This so called boundary layer can be over 2 inches thick in the middle on the windward side and in the leech on the leeward side of the sail. When the velocity of the air stream decreases to a certain point the boundary layer separates from the sail. The telltales indicate how the boundary layer is behaving at different moments in different parts of the sail. The separation of the boundary layer always means a significant loss in power and is to be avoided at all cost. This is why the telltales are so useful, and why we try to trim the sail so that they always stream steadily aft.
Separation bubbles The sail is often compared to a wing of an airplane.
A sail, however, has one aerodynamic characteristic that makes the comparison feeble. In the luff of a sail the airflow always gets separated from the sail surface on one side or the other, creating a separation bubble. When the airflow meets the luff of the sail in too steep an angle, it separates and a separation bubble forms. The bubble is like a small eddy where air circulates around in the same spot. The tighter the sail is sheeted the longer the separation bubble will be.