Sails generate most of their propulsive force from the smooth flow of air along the leeward side or the sail. A force similar to the lifting force of an aircraft wing is generated by the smooth flow of air over the leeward side of the sail. However, unlike a rigid wing the sail is a flexible membrane and depends upon airflow generated pressures, and fabric cut to specific patterns, to form and maintain this membrane in its three dimensional airfoil shape. Airflow striking the windward surface of the sail creates a region of higher air pressure which "pushes" against the sail surface while airflow deflected around the leading edge of the sail and flowing smoothly across the leeward side creates a region of lower air pressure which "pulls" the cloth to leeward. The push and pull actions resulting from the airflow/sail interaction add together to provide the force necessary to propel the boat. The magnitude of this propulsive force is maximum when the angle between the airflow direction and the plane of the sail falls within a narrow range of values (generally on the order of 5 to 10 degrees). Luffing occurs when the air flow strikes the luff of the sail at too small an angle causing the luff to flutter and the sail to lose power This condition is readily recognized and is easily corrected by either trimming the sails in or by turning away from the wind until the luffing stops and the boat regains speed.

More treacherous is the condition where airflow strikes the luff at too large an angle and becomes turbulent across the leeward surface resulting in a marked decrease of propulsive force. When in this condition the sail is said to be "stalled". The only indication of a stalled sail (assuming no tell tails are used) is that the boat is not moving at the speed it normally would under existing wind conditions, and other boats are passing as if it were anchored. The best way to test for, and to correct for, a stalled condition is to ease the sail slightly or to point slightly higher into the wind and see if the boat picks up speed (note: the operative word is slightly). If boat speed increases then you know the sails had been stalled so continue making small corrections until the boat falls back into the "groove" and back into the race. If no improvement in boat speed is seen then you should look elsewhere for the cause of your slowdown (i.e.: weeds caught on the underbody, a jammed sheet keeping a sail out of trim, etc.). Remember the old saying --when in doubt-- let it out!