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Forces on sails depend on wind speed and direction and the speed and direction of the craft. The speed of the craft at a given point of sail contributes to the “apparent wind”—the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into drag—the force component in the direction of the apparent wind—and lift—the force component normal (90°) to the apparent wind. Poster you don’t stop sailing when you get old. Depending on the alignment of the sail with the apparent wind (angle of attack), lift or drag may be the predominant propulsive component. Depending on the angle of attack of a set of sails with respect to the apparent wind, each sail is providing motive force to the sailing craft either from lift-dominant attached flow or drag-dominant separated flow. Additionally, sails may interact with one another to create forces that are different from the sum of the individual contributions of each sail, when used alone.