A roach is the extended, curved portion of a sailboat's sail—usually the mainsail—that protrudes beyond a straight line drawn between the head (top) and the clew (aft corner). It increases total sail area for more power and aerodynamic efficiency, typically requiring battens to maintain its shape and support the extra fabric.

Key Aspects of a Roach:
Location: Found on the leech (trailing edge) of the sail.

Purpose: The convex shape acts like a wing, providing extra power, particularly in light air or for reaching.

Support: Battens (long, thin, usually fiberglass rods) are inserted into pockets to prevent the extra fabric from flapping (flogging) and to maintain the sail's shape.

Types: Ranges from a "no roach" (straight leech) to a "full roach" or square-top main.

Rigging Considerations: A large roach can interfere with the backstay, potentially causing chafe damage to the sail when tacking.

Often seen on modern cruisers and multihulls, a high-roach sail offers improved performance but requires specialized handling, such as ensuring battens are supported for easy, clean furling.