The Porpoise class were submarines built for the United States Navy in the late 1930s, and incorporated a number of modern features that would make them the basis for subsequent Salmon, Sargo, Tambor, Gato, Balao, and Tench classes. Based on the Cachalots, enlarged to incorporate additional main diesels and generators, the Portsmouth boats were all riveted. In general, they were around long and diesel-electric powered. Displacement was 1,934 tons submerged for the first four boats, 1,998 tons for the later ones. The Porpoise class consisted of the following subclasses: