Relative wind (sail NA)

The relative (apparent) wind on a sailing yacht is the vector sum of the true wind and the wind created by the boat’s own motion, V_app = V_true - V_boat in vector form, and it is the wind the sails and instruments actually feel. Because the boat-motion vector adds in, the apparent wind is stronger and drawn forward of the true wind, increasingly so as boat speed rises, which is why a fast craft sailing across the wind still trims its sails well forward. The apparent-wind angle and speed set the sail’s angle of attack and therefore its driving and heeling force; the apparent-wind triangle is the starting point of every velocity prediction program (VPP) solving for equilibrium boat speed.