Abstract: The present invention relates to a surface effect ship including a catamaran type supported structure with two side hulls connected by a central box structure, able to operate either as a buoyant displacement vehicle or as an air cushion supported vehicle. The ship includes, within the hulls, continuous longitudinal stiffeners mainly to bear longitudinal bending stresses within the central box structure connecting the two side hulls and built-in within these hulls, transverse beams mainly to bear transverse bending stresses, and at least two reinforced transverse bulkheads located in the bow and the stern of the ship respectively to bear transverse bending stresses applied on the bow and the stern of the ship, resulting from twist moments applied diagonally to the ship. Each bulkhead includes a transverse beam within the box structure with a vertical web and extending down within the hulls and strengthened within the box by upper and lower horizontal flanges.