Abstract: A catamaran boat comprises two pontoons which are rigidly interconnected in laterally spaced relationship by a plurality of transverse beams. An air guiding sheet is rigidly connected to the bottom surfaces of the transverse beams and defines an air tunnel extending above the water between the two pontoons. The air guiding sheet is upwardly curved at its forward end and is provided with one or more transversely extending downwardly projecting ribs or bulges which result in a compression of the air in the tunnel when the boat is moving through the water. This substantially increases the lift exerted on the boat, permitting it to ride at a higher level and achieve higher speeds. The inner vertically extending walls of the spaced pontoons are disposed at an angle to the vertical, with each inner wall extending outwardly toward the outer wall by a slight angle. This configuration permits the boat to traverse shallower waters than has been heretofore possible with catamaran type boats.