Shinano and Taihō being sunk by U.S. submarines, and Hiyō by air attacks. The IJN also attempted to build a number of fleet carriers called the Unryū-class, mostly based on the older Hiryū design rather than the newer Shōkaku or Taihō for the sake of reducing construction cost and time. Most carriers were still under construction or cancelled by the end of the war, while the few completed ships never embarked air groups due to severe shortages of carrier-qualified aircrew. Destroyers Japanese World War II destroyers (駆逐艦 Kuchikukan) included some of the most formidable destroyers of their day. This came as a nasty surprise to the Allies, who had generally underestimated Japanese technical capabilities. The Japanese had reassessed their naval needs in the mid-1920s and, placing an emphasis on ship and weapons technology and night fighting expertise, developed a completely new destroyer design. Subsequent development from one destroyer class to the next was not, however, a smooth progression. Aside from the usual changes arising from experience, serious design faults also came to light and naval treaties imposed restrictions. As a result, the early "Special Type" destroyers required significant changes and the specifications of subsequent classes was reduced in one way or another. Naval treaties were later abrogated in 1937 and so destroyer development continued without regard to limits. Generally speaking, the Imperial Japanese Navy (IJN) requirements gave rise to warships that were substantially larger than their European or American equivalents, often well-equipped with heavy torpedo armament for surface engagements but with less emphasis on anti-aircraft or anti-submarine armament. In the early war years, their advantages were exploited against the often second rate and poorly coordinated Allied ships stationed in the region such as at the IJN victory in the Battle of the Java Sea. The Japanese did not, however, continue to install new technology, such as radar, to match their opponents, and destroyer numbers were eroded steadily in the latter half of the Pacific War. The Japanese emphasis on capable but expensive fleet destroyers had neglected the need for large numbers of cheaper escort vessels (destroyer escorts or frigates) to defend critical merchantmen, a need learnt by both the Royal Navy and the United States Navy in the Battle of the Atlantic. In recognition that quantity was as important as quality in some roles, design policy was therefore modified to produce units that were easier to build and operate. Despite