How do sails propel a boat forward what principle does a sail cause and object to move?

Answer 1

A sail can act in different ways.

It can catch the wind straight on which causes the boat to be dragged along, like holding a bag in front of a fan.

Or it can catch the wind at an angle. This makes the sail work like an aeroplane wing. The wind makes the sail curve and air rushes over the front (concave or indented side) and over the back (the convex or bowed out side).

The air passing over the back, bowed out side moves faster than the air on the front so the pressure on that side is lower. This causes lift which pulls the boat in that direction.

(research lift of wings to get a full understanding how this works)

This lift effect can be used to move a boat in a different direction to the way the wind is blowing. This is why boats tack or zig zag sometimes rather than go in a straight line. Learning to set the sails and how to manoeuvre when the wind isn't blowing the way you want to go. Is more like an art than a science. The basics can be picked up quickly but it takes many years or even a lifetime to become an expert. They act as aerofoils, much like the wings of an aeroplane, and work by the air pressure difference on the surfaces of the sails.