Page "Sailing" Paragraph 4

The air interacting with the sails of a sailing vessel creates various forces, including reaction forces.

If the sails are properly oriented with respect to the wind, then the net force on the sails will move the vessel forward.

However, boats propelled by sails cannot sail directly into the wind.

They must tack ( turn the boat through the eye of the wind ) back and forth in order to progress directly upwind ( see below Beating or " Working ").