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 76abbr=onNaNabbr=on, no torpedoes, and with a maximum speed of no more than 20kn. 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 battleships which had been reassigned to coastal defense duties. Immediately before the start of then Pacific War, the Imperial Japanese Navy suddenly decided to give more priority to convoy escorts, possibly in light of the ongoing successes of German U-boats against British shipping in the Atlantic. As the Shimushu class was not suited for mass-production and took too long to build, the 1941 Rapid Naval Armaments Supplement Programme authorized thirty modified versions of the Shimushu class, which were designated the Etorofu class.[1] However, sixteen of the projected thirty ships were subsequently re-ordered to the subsequent Mikura, Hiburi or Ukura designs.
Production began between February 1942 and August 1943. Despite simplification, the design was still too complex for mass production and one of the ships was not completed until early 1944.[1]
The Etorofu class was almost identical to the Shimushu class but with a simplified bow, stern and bridge structure to facilitate production. The ships measured 77.72m (254.99feet) overall, with a beam of 9.1m (29.9feet) and a draft of 3.05m (10.01feet).[2] They displaced 870sp=usNaNsp=us at standard load and 1020sp=usNaNsp=us at deep load. The ships had two diesel engines, each driving one propeller shaft, which were rated at a total of 4200bhp for a speed of . The ships had a range of 8000nmi at a speed of .[3]
As with the Shimushu class the main battery of the Etorofu class consisted of three Type 3 1201NaN1 guns in single mounts, one superfiring pair aft and one mount forward of the superstructure. Anti-aircraft protection was by four Type 96 251NaN1 anti-aircraft guns in two twin-gun mounts abreast the bridge. However, for a ship supposedly designed for convoy escort, only one Model 94 depth charge launcher was installed on the quarterdeck along with a Model 3 loading frame. The number of depth charges was initially 36, but this was increased to 60 while the ships were still in production, which necessitated the deletion of the two paravanes initially in the design for minesweeping. The ships were also equipped with a Model 93 sonar and a Type 93 hydrophone.[1]
During the Pacific War, the number of Type 96 anti-aircraft gun was increased to five triple-mounts and a varying number of single-mounts, up to 15 in total by August 1943. A Type 22 and Type 13 radar were also installed. A Type 97 811NaN1 trench mortar was also installed front of the bridge[3]
The Etorofu class proved to be an inadequate design by the time the final units entered service in 1944. Their speed was slower than most submarines, and with only one depth charge projector, their combat capability against the increasingly effective United States Navy submarine forces was ineffective. The Etorofu-class vessels were mostly deployed to the South China Sea or East China Sea as convoy escorts, but few recorded any attacks against Allied submarines. Conversely, six of the 14 ships in the class were sunk by American submarines. Of the six survivors, three were used as repatriation ships after the war, and were subsequently given as prize of war to Allied navies.[1]
Thirty ships (numbered #310 to #339) were included in the Rapid Naval Armaments Supplement Programme in 1941. These are listed below with the shipyard to which each was allocated:
| Number | Name | Builder | |
|---|---|---|---|
| Etorofu | Hitachi-Sakurajima Shipyards | |
| Matsuwa | Mitsui-Tamano Shipyards | |
| Sado | NKK-Tsurumi Shipyards | |
| Oki | Uraga Dock Company | |
| Matsure | Hitachi-Sakurajima Shipyards | |
| Iki | Mitsui-Tamano Shipyards | |
| Tsushima | NKK-Tsurumi Shipyards | |
| Wakamiya | Mitsui-Tamano Shipyards | |
| Hirado | Hitachi-Sakurajima Shipyards | |
| Fukae | Uraga Dock Company | |
| Mikura | NKK-Tsurumi Shipyards | |
| Amakusa | Hitachi-Sakurajima Shipyards | |
| Miyake | NKK-Tsurumi Shipyards | |
| Manju | Mitsui-Tamano Shipyards | |
| Awaji | Hitachi-Sakurajima Shipyards |
| Number | Name | Builder |
|---|---|---|
| Kanju | Uraga Dock Company |
| Nōmi | Hitachi-Sakurajima Shipyards |
| Kurahashi | NKK-Tsurumi Shipyards |
| Hiburi | Hitachi-Sakurajima Shipyards |
| Chiburi | NKK-Tsurumi Shipyards |
| Kasado | Uraga Dock Company |
| Yashiro | Hitachi-Sakurajima Shipyards |
| Ukuru | NKK-Tsurumi Shipyards |
| Daitō | Hitachi-Sakurajima Shipyards |
| Kusagaki | NKK-Tsurumi Shipyards |
| Okinawa | NKK-Tsurumi Shipyards |
| Amami | NKK-Tsurumi Shipyards |
| Aguni | NKK-Tsurumi Shipyards |
| Shinnan | Uraga Dock Company |
| Shōnan | Hitachi-Sakurajima Shipyards |
While fourteen of the above ships were completed to the Etorofu design, eight ships - Mikura (#320), Miyake (#322), Awaji (#324), Nōmi (#326), Kurahashi (#327), Chiburi (#329), Yashiro (#331) and Kusagaki (#334) - were altered to be built to the Mikura design; three ships - Hiburi (#328), Daitō (#333) and Shōnan (#339) - were altered to be built to the Hiburi design; and five ships - Ukuru (#332), Okinawa (#335), Amami (#336), Aguni (#337) and Shinnan (#338) - were altered to be built to the 'Ukuru design. The fourteen completed to the (original) Etorofu design were as follows:
| Kanji | Name | Builder | Laid down | Launched | Completed | Fate |
|---|---|---|---|---|---|---|
| 択捉 | Hitachi-Sakurajima Shipyards | 23 February 1942 | 29 January 1943 | 15 May 1943 | Ceded to the United States, 1946. Scrapped 1947 | |
| 松輪 | Mitsui-Tamano Shipyards | 20 April 1942 | 13 November 1942 | 23 March 1943 | Sunk by on 22 August 1944, Hidai Bay, Philippines [14-15N, 120-25E] | |
| 佐渡 | NKK-Tsurumi Shipyards | 21 February 1942 | 28 November 1942 | 27 March 1943 | Sunk by on 22 August 1944, Hidai Bay, Philippines [14-15N, 120-25E] | |
| 隠岐 | Uraga Dock Company | 27 February 1942 | 20 October 1942 | 28 March 1943 | Ceded to Republic of China Navy as Gu An (固安) in August 1947, captured by PLAN and renamed Chang Bai (长白) in 1949, scrapped 1982 | |
| 六連 | Hitachi-Sakurajima Shipyards | 25 July 1942 | 10 April 1943 | 31 July 1943 | Sunk by on 2 September 1943, Philippine Sea [08-40N, 151-31E] | |
| 壱岐 | Mitsui-Tamano Shipyards | 2 May 1942 | 5 February 1943 | 31 May 1943 | Sunk by on 24 May 1944, 150 miles W of Sarawak | |
| 対馬 | NKK-Tsurumi Shipyards | 20 June 1942 | 20 March 1943 | 28 July 1943 | Ceded to Republic of China Navy as Lin An (臨安) 31 July 1947, scrapped 1963 | |
| 若宮 | Mitsui-Tamano Shipyards | 16 July 1942 | 19 April 1943 | 10 August 1943 | Sunk by on 24 May 1944, East China Sea [28-38N, 122-05E] | |
| 平戸 | Hitachi-Sakurajima Shipyards | 2 November 1942 | 30 June 1943 | 28 September 1943 | Sunk by on 12 September 1944, South China Sea [17-54N, 114-49] | |
| 福江 | [4] | Uraga Dock Company | 30 October 1942 | 2 April 1943 | 28 June 1943 | Ceded to UK in July 1947, scrapped |
| 天草 | Hitachi-Sakurajima Shipyards | 5 April 1943 | 30 September 1943 | 20 November 1943 | Sunk by Royal Navy aircraft 9 August 1945, Onagawa, Japan | |
| 満珠 | Mitsui-Tamano Shipyards | 15 February 1943 | 31 July 1943 | 30 November 1943 | Scrapped 1946 | |
| 干珠 | Uraga Dock Company | 8 April 1943 | 7 August 1943 | 30 October 1943 | Scuttled after mine damage, 15 August 1945, near Wonson, Korea [39-10N, 127-27E.] | |
| 笠戸 | Uraga Dock Company | 10 August 1943 | 9 December 1943 | 27 February 1944 | Scrapped 1948 | |