Z51 was the only ship of the Type 1942 destroyer class built for the Kriegsmarine. She was the only destroyer built by the Kriegsmarine to use diesel fuel, which would have given it a longer range and readier access to refueling compared to fuel oil. The Type 1942 destroyer class went through a long period of development, including four total designs. Z51 was ordered on 25 November 1942, and launched on 2 October 1944, far before being finished, likely to clear her slipway for U-boat construction. She was left floating outside Bremen and was sunk in an air raid on 21 March 1945. Germany attempted to communicate the plans for the Type 1942 destroyer to its ally the Empire of Japan using a submarine, U-234, however Germany surrendered before the submarine reached Japan. Z51 was broken up between 1948 and 1949.
Background Interbellum Following the end of World War I Germany signed the Treaty of Versailles, which put strict limits both on the size and displacement of warships that she could possess. During the Interwar period, the period between the first and second world wars, the average size of Allied ships and their armaments in almost all warship categories grew substantially. As a result of the treaty, Germany felt that her ships could not compete with those of the Allied navies and began to ignore the treaty, at first covertly, and later openly after Adolf Hitler, the (leader) of Nazi Germany, publicly denounced it in March 1935. The displacements of all German ships at the time were purposefully understated to have their official sizes comply with the treaty. At first, changes to German destroyers were made with the goal of being able to contain French and Polish destroyers, effectively making the destroyers double as small cruisers. This was exemplified by the jump in the displacement of the Type 1934 destroyers from as planned in 1932 to when contracted. While these changes were made in order to allow them to overpower their French and Polish counterparts, which displaced and respectively, later it was necessary that these destroyers be able to match British destroyers, a much more difficult goal.
Due to the comparatively small number of German shipyards, compared to the British or French, Germany adopted a policy of heavily arming her destroyers to compensate for their low numbers, so that they bore similar armament to French and Polish light cruisers. Several negative consequences resulted from this, such as making them slower and overweight. Although German heavy destroyers matched British light cruisers in armament, they were much less seaworthy and their fire control was inferior.
Plan Z Plan Z was a German naval rearmament plan that started in 1939, and involved building ten battleships, four aircraft carriers, twelve battlecruisers, three pocket battleships, five heavy cruisers, forty-four light cruisers, sixty-eight destroyers, and 249 submarines. These ships were to form two battle fleets: a "Home Fleet" to tie down the British war fleet in the North Sea, and a "Raiding Fleet" to wage war upon British convoys. Erich Raeder, the Grand Admiral of the Kriegsmarine, was assured by Hitler that war would not start until at least 1945. Raeder had wanted the deadline for the completion of Plan Z to be extended to 1948, but Hitler insisted on 1945, although Hitler privately wanted to be at war with the Anglo-French alliance by 1942. World War II began in 1939, meaning that very few of Germany's heavy ships would be finished at that point. Germany's main naval opponents were France and England. Compared to the number of ships Germany had upon entry into the war (in parentheses) the two countries had combined totals of: 22 battleships (two), seven carriers (none), 22 heavy cruisers (four), 61 light cruisers (six), 255 destroyers (34), 135 submarines (57, of which less than half could actually serve in the Atlantic or the North Sea). Due to the clear advantage her enemies had, Raeder remarked that the Kriegsmarine could not hope to win, and thus the only course for them was to "die valiantly".
Destroyer function The function of the destroyer was defined by its evolution: around the 1870s, nations that could not directly threaten the British Royal Navy began to invest in torpedo boats, small and agile ships which used their torpedoes to deliver enough damage to pose a tactical issue to enemy fleets. Near the turn of the 20th century, British and German torpedo boats grew in size to the point of creating a separate line of sea-going torpedo craft, "torpedo boat destroyers", or simply destroyers, designed in part to counter torpedo boats themselves. Experience in World War I showed that destroyers very rarely engaged capital ships, but more often fought other destroyers and submarines; because of this, destroyers were partially re-focused towards escort and anti-submarine services. During the war, they were used as "maids of all work", fulfilling virtually every role to some degree, and, unlike capital ships, which rarely left port during the war, served in numerous operations. By the end of the war, destroyers were perceived as one of the most useful classes of ships.
During World War II, destroyers served essentially the three basic functions they had in World War I: to act as screening ships to defend their fleets from those of an enemy, to attack an enemy's screening ships, and to defend their fleet from submarines. There was an increased desire to introduce anti-aircraft measures to the destroyers, although many nations struggled to do so effectively. How destroyers were actually used varied by country. Germany did not use her destroyers to defend against submarines, hence their lack of strong anti-submarine armament. Germany relied on a massive fleet of trawlers that had been requisitioned and refitted as minelayers instead. British destroyers were built for escorting fleets, defending them from enemy aircraft and sinking submarines. German destroyers were built to escort fleets, or act as torpedo boats. The role of the destroyer began to vary more widely as World War II progressed, with five parallel evolutions: the all-purpose destroyer (all countries), the anti-submarine destroyer (the United States and the United Kingdom), the anti-aircraft destroyer (Japan and the United Kingdom), the small destroyer (Germany and Italy), and the super-large destroyer (France).
Development The Type 1942 destroyer class marked a major change from previous destroyer concepts produced by the Kriegsmarine, as it was the first design to be given diesel propulsion. This was done because of the longer endurance offered by diesel, and likely also because diesel fuel was more readily obtainable for Nazi Germany. Between 1928 and 1935, MAN Augsburg built a light, double acting, two-stroke diesel motor, which met German navy specifications. The motors were then tested on the German cruiser Leipzig and the training ship Bremse. The motors encountered initial teething problems, which although swiftly overcome, allowed those supportive of steam engines to deny diesel propulsion any research funding in the critical building period from 1935 on. It was not until 1938 that the Kriegsmarine renewed its interest in diesel propulsion. However, the advent of World War II largely led to the suspension of research, except for the Type 1942 destroyer class, which was to use six MAN type 12Z32/34 engines.
Diesel engines Six MAN type 12Z32/34 engines were ordered in 1943, but only four were completed by the end of the war in 1945. Test work on the engines was abandoned in February 1945, due to lack of fuel. The type 12Z32/34 engine was 24-cylinder, placed in a 2 × 12 configuration, had a bore, a stroke motor, and was a 'V' form, fast-running, double-acting, two-stroke, diesel engine made of welded steel. It had a maximum of 600 revs per minute, a piston speed of , maximum output of , medium effective pressure of , and a maximum fuel demand of . It had a dry weight of , including auxiliary machinery, but not the oil cooler. The six engines combined would give the Type 1942 destroyer class a range to match the pocket battleships and U-boats.
Sub-designs The Type 1942 design was expanded to three sub-designs as it was developed: 'A', 'B', and 'C'. The original plan had a six-diesel engine, three-shaft configuration, with an armament of four guns, with two superfiring on the fore and two superfiring on the aft. The design attracted much criticism, thus a new sketch was drawn, 'A', with a twin 12.7cm turret on the fore and aft, with a single 12.7cm turret superfiring over both. This design also drew criticism. By December 1943, the Führer der Zerstörer (FdZ), the commander of destroyer development and deployment, demanded that the fore single superfiring turret be removed, in order to reduce the length, improve speed, and reduce bow weight. They also mandated that the aft single 12.7cm guns be able to elevate up to 75°. However, on 22 January 1944, after a meeting between the FdZ and Naval Group North, the FdZ dropped their demand for the fore single turret to be removed, after being informed that the speed loss due to it was marginal.
At the same conference, the new 12.7 cm KM/41 naval gun was also discussed, as it was the best quick-firing gun design of the time. It was pointed out that the 12.7cm KM/41 had a severe lack of anti-aircraft abilities, due to the fact that none of the mountings were tri-axial, and indeed, only the single turret guns were even bi-axial. Additionally, the twin turrets could only elevate to 52°, and the single turrets only to 75°. Because of this, questions on the worth of stabilization for the directors and rangefinders were raised, whereupon it was decided that it would be up to the Shipbuilding Commission to decide if the stabilization would come with too great a weight penalty and to omit if they felt it did. Due to the severe lack of anti-aircraft defense, it was decided to replace the LM/44 twin mounted AA guns with twin mount AA guns if possible, although the 2cm AA guns were considered an acceptable interim measure. Another point of consideration was the matter of the new Gerät 58 AA gun, which was designed for U-boats. The idea of mounting these guns on the Type 1942 destroyer class was discussed at length, although it had major issues. One such issue was that the new 5.5cm guns weighed between and , inclusive of directors and sights, which meant that the four guns could be replaced by only two 5.5cm guns. Another issue raised was that room would have to be made for the new 5.5guns. It was decided that the aft single 12.7cm gun and two twin 2cm guns would be removed to make room for two 5.5cm guns, although it was noted that the 12.7cm guns would likely arrive before the 5.5cm guns, and thus the 12.7cm guns would be allowed on a temporary basis. It was also decided that the remaining 3.7cm AA guns would be replaced by 3cm guns, in order to simplify ammunition supply.