Shimushu-class escort ship

The Shimushu-class escort ships (占守型海防艦) 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 "A-class" coastal defense vessel (甲型海防艦).

Background
The Shimushu-class kaibōkan, as with the Chidori-class torpedo boat 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 77.72 m overall, with a beam of 9.1 m and a draft of 3.05 m. They displaced 860 LT at standard load and 1020 LT at deep load. The ships had two diesel engines, each driving one propeller shaft, which were rated at a total of 4200 bhp for a speed of 19.7 kn. The ships had a range of 8000 nmi at a speed of 16 kn.

The Shimushu-class was well armed for its size. The main armament of the Shimushu class consisted of three Type 3 120 mm guns in single mounts, one superfiring pair aft and one mount forward of the superstructure, which had been taken from old destroyers. The minesweeping role was accomplished by two paravanes on each beam. The ASW role was accomplished with 18 Type 95 depth charges, but this was doubled in May 1942 when the minesweeping gear was removed and a Type 93 sonar was installed. The anti-submarine weaponry later rose to 60 depth charges with a Type 97 81 mm trench mortar and six depth charge throwers.

Anti-aircraft protection was by four Type 96 25 mm anti-aircraft guns in two twin-gun mounts abreast the bridge. This was later increased by August 1943 when the twin-mounts were replaced by triple-mounts, and an additional three triple-mounts, one in front of the bridge and two behind the smokestack were added. A Type 22 radar was also installed in 1943 and a Type 13 in 1944.

Operational service
Despite being designed for operations in northern waters, the lead vessel of the class, JAPANESE ESCORT Shimushu was used primarily in southern waters for convoy escort. She survived the war and was given as a prize of war to the Soviet Navy, which continued to use her until 1959. JAPANESE ESCORT Ishigaki was sunk by a submarine in 1944. JAPANESE ESCORT Hachijo and JAPANESE ESCORT Kunashiri served in the northern Pacific, and both were scrapped after the end of the war.

Ships in class
The four ships were ordered as ships #9 to #12 under the 3rd Naval Armaments Supplement Programme in 1937.