How a sail makes a boat move forward
Now, if you remember your physics classes really well, you might ask: "Yeah, but if the force is pointing this way, how come the boat doesn't drift to the side ?"... and you would be just right !
The trick is that a sailboat has a keel underwater, that produces a resistance in the opposite direction ( green ) of the sail's side component force ( red )
These two forces balance each other and, hence, are cancelled
The only remaining force is the one forward, which is why a sailboat does not drift, but instead sails forward
Hint: Many beginning sailors think "The tighter the lines, the faster the boat"
This is actually generally the opposite
Indeed, as just seen, a sail force is perpendicular to the plane of the sail
Therefore, the more you "open" a sail, the more its resulting force ( black ) is oriented forward and the "longer" gets its forward component force ( blue ), which makes the boat sail faster
Compare the length of the blue arrow below with the blue arrow on the latest figure, the black force did not change in length, but, because it's pointing more forward, its forward component ( blue ) increased significantly
Therefore, try to open your sails, whenever possible, in order to gain more speed
