This paper is focused on the history of a specific MAS (Motoscafo Armato Silurante) project among the many that were developed. The MAS was a class of fast torpedo armed vessel used by the Regia Marina during World War I up to World War II. During the two World Wars the general design of the MAS, however, was changed. From 1932 to 1937 the Baglietto shipyard developed, among the many projects, two different prototypes: the MAS-431 and the Motor Torpedo & Gun Boat “Stefano Turr”. The first project was a small and very fast ship that represents the evolution of the MAS of the first World War, summarizing the best of the experiences gathered up to that moment. The “Stefano Turr” project was a large boat of over 50 tons of displacement that, besides dimensions notably superior to those of the MAS-431, has a stepped hull similar to the MAS-431. Unfortunately, the “Stefano Turr” project not gave satisfactory results in terms of performance. This paper tries to investigate the reasons for the lack of success of the hull performance using modern tools as the Computational Fluid Dynamics (CFD) approach.