Sailing across or towards the wind, a sail works like an aircraft wing: the curved sail turns the airflow, creating lower pressure on its leeward side and higher pressure on its windward side, which pulls the sail forward and sideways. Sailing with the wind behind, the sail is mainly pushed.
A sail works in two different ways, depending on the point of sail.
Across and towards the wind, the sail is a wing. When it is set at a small angle to the wind it fills into a curved shape, rather like an aircraft wing stood on its end. The wind divides at the front edge of the sail, the luff, and flows smoothly round both sides. The sail’s curve and angle turn the airflow: on the leeward side the air speeds up and its pressure falls, and on the windward side it slows and its pressure rises. That pressure difference is lift, and it pulls the sail towards its leeward side and slightly forward.
Lift only works while the air is flowing smoothly over both sides. If the sail is let out too far, or the boat steers too close to the wind, the sail meets the wind edge-on and the luff flaps. If the sail is pulled in too far, the air on the leeward side can no longer follow the curve, breaks away, and the sail stalls. In both cases the sail produces less drive. The telltales on the jib show which is happening.
With the wind behind, the sail is mainly pushed. On a run the sails are let right out across the wind, and the wind simply pushes on them. This is less efficient than lift, which is one reason a boat is usually faster on a broad reach than on a dead run.
Most of the force from a sail working as a wing acts sideways rather than forwards. The keel or centreboard resists that sideways force and leaves the forward part, the drive, to move the boat ahead. The sideways force acting high up the mast is also what makes the boat heel.
A popular explanation says the air on the curved side speeds up because it “has further to go” and must arrive at the back of the sail at the same time as the air on the other side. That isn’t the real reason, and in practice the leeward air arrives first. The speeding up comes from the way the curved, angled sail turns the flow, as on a wing.
More in the book: Chapter 3, How sailing works