How is it possible that a sailboat can go faster then the wind?

Answer 1

The explanation involves the relative positions of the wind, sail and keel of the sailboat but essentially of atmospheric pressure (air pressure at sea level). This is approximately 15 pounds per square inch. In general the keel will be aligned at an angle into the wind (not directly) while the sail is placed parallel to the wind direction. The air pressure on one side of the sail (which is shaped like a wing) will be be reduced and the resulting force from the opposite direction will be transmitted to the spar (and thus the boat). Since the keel prevents the boat from moving directly in that direction, the boat will be forced to"slide up" into the wind. As it moves into the wind the movement relative to the wind increases still more increasing the pressure imbalance thus increasing the effect. A sail can be many sq. inches in area so that when the pressure is reduced on one side of the sail the force imbalance can be considerable. Much, much more than the wind's push against the boat. There are other factors such as friction effects of air and water but fundamentally it's air pressure and how the sail shape and positioning creates the pressure differential. The effect is even more dramatic for ice sailing. Wind speeds of 20 mpg can create boat speeds exceeding 60 mph. Every year there are serious injuries and even fatalities due to excessive speeds involving ice sail boats.