The leech is the trailing or back edge of a sailboat sail, spanning from the top corner (head) to the lower back corner (clew). It is a critical component for aerodynamics, controlling sail twist, and shaping the sail's draft. It often features a adjustable "leech line" to prevent fluttering damage and battens to support its shape.

Key Characteristics and Functions
Location: Runs along the back of the sail between the head and the clew.

Shape Control: The leech’s tension is managed via the boom vang, mainsheet, and traveler, which determines the amount of "twist" (difference in angle between the top and bottom of the sail).

Aerodynamics: A tighter leech (less twist) keeps the sail flatter for upwind performance, while a looser leech (more twist) is used in lighter winds to produce a fuller, more powerful shape.

Leech Line: A small line running inside the hem of the trailing edge that can be tightened to stop "leech flutter," a flapping motion that can cause permanent damage to the sail cloth.

Battens: Stiffening elements often inserted into pockets along the leech to maintain its shape and prevent it from curling forward.

Properly managing the leech is essential to reducing drag and maximizing lift.