The were a class of kaibōkan (equivalent to US destroyer escorts or British frigates) built for the Imperial Japanese Navy just prior to World War II. Four ships out of an initially planned 16 vessels were completed. The class was also referred to by internal Japanese documents as the . Background The Shimushu-class kaibōkan, as with the torpedo boat, was a consequence of the 1930 London Naval Treaty, which placed limitations on the total destroyer tonnage the Imperial Japanese Navy was permitted. One way in which the treaty could be circumvented was to use a loophole in the treaty which permitted ships of between 600 and 2,000 tons, with no more than four guns over 76mm, no torpedoes, and with a maximum speed of no more than 20 knots. A new class of vessel was designed to use this loophole, and was given the obsolete designation of kaibōkan (Kai = sea, ocean, Bo = defence, Kan = ship), which had previously been used to designate obsolete warships which had been reassigned to coastal defense duties. However, due to many other priorities, the budget for this new class was not secured until the 1937 3rd Naval Armaments Supplement Programme. Design Due to the low priority of the project, the design of the Shimushu-class was subcontracted out to a private firm, Mitsubishi Heavy Industries. The Imperial Japanese Navy had intended for a simple design whose primary role was patrol and fisheries protection of the remote and sub-Arctic Kurile Islands, which would free destroyers currently used for this role for higher priority missions. The secondary role of the new class was to be minesweeping, and convoy escort was considered a minor third priority. However, the design developed by Mitsubishi was more complex than the Imperial Japanese Navy had anticipated, with a double-curved bow and a forecastle deck which improved seaworthiness in the rough northern seas. The hull was also reinforced and insulated against the cold weather. The consequence was that the design was not suited to prefabrication or mass production, and construction times which created problems when more vessels were needed in a short time after the start of the Pacific War. The ships measured overall, with a beam of and a draft of . They displaced at standard load and at deep load. The ships had two diesel engines, each driving one propeller shaft, which were rated at a total of for