roach
Sail material that forms a concave curve rather than straight leech (aft edge) of a sail to increase the sail area over that of a simple triangular sail.
roach: the curve that gives a sail more bite
A roach is the curved outward bulge along the trailing edge (leech) of a sail, extending beyond the straight line that would connect the head and clew. Unlike a sail cut with a simple triangular leech, a roached sail has a concave curve cut into the after edge, with material that projects outward. This extra cloth increases the effective sail area without lengthening the boom or adding height to the mast.
Roaches are cut into mainsails and jibs using a curved batten pocket sewn into the leech. The battens, typically made of fiberglass, wood, or carbon fiber, run horizontally through pockets stitched parallel to the leech and hold the curve in place under wind pressure. A typical roach might project 6 to 12 inches outward on a cruising mainsail, depending on sail size and design. Racing sails often have more aggressive roaches, sometimes reaching 15 percent or more of the foot length.
How roach works
When the sail fills with wind, the battens support the roached portion so it does not collapse or fold back against itself. The curve creates additional draft and area in the upper and mid-sections of the sail, where aerodynamic efficiency is highest. This extra area translates to more driving force without the structural penalties of a taller mast or longer boom. However, roaches require proper batten tension and rigidity to function; if battens are too weak or loose, the roach will flutter and the sail will lose effectiveness.
Roaches introduce complications in sail handling. A heavily roached sail has less overlap with the lower shrouds and spreaders, which can reduce chafe but demands careful battens during jibes and tacks. When lowering or furling a roached mainsail, the battens can catch on the boom or gallows frame if not managed deliberately. Repair is also more involved, as replacing a roached leech requires careful reinstallation of battens and their pockets.
The term likely derives from the resemblance of the curved leech to a roach's back or the shape a roach makes when viewed from the side. In older sailmaking, before modern batten technology, roaches were gentler curves or absent altogether. Modern roaching is a direct result of improved batten materials and reinforcement techniques developed through the latter half of the twentieth century.