A roach is the extra, convex curve of sail material extending beyond a straight line between the head and clew on the leech (trailing edge) of a sail. It increases sail area and improves aerodynamic efficiency (lift). Roach is supported by battens to prevent flapping and requires proper trimming to avoid interference with the backstay.

Key Characteristics of Sail Roach
Purpose: The primary purpose is to add sail area and create a more efficient elliptical shape for better performance, particularly upwind.

Shape: It represents a, usually curved, addition to the triangular shape of a sail, creating a rounded "belly" on the leech.

Support: Because the roach extends beyond the straight-line structural points of the sail, it needs to be supported by battens (flexible rods) to prevent the fabric from fluttering (flogging).

Backstay Interaction: A large, or "roachy," mainsail often extends back close to the backstay, which can cause chafing during tacks, especially on modern, performance-oriented boats.

Types:
Full Roach: A pronounced curve often seen on racing boats, demanding full-length battens for support.

Partial/Moderate Roach: A smaller, more conservative curve often used in cruising sails for a balance of power and ease of handling.

No Roach (Hollow Leech): When the leech is concave rather than convex, often found on traditional or some offshore sails, reducing the need for long battens.