K-56 was one of a dozen double-hulled K-class submarine cruisers built for the Soviet Navy during the late 1930s. The submarine's construction was interrupted by the Axis powers' invasion of the Soviet Union in June 1941 (Operation Barbarossa) and the subsequent Siege of Leningrad, delaying her commissioning into the Baltic Fleet until 1942. She could not do anything other than training until the siege was broken and Estonia liberated in 1944. Her three wartime patrols were not very successful.

After the war, K-56 was transferred to the Northern Fleet in 1948. She became an experimental ship in late 1956 and was sunk during a nuclear test in the Arctic the following year. She was stricken from the navy list in 1957.

Design and description Despite the unsuccessful built in the early 1930s, the Soviet Navy still dreamed of cruiser submarines capable of attacking enemy ships far from Soviet territory. In 1936 it received approval to build them with the addition of minelaying capability (Project 41). The boats displaced surfaced and submerged. They had an overall length of , a beam of , and a draft of at full load. The boats had a maximum operating depth of . Their crew numbered 66 officers and crewmen.Budzbon, Radziemski & Twardowski, p. 142

For surface running, the K-class boats were powered by a pair of 9DKR diesel engines, one per propeller shaft. The engines produced a total of , enough to give them a speed of . When submerged each shaft was driven by a PG11 electric motor for . The boats had a surface endurance of at and at submerged.

They were armed with six torpedo tubes in the bow and four were in the stern, two internal and two external. They carried a dozen reloads. A dual-purpose minelaying/ballast tank was located under the conning tower that housed 20 chutes for EP-36 mines which also served as outlets for the ballast tank's Kingston valves. This arrangement proved problematic as this was the location of the greatest structural loads in the hull and the mines were sometimes pinched in the chutes as the hull flexed. Another issue was that the chutes would sometimes jam when debris was drawn in with ballast water. The boats were also equipped with a pair of B-24PL deck guns fore and aft on the conning tower and a pair of 21-K guns above them.Polmar & Noot, p. 265

Construction and career K-56 was laid down on 17 October 1937 by Zavod No. 189 in Leningrad and launched on 29 December 1940. When the Axis Powers invaded on 22 June 1941, the submarine was 87 percent complete as she was expected to be completed in September. The invasion delayed her completion until 29 October 1942. She was commissioned into the Baltic Fleet on 12 November. Strong Axis defenses prevented her first war patrol until October 1944 after the Baltic offensive liberated Estonia.Rohwer, p. 361

K-56 patrolled off the East Prussian coast and missed one ship on 26 October off Brüsterort. She departed for her next patrol in mid-December. The boat missed the training ship on 23 December, missed another ship two days later, damaged the merchant ship on 26 December and torpedoed the Swedish steamship Venersborg three days later. K-56's next patrol was not until late March 1945; she missed the submarine chaser UJ 1201 on 4 April north of the Stolpe Bank, but sank the Swedish lugger Ramona with her guns on 11 April.Rohwer, pp. 361, 374, 401, 410

K-56 was transferred to the Northern Fleet in 1948 and was renamed B-9 the following year. She became an experimental ship on 30 December 1956 and was sunk on 24 September 1957 at Novaya Zemlya during a nuclear test. The boat was stricken on 16 October 1957.Budzbon, Radziemski & Twardowski, p. 144

Claims + Ships sunk by K-56 Date Ship Flag Tonnage Notes 29 December 1944 Venersborg 1046 GRT Merchant ship (torpedo) 11 April 1945 Ramona 57 GRT Fishing boat (artillery) Total:1,103 GRT

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