The Soviet Navy recommissioned the battleship as the Novorossiysk (Новороссийск). Novorossiysk was based at Sevastopol from July 1949, serving as a flagship of the Black Sea Fleet and later as a gunnery training vessel. On 29 October 1955, the Novorossiysk was moored in Sevastopol Bay, 300 meters (1,000 feet) from shore and opposite a hospital. At 0130 hrs., an explosion estimated to be the equivalent of 1,200 kilograms of TNT under the bow of the ship pierced all decks from the bottom plating to the forecastle deck. In the forecastle deck there was one hole which measured 14×4 meters in size. The damage extended aft from the bow 22 meters.
The ship sank slowly from the bow, capsizing at 0415 hrs., 2 hours and 45 minutes after the explosion, and 18 hours later became fully submerged. The capsizing resulted in the death of 608 sailors, most of whom were staying in the ship's compartments. It became the worst disaster in Soviet naval history. Because of the politics of the Cold War, the fate of the Novorossiysk remained clouded in mystery until the late 1980s.
The cause of the explosion is still unclear. The official cause of the sinking, regarded as most probable, is a magnetic RMH naval mine, laid by the Germans during World War II. During the next two years after the disaster, divers found 19 German mines on the bottom of Sevastopol Bay. Eleven of the mines were as powerful as the estimated blast under Novorossiysk. There is, however, some doubt that the blast was caused by a mine. The area where Novorossiysk sank was considered swept of mines, and other ships had used the area without triggering the mine. Some experts place the maximum battery life of the magnetic mines at 9 years, and thus contend that such a mine would be unlikely to trigger by the time of the explosion. Another problem some experts claim is that the size of the crater (1 – 2.1 m deep) was too small for such a big mine. On the other hand, according to some research, damage to the ship corresponded to an explosion equivalent to 5,000 kilograms of TNT.
A more theatrical conspiracy explanation was that Italian frogmen were avenging the transfer of the formerly-Italian battleship to the USSR. Covert action by the Italian special operations unit Decima Flottiglia MAS has often been surmised, and there were rumors that not long thereafter a group of Italian Navy frogmen received high military awards. However, no concrete evidence for this hypothesis has surfaced. Another theory states that explosives were hidden in the ship before she was given to the Russians. No evidence of sabotage has been found, though Soviet enquiries did not rule out the possibility because of the poor safeguarding of the fleet base on the night of the explosion. The goal of covertly destroying the battleship would be a small prize compared to the risk of provoking war if discovered, so the motive of such an Italian operation is questionable and does not support these theories. There is also a conspiracy theory that Novorossiysk was sunk by Soviet secret service divers in order to blame Turkey for the sabotage as justification to take control of Bosporus and Dardanelles, and that the plan was eventually abandoned. There is no strong evidence to support this hypothesis.
The enormous loss of life was directly blamed on the incompetent actions of her captain, Fleet Commander Vice Admiral Victor Parkhomenko. Among other underestimates of the danger to his ship, he did not know the conditions of the sea bottom, believing that the ratio between the sea depth (17 meters) and the ship's beam (28 meters) would prevent capsizing. However, the bottom was soft ooze, 15 meters deep, which offered no resistance. It was also reported that the commander displayed conceit and groundless calmness during this critical situation, and had even expressed the wish to "go have some tea".
Because of the loss of Novorossiysk, the First Deputy Minister of Defence and Commander-in-Chief of the Navy Nikolai Gerasimovich Kuznetsov was fired from his post in November 1955, and in February 1956 was demoted to the rank of vice admiral and sent to retirement without the right to return to active service in the Navy. Kuznetsov was later reinstated.
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