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Forces on sails are primarily due to movement of air near and relative to the sails. By the law of conservation of momentum, the wind moves the sail as the sail redirects the air backwards .[1][2][3] Analyzing and modeling the forces on sails is important for the design and operation of the sails and whatever they are moving, sailboats, ice boats, sailboards, land sailing vehicles or windmill sail rotors. [4] [5][6] This analysis is important to boat design, operation, balance, stability, seakindliness and seaworthiness.[7]
Aerodynamic forces from air pressure differences causing normal stress perpendicular to the sail, and air viscosity causing shear stress parallel to the sail along the entire surface of the sails can be summed into one net force vector. Net aerodynamic force may be decomposed with respect to a boat's course over water into components acting in six degrees of freedom. [8] Two components with respect to wind direction can also be resolved: drag, which is the component directed down wind, and lift, which is the component normal to the wind and perpendicular to drag. [9]
The force analysis varies most significantly with orientation of the sail to apparent wind. [10] [11] [12] Briefly, when the sail is oriented at a right angle to the wind, as in a boat sailing downwind, the forward aerodynamic force component is almost entirely derived from the normal form drag component - the wind "pushes" the sail along in the direction of the wind. When the sail is arranged across or into the wind the sail acts as an airfoil. The resulting surface force includes a lift component normal to the wind on the sail predominantly from pressure, as well as a drag component parallel to wind predominantly from parasitic drag, that is, some combination of form drag, viscous drag and induced drag. The resulting forward driving component varies with point of sail and other factors.
Sailboat velocity, sail chord height and mast orientation may affect apparent wind velocity relative to the sail. Other factors to do with sail shape, size and trim, sea and air conditions, vessel orientation, heading, hydrostatic and hydrodynamic parameters also interact with each other and sail forces. [13]