Soviet K-class submarine
The K class were the largest submarines built for the Soviet Navy in the World War II era. Design. The design was approved in 1936 as a long range "cruiser submarine" with a heavy torpedo and gun armament. The boats could operate as a "fleet submarine" working with the battle fleet or as long range commerce raiders. They were a significantly improved version of the and overcame most of their shortcomings (Conway's Fighting Ships). The double hull was divided into seven compartments. It was originally planned to carry a small floatplane for scouting but this  concept was abandoned when the planned aircraft proved too flimsy. Yakubov and Worth state that these were the most successful Soviet submarines of the World War II era, with high speed and good seakeeping. This class of submarine possessed better ventilation and air conditioning systems than any other class of Soviet submarine in World War II. They had amenities such as a bunk for every sailor, small cabins for each officer, showers, electric heaters (this class was designed to operate primarily in the Arctic), and an electric galley. The hull provided spacious accommodation. Diving time was 60 seconds. American naval  constructors inspected "K-21" in 1944, and thought the design to be workmanlike but technically inferior to contemporary American boats such as the s An improved design, the KU class, which was to be of welded construction was in planning in 1941. 24 KU boats were planned, but none were started. Boats. All twelve boats were built by Marti Yard / Ordzhinikidze Yard, Leningrad, on the Baltic Sea, for the Soviet Northern Fleet. "K-1" to "K-23" were transferred before the German Invasion, and "K-51" to "K-56" were trapped in Leningrad during the blockade; they were completed after the war and transferred to  the Arctic