Type S medium submarines (unofficially nicknamed Stalinets) were one of the most widely produced and employed submarine class in the Soviet Navy during World War II. IX series - Only three ships were built in this group, S-1, S-2, and S-3, using partially German-supplied machinery. The boats were of semi-double hull type, with riveted pressure hull and welded light hull sections in superstructure and extremities for improved seaworthiness. The sail was medium sized and oval in plane, to reduce water drag. It housed conning tower, bridge, periscope fairings, and 45 mm (1.77 in) anti-aircraft gun. IX-bis series - Instead of German engines, domestically produced 1D turbo-diesels were installed. Unlike their foreign counterparts, they had (for the same power) slightly higher speeds and were non-reversible.