Parts of a sail
Roach
The extra sail area outside the straight line from head to clew.
DutchGillingour Dutch loft →
SpanishAlunamientoour Spanish loft →
Draw a straight line from the head to the clew. Everything the leech bulges out beyond that line is the roach — the shaded area in the diagram.
Roach is free power: area added high up, where the wind is stronger and cleaner, without making the boat any longer or the mast any taller. But cloth hanging outside the straight line has nothing to hold it up, so roach only exists if battens support it. More roach means more batten.
How much roach a sail can carry is set by the boat, not by taste: the backstay has to clear the leech when you gybe. Boats with swept spreaders and no backstay — most modern cruising boats and nearly all catamarans — can carry a lot. A boat with a fixed backstay cannot.