Ironically, the first attempt by the Japanese to produce a modern and workable light cruiser design had evolved into the Tenryu class of large destroyers. While these ships fulfilled the role of a light cruiser in traditional Japanese thinking, they did not accommodate the other tasks that were required of that type of ship. Although their gun armament was entirely satisfactory for destroyers a battery of four inch guns on a cruiser appeared to be a throwback to the designs and standards that had been current before the First World War. A real light cruiser design was required.
Before design could start on the new ship, an important question had to be asked. Exactly what were these ships supposed to do? This was a first overture of a question that was to bedevil the Japanese Navy for the next twenty years. All to soon, the question "what were these ships supposed to do" became "what is the Navy supposed to do" and it was the failure to provide a plausible answer to that question which eventually doomed the Japanese Navy to a slow death from financial strangulation. All that, however, lay in the future. The immediate problem was to determine what roles a future light cruiser would perform. The destroyer flotilla flagship and support role was assigned to the Tenryu class, scouting was now undertaken by aircraft. However, there were three distinct roles for which a light cruiser would be suitable.
The first was trade protection. Japan was already heavily dependent upon merchant shipping for food, raw materials and energy supplies. The Navy recognized that disruption to those supplies would have disastrous possibilities and was an eventuality that had to be guarded against. The primary likely enemies were the fleets of Australia and India, both of which had been reinforced by the Imperial Gift. They were largely destroyer fleets though; although they had heavier warships, the major ships were too few in numbers and therefore too valuable to commit to a trade war. A 6 inch gunned light cruiser would be adequate to provide cost-effective deterrence against such destroyer attack. Adding a heavy battery of torpedoes would provide a capability against heavier ships. Oddly, indeed incredibly, the idea of a submarine campaign against Japan's merchant fleet seems never to have crossed the Japanese fleet planner's minds.
The second role was coastal operations in China. By 1945/46 the China War was starting to wind down. China had essentially been defeated and her nominal allies were too heavily involved in their own wars to provide adequate help. However, the conquest of China would leave Japan with an immense length of coast to patrol. If capability in this area was not provided, that coastline would be wide open for smuggling, infiltration and general maritime mayhem. A force of light cruisers couldn't be everywhere but they could provide heavy firepower to support smaller and less capable forces.
The third role was showing the flag and enforcing Japanese claims to what she regarded as her indisputable rights in the Pacific. Japan had extensive island possessions in the Pacific, most notably around her great naval base of Truk. Throughout the 1950s and into the 1970s, Japan sought to expand these possessions by occupying and garrisoning remote outcrops and semi-submerged reefs. This policy was finally ended when one such expedition resulted in the famous Pescadores Incident but for twenty years, the new light cruisers would be at the forefront of Japanese expansion in this area.
The obvious starting point of the new design was the existing Oyodo class. These strange light cruisers were armed with six 6.1 inch L60 guns in two triple turrets forward but with the entire stern occupied by a large hangar and a catapult. Some thought was given to rebuilding these ships along normal lines but the work involved was too great. The existing pair of Oyodos were used as experimental ships in the development of ship-borne autogyros and helicopters and eventually ended up serving as large, well-gunned destroyers. However, their use as a design base had several important implications. It strongly implied the use of the 6.1 inch L60 triple turret for the new ships only there was a problem. 60 guns of this type had been built, equipping 20 triple turrets that had armed the Mogami class cruisers. After those cruisers were rearmed with twin eight inch turrets, four of the surplus turrets had been used as the secondary armament on each of the four Yamato class battleships and the remaining four turrets had been split between the two Oyodos. If the new light cruisers were to carry this gun, production had to be restarted
This raised another issue. For years, it appeared that each new class of Japanese ship had been armed with guns developed specifically for it. The result was a fleet filled with ships carrying different guns and requiring different munitions. The number of each type built was small and it was hard to keep an adequate number of gunners trained on each. This was too inefficient to be allowed to continue. A major step forward had been to standardize future destroyer construction on the 3.9 inch L65. To standardize future light cruisers on the 6,1 inch L60 seemed equally sensible. This was to have a major impact on later design practice.
The new Yubari class light cruisers mounted a standard battery of nine 6.1 inch L60 guns in three triple turrets, two forward, one aft. This appeared a little light for what was essentially a 9,000 ton ship but the excellence of the 6.1inch gun made up for any deficiency in numbers. The second part of the ship's primary armament was her battery of torpedo tubes, 16 24 inch tubes in four quadruple mountings with a complete set of reloads. The anti-aircraft armament consisted of eight 3.9 inch L65 guns in four twin turrets. The four twin turrets were arranged in a diamond pattern with the two midships mounts being theoretically capable of cross-deck firing. Finally, the ships received eight triple 25mm anti-aircraft mounts. These were a new pattern equipped with a radically-improved squirrel cage ammunition feed that significantly enhanced their capability. Finally, the ships were equipped with two catapults and aircraft handling facilities aft.
The first three Yubari class cruisers were laid down in 1947 and commissioned in 1951/52. They were followed by two more batches of three, laid down in 1948 and 1949 with the class being completed by a tenth cruiser laid down in 1950 and completing in 1955. In contrast with earlier Japanese light cruisers, they proved to be highly satisfactory ships, well-armed and well-suited for their primary roles. During the late 1950s, they received a light modernization that saw their catapults replaced by a helicopter deck and, later by that deck used to mount anti-ship missiles. There were plans for a much more extensive rebuild in the early 1960s that would have seen them converted into missile cruisers but these fell foul of the Japanese Navy's Strategic Review of 1959-61. by the time decisions on their reconstruction had been taken, the Show Restoration Coup of 1965 saw the Navy slide into relative insignificance. The Yubari class continued to soldier on until age and machinery problems caught up with them. The first of the class were withdrawn from service in the late 1980s with the final member of the class decommissioning in 1995.