An outhaul is a critical piece of running rigging on a sailboat that adjusts the tension on the foot of the mainsail along the boom, controlling the sail's depth or "draft" in its lower third. It acts as a sail-shaping tool to optimize performance: tight for flat sails in heavy wind and loose for full sails in light wind.

Key Features and Functions:
Mechanism: It consists of a line, often with a block/pulley system, attached to the clew (lower aft corner) of the sail.

Tensioning (Flat Shape): Pulling the outhaul tight extends the sail towards the end of the boom, reducing draft (depth) and drag, which reduces heeling in stronger winds.

Easing (Full Shape): Loosening the outhaul allows the clew to move toward the mast, increasing the draft for more power in lighter winds.

System Types: Outhauls can be simple, or more complex internal systems (up to 14:1 purchase) for larger boats, sometimes utilizing shock cords for easier release.

Applications: It is used for fine-tuning sail shape to balance the boat. It is most effective on the lower 30% of the sail.