An apparatus and method for reducing surface friction drag on the hull of a surface vessel by entraining air into water flowing in the boundary layer flow along the hull. A hemispherical protrusion creates flow separation by diverting a portion of the water flow away from the surface of the hull, which results in a reduced pressure region downstream from the protrusion. A vent located on the protrusion allow air to flow into the reduced pressure region. The air in the reduced pressure region is entrained into the water due to turbulent mixing from vortices shed on to the rear of the protrusion.