Abstract: A catamaran vessel includes two hulls which are mutually connected by a cargo deck. A superstructure includes passenger spaces and is supported by a web structure which extends generally along each hull. Each longitudinal side wall of the superstructure is formed from a latticework beam whose plane is oriented generally vertically and whose longitudinal axis extends essentially horizontally. The latticework beams are fixedly connected to the wall structures. The floor and ceiling structures of the superstructure are connected to the latticework beams so as to greatly stiffen the superstructure and to render the beams highly resistant to bending forces. Consequently, the spaces defined between the longitudinal profiles of the beams and the struts are essentially free of deformation forces when the vessel is at sea. The spaces can be used essentially completely for windows, and the wall structures can be given a relatively low natural bending resistance.