In sailing, a stall occurs when the smooth airflow over the leeward (downwind) side of a sail breaks up and becomes turbulent, resulting in a dramatic loss of lift and a significant increase in drag. It is often described as the moment the sail stops producing efficient, forward-driving force, causing the boat to lose speed and drift sideways.

Key Aspects of a Stalled Sail
Cause: Stalling typically happens when the sail's angle of attack is too large for the speed of the wind—usually from trimming the sail in too hard (over-sheeting) or steering too close to the wind (heading up too high).

Symptoms:
Windward Telltales Lift: The telltales on the side closest to the wind start flying upward or spinning.

Luffing/Flapping: The front edge of the sail begins to shake or "luff".

Loss of Power: The boat feels sluggish, stops accelerating, and begins to drift sideways (leeway) rather than moving forward.

Foil Stalling: The same principle applies to underwater foils (rudder, centerboard/keel). If the boat turns too quickly, the water flow breaks away from the rudder, causing it to lose steering effectiveness, which is also termed a stall.