A system (16) for providing an air lubricating layer between a substantially flat bottom (6) of the hull (4) of a vessel (1) and water flowing under the bottom as the vessel is moving through the water. The system has a cavity (33) with a deflector. The deflector has an elongate part (29,34,34′) extending in the length direction of the cavity over at least at least 5%, preferably at least 10% of the cavity length L, wherein a projected area of the at least one deflector member (24,24′-26,26′) on the interface plane (30) covers at least 25%, preferably at least 50%, most preferably at least 75% of the surface area of the interface plane.