The outward and rearward curve of the leech of a mainsail, an unnatural shape which is maintained by Battens. Roach makes a pretty shape, and gives untaxed area to the racing man whose sail area is calculated on straight corner-to-corner measurements. Little aerodynamic advantage can be expected, except in fully-battened sails of high Aspect ratio where the sail can be given a markedly elliptical profile. In that case the resulting aerofoil can be very efficient for fast close-hauled sailing. Otherwise, one is better off without roach or battens.