In the run up to World War 2 (1939-1945), the Soviet Navy adopted the P-class diesel-electric submarines which numbered just three of the four planned boats. However, these were not a success as they were inherently limited in a number of ways. The Navy, still searching for an effective ocean-going submarine, was granted a new design in the K-class which was being planned even as the P-class were being built. In all ways, the K-class was a substantial improvement over the failed P-class boats and became one of the more critical naval components of the Soviets for their time in World War 2. Twelve boats were completed in all with five bring lost in action. Seven lived to see retirement and a single example was preserved.
As built, the K-class displaced 1,500 tons when surfaced and 2,600 tons when submerged. Overall length ran 97.65 meters with a beam measuring 7.4 meters and a draught running down to 4.5 meters. Propulsion power was from a diesel-electric arrangement in which the diesel engines provided 8,400 horsepower for surface running and the electric motors generated upwards of 2,400 horsepower for undersea travel. Range was out to an impressive 14,000 nautical miles and the double-hull design was tested to depths of 230 feet. Aboard was a crew of sixty-seven which included ten officer-level personnel.
Armament was improved over that of the P-class. Leading the way were six torpedo tubes mounted at the bow. This was augmented by a pair of torpedo tubes facing the stern and a further two torpedo launchers were fitted externally to face the stern. For surface warfare, the boat was equipped with 2 x 100mm deck guns and carried 2 x 45mm guns to fend off attacking aircraft. Some twenty naval mines could be carried for denying strategic waterways.
Compared to the preceding P-class, the K-class was able to venture further away from shore and achieve good dive times and ocean-going speeds. They were more controllable in rough sea states as well and held better crew comforts for their time. Their armament fit was also considerably greater than that of previous Soviet boat designs making the K-class an overall improvement for the Navy.
The boat's profile was cleaned up from the earlier P-class and featured a relatively level deck line and more contained sail. The sail was set ahead of midships and was stepped to accommodate the deck gun artillery. The bow was well-pointed and slightly raised as was the stern section. Under the stern was the rudder control straddled by the twin-propeller arrangement.
The completed boats were K1, K2, K3, K21, K22, K23, K51, K52, K53, K54, K55 and K56 and they were launched from December 1936 into 1941, destined for service with the Soviet Northern Fleet. K1, K2, K3, K22 and K23 were sunk during the fighting of World War 2 while K51, K52, K53, K54, K55 and K56 were caught during the siege of Leningrad (1941-1944) and not finished until after the war concluded. K21 survived the war to become a training platform before being preserved. K51, K52, K53, K54, and K55 all survived their wartime exposure and were ultimately decommissioned and scrapped. K56, launched in 1940, was expended during nuclear trials in 1957.
The K-class boats were to have been evolved under the KU-class initiative which were to feature welded hulls. The class was in-the-works as soon as 1941 with some twenty-four boats set to make up the new and improved group but this program was not furthered.