Abstract: An aerodynamic device mounted to a mainsail at the area of maximum air activity includes a venturi and a plurality of vertically disposed strips or offsets defining extending fins. The flow axis of the venturi is substantially parallel to the foot of the mainsail and the vertical fins are disposed in a substantially horizontal row along the venturi flow axis. Certain ones of the fins are positioned respectively forward and after the venturi and other ones of the fins are superimposed on the venturi. The aerodynamic device is operative to reach the free air layer beyond the zero activity and turbulent air layers for greater air drive and improved on-wind direction.