ĭuring 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. The ships were also equipped with a Model 93 sonar and a Type 93 hydrophone. 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. 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. Anti-aircraft protection was by four Type 96 25-millimeter (1.0 in) anti-aircraft guns in two twin-gun mounts abreast the bridge. Īs with the Shimushu class the main battery of the Etorofu class consisted of three Type 3 120-millimeter (4.7 in) guns in single mounts, one superfiring pair aft and one mount forward of the superstructure. The ships had a range of 8,000 nautical miles (15,000 km 9,200 mi) at a speed of 16 knots (30 km/h 18 mph). The ships had two diesel engines, each driving one propeller shaft, which were rated at a total of 4,200 brake horsepower (3,100 kW) for a speed of 19.7 knots (36.5 km/h 22.7 mph).

They displaced 880 metric tons (870 long tons) at standard load and 1,040 metric tons (1,020 long tons) at deep load. The ships measured 77.72 meters (255 ft 0 in) overall, with a beam of 9.1 meters (29 ft 10 in) and a draft of 3.05 meters (10 ft 0 in). The Etorofu class was almost identical to the Shimushu class but with a simplified bow, stern and bridge structure to facilitate production.

Despite simplification, the design was still too complex for mass production and one of the ships was not completed until early 1944. Production began between February 1942 and August 1943. However, sixteen of the projected thirty ships were subsequently re-ordered to the subsequent Mikura, Hiburi or Ukura designs.

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. 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. 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. 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 76 mm (3 in), no torpedoes, and with a maximum speed of no more than 20 knots (37 km/h 23 mph). The Shimushu-class kaibōkan, as with the Chidori-class 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. The class was also referred to by internal Japanese documents as the 'Modified A-class' coastal defense vessel ( 甲型海防艦, Kō-gata kaibōkan). Eight of the fourteen ships were sunk during the war. The Etorofu-class escort ships ( 択捉型海防艦, Etorofu-gata kaibōkan) were a group of fourteen kaibōkan escort vessels built for the Imperial Japanese Navy during World War II. 36 × depth charges initially, but later up to 60.

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