Abstract: Rows of closely spaced apertures are installed in a sail to control boundary layer air flow over a sail to laminar flow type by "bleeding" high energy air from the windward or concave side of the sail through the apertures to the convex or leeward side of the sail. The "bleed air" vents the windward boundary layer air through the apertures, gently lifts the leeward or convex side boundary layer off the sail surface; then new laminar boundary layers are formed, on each side of the sail, downwind of the aperture. The increase in air energy adjacent to the sail surface maintains desirable air flow velocity and pressure gradients on the sail that prevents flow separation. The apertures permit large camber of sail contour and high angles of attack without stalling. The air "bleeding" action continuously removes old boundary layer air and creates new laminar boundary air layers on the sail.