Abstract: A catamaran which has a centre tunnel, the opposite sides (105a, 105b) of which form asymmetrical pontoons, which are mirror images of each other, and which pontoons have buoyancy, which has been adapted so that when the catamaran moves in water, the centre tunnel functions as a combined water and air tunnel. When the catamaran is stationary in water, the ceiling (108) of the centre tunnel is in water. The ceiling of the centre tunnel further curves in a cylindrically convex manner downwards when going from the bow (116) to the direction of the stern (118) only after an essentially horizontal portion (401) of a distance (d), which essentially horizontal portion is located between the pontoons.