To understand how a boat moves upwind, one must look at the sail not as a bag catching air, but as an airplane wing standing vertically. This relies on . As wind flows across the curved surface of a taut sail, the air traveling along the outside (the "back" of the sail) must move faster than the air on the inside. This creates a zone of low pressure on the outside of the sail, effectively "pulling" the boat forward and sideways. The Role of the Keel
Should I include a section on or stay focused on sailing ? Into the Wind
If a sail only generated lift, the boat would simply slide sideways across the water. This is where the or centerboard—the heavy fin beneath the hull—comes into play. The water provides resistance against the keel, counteracting the sideways force of the wind. The result of these two opposing forces (the pull of the sail and the resistance of the keel) is a resultant force that drives the boat forward. The Art of Tacking To understand how a boat moves upwind, one