How Do Sailboats Sail into the Wind?
Sailboats cannot sail directly into the wind. They can, however, sail upwind by following a zigzag course so that the wind pushes first from one side of the sail, then the other. This is called tacking. In sailing upwind, the sails are set so that they catch some of the wind.
And because the keel, or center board, keeps the boat from slipping sideways, it is pushed forward. After tacking in one direction for a distance, the boat then comes about and tacks in the other direction. By repeating this zigzag maneuver, the boat finally reaches its destination.
But this reverse movement is possible because a moving boat’s sail is shaped as an airfoil like the wing of a plane. When air moves over a plane’s wing, from front to back, wind flowing over the top of the wing has to travel farther than wind flowing under the wing’s bottom surface. This creates a pressure difference that lifts the plane.
On a sailboat, wind blowing against the boat at an angle inflates the sail, and it forms a similar foil shape, creating a difference in pressure that pushes the sail perpendicular to the wind direction.
Together, the forces of drag, from the water, and the pressure from the wind against the sail itself push the craft forward. It moves at an angle opposite the direction of the wind, called windward in sailing terminology.
Windward sailing also does not work if a boat is pointed directly opposite the wind direction, according to The Physics of Sailing.
Wind has to be moving against the boat at an angle of at least 40 degrees for most vessels. Angling too sharply into the wind causes the forces on the boat to become unbalanced, and moves the boat sideways in the water.