How Does a Boat Sail Upwind?
If your destination lies upwind, how do you sail there? Unless the wind is blowing from directly astern (over the back of the boat), the sails propel the boat forward because of “lift” created by wind blowing across them, not by wind pushing against them. As you steer more toward the wind direction, you trim the sails in tighter to keep them full, and keep generating lift. But sail too close to the wind and the sail will “luff”— the forward edge will start to flutter in and out and the boat will slow down. Turn more into the wind and soon the whole sail will be flapping like a bed sheet hanging out to dry. But keep turning through the wind and soon the sail will fill on the other side of the boat. This is called “tacking.
