Sandy Goodall - Sailmaker, Designer and Sailing Principles Instructor, Victoria, British Columbia, aboard the Sif, Discussing how sails work
As you can see, the sail is not flat but has quite a distinct three dimensional curvature or shape. That's essential for the sail to be able to convert wind and energy into forward motion. Briefly, the way a sail works is that it's an obstruction to the oncoming wind and air is much like water, it acts like a fluid so when it approaches a sail it has to split and divide so that the air flows down one side of the sail and some flows down the other. And in quite a complex process which involves the air's viscosity or stickiness, the tendency to stick to what it's touching, a certain type of turbulence ensues and the air actually rotates around the sail and in such a way as to accelerate the flow over one side relative to the speed of the flow on the other side. That creates a difference in pressure, the upshot of which is kind of a suction or pull on one side of the sail and that is what actually draws the sail, draws the boat along.