ''et al'' show that draft is a much more significant factor affecting sail propulsive force than the position of maximum draft.<ref name=Principles>

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