Which properties of air are instrumental in propelling a sailboat?

Answer 1

The air in the troposphere (lowest layer) moves from regions of higher pressure to regions of lower pressure. This movement is necessary for wind.

The air has mass. The vector sum of the momentum all of the molecules of the moving air is the force which fills the sails and propels the boat.

The angle at which the moving air hits the sail is important in determining the direction of the craft.

The wind also interacts with the surface of the water creating waves. In very strong (greater than gale force) the reaction of the boat to the waves maybe be greater than the direct force from the wind itself.