Sails work by “catching the wind” only when the boat is sailing directly downwind.
The rest of the time, a sail is essentially an airplane wing standing on end, and works the same way:
When properly trimmed (adjusted or positioned), the sail’s leading edge—the luff—points into the wind, creating higher pressure on the windward side (the side facing the wind) and lower pressure on the leeward side (the side away from the wind).
The sail “lifts,” or moves, toward the lower-pressure zone, pulling the boat along with it. This works because the sail isn’t a flat sheet of cloth, it’s curved, like a wing. (The curvature, or “draft,” is built-in by the sailmaker, through careful cutting and sewing of the narrow panels that make up the sail.)
Not all of the lift developed by a sail moves the boat ahead. Since the direction of lift is roughly at right angles to the sail, some of it tries to pull the boat sideways, too—but the shape of the hull and keel creates a high resistance to the sideways force, and the boat moves ahead.
How much of the total lift acts to pull the boat forward and how much sideways depends on the “point of sail,” the angle between the boat and the wind. Sailing “closer to the wind” means there will be more sideways component, because the sail is trimmed in closer to the centreline of the boat.
Because of this, when “beating” into the wind, most sailboats move a little bit sideways as well as ahead. Sailors call this “making leeway,” and always take it into account when navigating or sailing in close quarters. To learn more, check out Beginner Sailing Tips
or visit a sailing school