If you’ve never thought about it, you might assume the wind always moves a sailboat by pushing it. This is true when a sailboat moves in the same direction as the wind. The wind fills up the sail and pushes the boat along. Running with the wind can be especially exciting when flying the big, bright spinnaker sail.
But how can a sailboat move in any direction other than with the wind? Simple. The wind pulls the boat. The principle is the same as the one that makes airplanes fly. It is called the Bernoulli Effect.
Two things are required to make the Bernoulli Effect take place: moving air (or fluid) and a surface curved on one side. When air moves over the curve (such as the rising curve of an airplane wing or the outward curve of the sail), that air moves faster than the air moving underneath the curve. This creates lower pressure on the top of the wing or the outside of the sail. This difference in pressure creates lift for the airplane and pull for the sailboat.
Because of this effect one of the sailor’s main jobs is to keep the curve of the sail correct. Sailors call this “trimming the sails.” Sailors adjust the trim by pulling in on or releasing the sheet secured to the clew of the sail. When the sail is correctly trimmed, it stays taut and works efficiently to move the boat. If it is not trimmed correctly, it luffs and spills wind. The boat does not move efficiently. For an excellent website with animation demonstrating the Bernoulli Effect and its use in sailing, visit http://www.seed.slb.com/en/scictr/watch/sailing/index.htm.