An outhaul is a line or tackle system used to adjust the tension along the foot (bottom edge) of a sailboat’s mainsail by pulling the clew (bottom aft corner) toward the end of the boom. It acts as a primary control for the sail's draft or curvature in the lower third.

Tighten (Increase Tension): Flattens the sail, reducing power and drag, ideal for heavy wind conditions.

Ease (Reduce Tension): Increases the draft (fullness/camber), adding power for light wind conditions.

Purpose: Allows sailors to optimize the sail's shape for different wind speeds and wind angles, improving boat performance and reducing weather helm.

It is an element of the running rigging that typically runs from the clew through a block at the boom end to a cleat, often allowing adjustment from the cockpit.