The Kronshtadt-class battlecruisers, with the Soviet designation as Project 69 heavy cruisers (), were ordered for the Soviet Navy in the late 1930s. Two ships were started but none were completed due to World War II. These ships had a complex and prolonged design process which was hampered by constantly changing requirements and the Great Purge in 1937.
They were laid down in 1939, with an estimated completion date in 1944, but Stalin's naval construction program was more ambitious than the shipbuilding and armaments industries could handle. Prototypes of the armament and machinery had not even been completed by 22 June 1941, almost two years after the start of construction. This is why the Soviets bought twelve surplus SK C/34 guns, and their twin turrets, similar to those used in the s, from Germany in 1940. The ships were partially redesigned to accommodate them, after construction had already begun, but no turrets were actually delivered before Germany invaded the following year.
Only Kronshtadts hull survived the war reasonably intact being 10% complete in 1945, although she was considered to be obsolete. The Soviets considered converting her into an aircraft carrier, but the idea was rejected and both hulls were scrapped in 1947.
Design The Kronshtadt-class battlecruisers had their origin in a mid-1930s requirement for a large cruiser () capable of destroying cruisers built to the limits imposed by the Washington Naval Treaty, of which the Soviets were not a signatory. Several designs were submitted by the end of 1935, but the Navy was not satisfied and rejected all of them. It asked for another design, displacing and armed with guns, in early 1936, eventually designated Project 22, but this design was cancelled after the Soviets began negotiations in mid-1936 with the British that ultimately resulted in the Anglo-Soviet Quantitative Naval Agreement of 1937 and agreed to follow the terms of the Second London Naval Treaty which limited battleships to a displacement of . The Soviets had been working on a small battleship design (Battleship 'B') for service in the Baltic and Black Seas and had to shrink it as a result of these discussions to a size close to that of the Project 22 large cruiser so that the latter was cancelled. Battleship 'B' was redesignated as Project 25 and given the task of destroying Treaty cruisers and German pocket battleships. The Project 25 design was accepted in mid-1937 after major revisions in the armor scheme and the machinery layout and four were ordered with construction to begin in late 1937 and early 1938. However, this decision occurred right before the Great Purge began to hit the Navy in August 1937 and two of the ship's designers were arrested and executed within a year. The Project 25 design was then rejected on the grounds that it was too weak compared to foreign ships and the whole program was cancelled in early 1938 after an attempt to modify the design with larger guns had been made.McLaughlin, pp. 100–05
However the Soviet Navy still felt a need for a fast ship that could deal with enemy cruisers and the original concept was revived as Project 69. They wanted a ship not to exceed 23,000 metric tons with a speed of and an armament of nine 254 mm guns, but the requirement proved to be too ambitious for the specified size and it increased to in the design submitted in June 1938. By this time, however, details were becoming available for the s and the ship was deemed inferior to the German ships. The State Defense Committee revised the requirements and specified a size about , an armament of nine guns, an armor belt thick and a speed about . A revised design was finished by October which was wargamed against the Japanese s, the French s as well as the Scharnhorst class. It was deemed superior to the Kongos at medium range and inferior to the Dunkerques at the same range, but generally superior to the Scharnhorsts, although it is doubtful that the Soviets were fully aware of the true specifications of the Kongōs as rebuilt or of the Scharnhorsts as the displacement of the latter had been given as , more than short of their true displacement. The Navy's Shipbuilding Administration thought that the original secondary armament of guns was too small and that the armor on the turrets, conning tower and the forward transverse bulkhead was too thin. A revised, 35,000-ton design with guns and extra armor was submitted to the State Defense Council in January 1939.McLaughlin, pp. 105–06
This was approved and the detailed design work began with the basic concept that the ship should be superior to the Scharnhorst-class ships and able to outrun the Bismarck-class battleships. At this time the horizontal protection was revised after full-scale trials revealed that a bomb would penetrate both a upper deck and a middle deck to burst on the main armor deck. So the middle deck was thickened to with the lower deck intended to catch any splinters penetrating the armor deck. This meant that the main belt had to be extended upwards to meet the main armor deck at a significant penalty in weight. The Defense Committee approved the sketch design on 13 July 1939, but the detailed design was not approved until 12 April 1940, after construction had already begun on the first two ships.McLaughlin, pp. 107, 109, 112
It was already apparent that the 305 mm guns and turrets were well behind schedule when Joseph Stalin asked the German representatives in Moscow on 8 February 1940 to negotiate a trade agreement if it would be possible to use the triple turrets in lieu of the triple 305 mm turrets of the Project 69 ships. They replied that the turrets were out of production, but new ones could be built. He then asked if twin turrets could be used instead. The Germans said that they would have to check back for the technical details. Krupp had six incomplete turrets on hand that had originally been ordered before the war to rearm the Scharnhorst-class battleships, but they were cancelled after the start of World War II when the Germans decided that they could not afford to have the ships out of service during the war. A preliminary purchase agreement was made to buy twelve guns and six turrets later that month, well before any studies were even made to see if the substitution was even possible. The Shipbuilding Commissariat reported on 17 April that it was possible so the agreement was finalized in November 1940 with the deliveries scheduled from October 1941 to 28 March 1943. The order also included rangefinders and searchlights.McLaughlin, pp. 109, 111
The Soviets never did get the detailed data required to redesign the ship's barbettes and magazines, but they did know that the 380-mm barbettes were bigger in diameter than that of the 305 mm turret as well as being taller than the Russian turrets. So the barbette of turret number two had to be raised to clear turret number one and the height of the conning tower had to be raised to clear turret number two. Similarly the anti-aircraft guns behind turret number three had to be raised as well.McLaughlin, p. 111 The new turrets required more electrical power which meant that the output of the turbo generators had to be increased to 1,300 kilowatts.Usov & Mawdsley, p. 382 All of these changes added over to the ships' displacement and the sketch design was completed by 16 October 1940, as Project 69-I (Importnyi—Imported), even though they still lacked data for the turrets and their barbettes. This was presented to the State Defense Committee on 11 February 1941, but the design was not approved until 10 April when it ordered that the first two ships be completed with German guns while the others would continue to use the 305 mm guns. The detailed design was supposed to be completed by 15 October 1941, but it was rendered pointless when the Germans invaded the Soviet Union in June.
General characteristics The Project 69-class ships were long overall and had a waterline length of . They had a beam of and at full load a draft of . As designed they displaced at standard and at full load. The displacement of the two Project 69-I-class ships increased to at standard load and at full load which increased the draft to at full load while the waterline length grew to simply because the extra draft submerged more of the sharply raked stem and spoon-shaped stern. The speed remained the same as the deeper draft was offset by a more efficient propeller form.McLaughlin, pp. 107, 112
The hull form was very full with a block coefficient of 0.61 which compared badly to the 0.54 of the Dunkerque, the 0.52 of the German or the 0.5266 of the American . This meant that a lot of horsepower was necessary to achieve even modest speeds. Stalin's decision that the Project 69 ships would use three shafts increased the shaft loading and reduced propulsive efficiency, although it did shorten the length of the armored citadel and thus overall displacement. The riveted hull was subdivided by 24 transverse bulkheads and used longitudinal framing in the citadel, but transverse framing for the structure fore and aft of the citadel. The metacentric height was for the 305 mm gunned ships, but dropped to in the 380 mm gunned ships. The tactical diameter was estimated at about .McLaughlin, pp. 107, 112, 115
The Kronshtadt-class ships were provided with two KOR-2 flying boats which would be launched by the catapult mounted between the funnels.
Propulsion The power plant was laid out on a unit system. The forward boiler room contained eight boilers and was followed by an engine room for the two wing propeller shafts. The second boiler room contained four boilers and was followed by a turbine room for the central shaft. The single-reduction, impulse-reduction geared steam turbines were an imported Brown Boveri design shared with the , but the factory in Kharkiv that was to build them never finished a single turbine before the Germans invaded. They produced a total of . Twelve 7u-bis water-tube boilers worked at a pressure of and temperature of . There were two other small boilers for harbor service and to power the auxiliary machinery. The electrical plant originally consisted of four 1200 kW turbo generators and four 650 kW diesel generators, but these were upgraded for the Project 69-I ships.McLaughlin, p. 109
Maximum speed was estimated at , using the revised propeller design, although forcing the machinery would yield an extra knot. The normal fuel oil capacity was , which provided an estimated endurance of at full speed. Maximum fuel capacity was which gave a range of at and at .McLaughlin, p. 107
Armament The main armament was initially going to consist of three electrically powered triple-gun turrets, each with three 54-caliber 305 mm B-50 guns. The turrets were based on the MK-2 turrets planned for the Project 25 large cruiser. The guns could be depressed to −3° and elevated to 45°. They had a fixed loading angle of 6° and their rate of fire varied with the time required to relay the gun. It ranged from 2.36 to 3.24 rounds per minute depending on the elevation.McLaughlin, pp. 107–08 The turrets could elevate at a rate of 10 degrees per second and traverse at 5.1 degrees per second. 100 rounds per gun were carried. The gun fired armor-piercing projectiles at a muzzle velocity of ; this provided a maximum range of .