Shimushu-class escort (1939)

Japanese ASW escorts: IJN Shimush, Kunashiri, Ishigaki, Hachijo 1940-59.

Albeit the Matsu and Tachibana of 1944-45 are often assimilated as the first “true” IJN escort destroyer, a first effort to produce something mid-way between a frigate and proper destroyer escort was already on the works since 1937, based on previous experience with torpedo boats and submarine chasers. The Shimushu or “A” class were the first of their kind, built 1939-41, just before the Pacific Campaign, and assigned coastal escort missions, from Japan to close destinations and back. The initial order for sixteen ship was cancelled for a cheaper and smaller design, the “B-class”. One was lost to a US submersible in 1944, the other three survived. One ended in Soviet service until 1959.

Development

IJN Shimushu on sea trials, colorized by irootoko Jr

1931 treaty Loopholes researches

The Shimushu-class kaibōkan (“escort ships”), shared the same roots as the Chidori class torpedo boat, in the 1930 London Naval Treaty, capping the IJN’s total destroyer tonnage. The naval staff, always eager to cheat on the Washington and London limitations, looked at a way to circumvent them using a loophole allowing ships between 600 and 2,000 tons with no more than four guns over 76mm, no torpedoes, and no more than 20 knots in speed. An internal program started in 1931 under the designation of kaibōkan (Kai = sea, ocean, Bo = defence, Kan = ship), already used for obsolete warships reassigned to coastal defense duties. However shifting budgetary priorities, had this new class not funded until the 1937 3rd Naval Armaments Supplement Programme, after Japan already left the 1936 naval conference.

Fishery Guard Ships

However these ships had also more concrete roots that just treaty loopholes, unlike torpedo boats. In postwar years, fishing disputes frequently occurred between the Soviet Union and Japan north of Japan, between the Kuriles, Sakhakin and Sea of Okhotsk. The Imperial Japanese Navy dispatched destroyers to protect fishery areas, but this was a bad use of ships needed for fleet work, and already in short supply. Plus the lack of cold-weather equipment (like de-icing the deck) made these deployments difficult. Construction of new “patrol ships” was considered in the First Replenishment Plan by 1931 already and subsequent 2nd Replenishment Plan but other priorities arose as seen above.

It was not before the Third Replenishment Plan in 1937 that work resumed on ships dedicated to fishery protection. Since the Navy wanted them for other naval tasks as well, they were re-rated and classified as “coastal defense ships” a category already assigned to much older vessels. Since these new vessels had to deal with Soviet patrol vessels, they needed to be on par in size but would bore the chrysanthemum emblem on their bow to be trely showcased as “warships.” This however was removed in July 1942.

Mass Production Cancelled

Still, the Shimushu had many shortcomings, such as main guns with low elevation, unable to do anti-aircraft fire. The Shimushu-class thus became the prototypes for the Etorofu and Mikura-class escort ships, produced in far larger numbers for convoy escort, also in the southern Pacific. The class initially was planned to count sixteen vessels, mostly to be built in civilian yards, but this was cancelled in 1940 as the ships were too complex to build (see below). Still, four more were provided by the programme of 1942 (Nos 790-793), but work never started. The Shimushu class were indeed cancelled as they were considered too large, complex, costly, requiring a large crew. In short, they were not suitable for mass production. Engineers at Mitsubishi Heavy Industries were indeed quite enthusiastic about this project, and they privileged technical excellence to impress the naval staff, over ease of construction.

Design of the class

The design was carried out not by the Naval Construction Department, busy already with many more urgent programmes, but by Mitsubishi Heavy Industries as a private design. The latter made ships capable of operations in northern seas, so with a tailored hull able to resist ice and deal with heavy weather, cold waters. The freeboard was high, the forecastle long and with a lot of flare to alleviate water splashing. She was equipped with extensive de-icing and heating systems. The hull at the waterline was reinforced to deal with ice and the chin was shaped and reinforced to just crush ice to some extent. The ship had also an excellent stability and measures taken in the bulkheads ensured the crew could move safely from the bow to the bridge and stern without going out, especially in rough weather.

The Shimushu class also had a powerful desalination system for a constant supply of fresh water. So much so the entire crew could enjoy bathing daily while at anchor. However the new ships needed to be armed as well, as their specialization for fisheries protection and patrol, meant her armament was still modest, and speed was low as it was not a primary requirement. She had diesel engines for extra range, making for low fuel consumption, as that was a main requirement however. Overall, of the ships had a very superficial similarities with the torpedo boats, they were much smaller, slower, not armed the same way, completely different animals.

Hull and general design

shimushu-pinterest
To be authorized still along 1930s treaty limits, the new ships were calculated to displace no more than 800 tonnes and were officially declared as such, despite all the hull reinforcements to deal with ice. The Shimushu class displaced 820-830 tonnes in reality, standard, but with later wartime additions, rose to 860, then 870 metric tons (860 long tons) at standard load and 1,040 metric tons (1,020 long tons) deeply loaded.
As for hull proportions, it was made to maximize speed despite having diesels, so they ended as the largest of all “escort ships” of the IJN, at 77.72 meters (255 ft) overall, for a beam of 9.1 meters (29 ft 10 in) making for a still favourable ratio of 1/9, and a moderate draft of 3.05 meters (10 ft).

The general outlook was very much unique, with a tall bridge located aft of the forecastle cut, not on it, while the prow had a moderate stem angle. So the only weight the forecastle had to carry was the “A” gun mount behind a generous wave breaker. There was a single, narrow structure running amidship from the base of the bridge, with a steamline face, down to “Y” gun position on deck. This structure supported “X” gun as well aft, so the artillery had better fire angles than torpedo boats that had to make room for their TT banks. There was a main tripod foremast just aft of the bridge, which also carried an open bridge and a main fire control rangefinder. There was a sjort pole mast aft of the single funnel amidship, raked, and preceded by a searchlight platform. Four utility boats were placed under davits amidshipn abaft the bridge and funnel.

Powerplant

The Shimushu class were classic warships with two shafts for agility, mated each to a diesel engine, rated to 2,100 bhp, and making for a total rating at 4,200 brake horsepower (3,100 kW). This enabled a top speed of 19.7 knots (36.5 km/h; 22.7 mph) on trials. Meaning when fully loaded in service, normal max speed averaged 15-16 knots. They however had a range of between 6,000 and 8,000 nautical miles (15,000 km; 9,200 mi) at a speed of 16 knots (30 km/h; 18 mph), but sources diverges on this. This was excellent for that category, and unlike tirpedo boats, they had “legs” perfect to patrol the long expanses of Japanese northern fishing areas, but that proved later handy for long escort patrol routes.

Protection

the blueprints
No armour, but as said above, ice protection was mandatory and the hull was much strenghtened at the waterline, wit extra plating, and a large cast part welded to the rest of the structure at the chin and stem to crush ice without damage, albeit there was a limit in ice thickness still. The remainder of the waterline, down to the poop, was also doubled. Otherwisen the originary protection measures were taken, with a machinery space divided into two separate rooms, one for each diesel, a separated room for an emergency extra diesel generator out of flooding reach, pumps, standard fire extinguishing equipments, de-icing equipments to melt ice forming in winter on deck, and bulkheading all along the hull, while still preserving access with well placed bulkhead doors unsuring the crew could move from one end to another, including an access through the engine rooms.

Armament

The Shimushu-class were armed as a baseline, like the Chidori class torpedo boats, with three Type 3 120-millimeter (4.7 in) guns, in single mounts: For stability, one was located on the forecastle (A) and the remainder in supoerfiring positions for X and Y on the structure and deck respectively. These were old guns already, taken from older destroyers converted for other uses or retired. They were added, at the demand of the IJN staff, a role of minesweeping. Thus, not onl she had a permanent mechanical minesweeping gear at the poop, she also had four paravanes on the superstructure walls aft amidships. The class also carried eighteen Type 95 depth charges, including six in side cradles and six reloads. For AA protection they had four Type 96 25-mm (1 in) anti-aircraft guns, in two twin-gun mounts, abreast the bridge on suspended wings supported by lattice struts.

12 cm/45 3rd Year Type naval gun

The 12 cm/45 gun was designed in 1895 as an indigenous variant of an Elswick Ordnance Pattern Y under the designation “Type 41”. In 1921 it was declined into an high-angle AA gun, the 12 cm/45 10th Year Type. The base model had a Single motion interrupted screw Welin breech block different from the Type 3 12 cm AA Gun used by the Army in 1943. On destroyers it was always shielded (photo), with the latter fitted with two opening panels. From 1922 the gun evolved into the 12 cm 11th Year Type naval gun (Model 1922) with a shorter gun barrel and horizontal sliding breech-block but reserved for TBs and submarines only. Manually loaded it fired a 20.3 kg (45 lb) high-explosive or illumination shell. The patrol conversions from 1943 also received an anti-submarine shell. This gun was ubiquitous for early IJN destroyers, also found on the Kamikaze, Kawakaze, Minekaze, Mutsuki, Wakatake class destroyer as well as the Chidori/Erotofu class TBs and the Shimushu and Tsukushi class escort/survey ships of the Hashidate class gunboats.

⚙ specifications 12 cm/45 3rd Year

Weight 3,240 kg (7,140 lb), Barrel length 5.4 m (18 ft) bore
Elevation/Traverse -7° +33° and 120°/120°
Loading system: Welin breech block, Hydro-pneumatic recoil for 5–6 rpm
Muzzle velocity 825 m/s (2,710 ft/s), range 16 km (10 mi) at 33°
Crew: 6. Round 20.3 kg (45 lb) 120 x 550 mm.R., sep. loading cased charge

25 mm Type 96

The type 96 25 mm AT/AA gun became the standard of the IJA and IJN, automatic cannon developed as variant of the French Hotchkiss 25 mm, dual-purpose and developed as AA on single, twin and triple mounts. Development worked on 1935. 25 mm Hotchkiss design evaluated and order placed for several mounts types for evaluation at Yokosuka Naval Arsenal. Led to the Type 94 and Type 95 and then the main model produced at Yokosuka Arsenal, the Type 96. Elements made with castings and not forging for larger production, Rheinmetall supressor, new mounts designed. Air-cooled gas operated with multiple rings (Hotchkiss patent).

Twin-mount in 1939, followed by triple mount in 1941 and single mount in 1943. Issues of the designed revealed in combat:
Slow elevation and traverse, ineffective sights, excessive vibration, limited magazine cap., blinding Flashes.
The 1944 single mount needed a single operator and a loader but had a better spiderweb sight.

⚙Tech specifications 25 mm Type 96

Barrel Lenght 1.5 m (4 ft 11 in) L/60
Barrel weights: Single 785 kg (1,731 lb)
Barrel weights: Twin 1,100 kg (2,400 lb)
Barrel weights: Triple 1,800 kg (4,000 lb)
Crew: 3, 7 and 9 respectively
Shell 25×163mm
Exact caliber 25 mm (0.98 in)
Action Gas operated
Elevation/Traverse -10°/+85° – 360°, manual
Rate of fire 200–260 rpm (cyclic)
Muzzle velocity 820 m/s (2,700 ft/s)
Effective range 6.8 km (4.2 mi) at 45° with HE shell
Maximum firing range 85° 3 km (9,800 ft) effective, 5.5 km (18,000 ft) max.
Feed system 15-round box magazine

Depht Charges

Standard Type 95, 30.5″ by 17.7″ (77.5cm by 45cm). 220 lb (100 kg) charge, Type 88 explosive (ammonium perchlorate and ferrosilicate). Fuse using a water inlet. 100 feet (30m) and 200 feet (60m) settings (after the revelations of a US Congressman).
Later increased to 324 lb (147kg), Type 97 explosive (70% TNA/30% HNDA), 300 foot (90m) setting.
-1944 Type 2: 230 lbs (105kg) Type 97 explosive settings 98, 197, 292, 390, 480 feet (30m, 60m, 89m, 120m, and 145m).
-1945 Type 2: 357 lbs (162kg) explosives.

Sensors

93-shiki (Type 93) sonar (1943)

2 kW 4400 lbs (2000 kg) model, FRQ 17.5 kHz, receiver gain 120 decibels 2, 4, or 8 seconds pulse interval, range up to 1640-3300-6600 yards (1500m-3000m-6000m) at 3°/10° resolution.

Type 22 Radar

The early Type 22 General Purpose Radar looks like characteristically like a 2910 lb (1320 kg) drum topped with a twin cornet amplificators, mounted mid-way top the mast.
Specs: Wavelength 10 cm, pwd 10 microsecond, PRF 2500 Hz, scan rate 5 rpm, PP 2 kW.
Range: 20 nautical miles (35 km) aircraft group, 10 nm (17 km) single, 13 nm(24 km) battleship.
Subject to land clutter. Horn and A scope display, accuracy 220 yards (200m)/3 degrees res. 1600 yards (1500 m)/40 degrees
300 sets produced installed on destroyers from the summer 1942. Generalized in 1944.

Type 13 Radar

Vertical 240 lb(110 kg) bedframe antenna, developed 1941, introduced mi-1943, with vertical dipole transmitter and Yagi mattress receiver. 1000 sets produced. Air defense, range 100 km (group), and single aircraft 50km.
Specs: Wavelength 200 cm, pw 10 ms, PRF 500 Hz PP 10 kW, Range: 30-60 nautical miles (50-100 km)

⚙ specifications (wartime)
Displacement	860 long tons (874 t) standard
Dimensions	77.7 x 9.1 x 3.05m (255 ft x 29 ft 10 in x 10 ft)
Propulsion	1 shaft, 1x diesels, 4200 shp
Speed	19.7 knots (22.7 mph; 36.5 km/h)
Range	Fuel 150t. 6000 nm/16 kts
Armament	3× 120 mm)/45, 10-15× Type 96 25 mm AA, 6× DCTs, 1 DCT (60), 80mm mortar
Protection	Hull strenghtened for ice conditions
Sensors	type 93 sonar, Type 22, 13 radars
Crew	150

Upgrades

After the outbreak of the Pacific War in 1941, they were eventually all sent to escort convoys on southern routes and ironically were never deployed to protect fisheries. Later in the War, they received extra anti-aircraft guns and depth charges, racks and throwers: In May 1942, the minesweeping gear was removed (not the paravanes), and a Type 93 sonar was installed. Later in 1943-44, six depth charge throwers were installed, located aft on the poop, close together with their reloading system down to the lower hull and a total of 60 depth charges. There was also a single Type 97 81 mm (3.2 in) trench mortar close by, that had greater range and could be traversed either side.

Anti-aircraft protection was also greatly enhanced. From the initial three twin mounts, by August 1943 they were replaced by triple-mounts (on wings abaft the bridge), with an additional three triple-mounts, one in front of the bridge, two behind the smokestack. In addition to the Type 93 sonar, they also gained a Type 22 radar installed in 1943 and Type 13 in 1944.

Career of the Shimushu class

IJN Shimushu 占守 (1939)

Shimushu was ordered from Mitsui-Tamano Shipyards in 1937, laid down on 29 November 1938, launched on 13 December 1939 and commissioned on 30 June 1940. She participated in the landings in Thailand, escorting the 55th IJA Infantry Division for the landings at Nakhon, on 8 December 1941. She escorted other invasion convoys in support of the Malaya, Sumatra, and Palembang campaigns and landings. Her sonar operator located the British battlecruiser HMS Repulse nearby on 29 January 1942, the first IJN vessel to locate her wreck and precise her location. She took part in the infamous HI-40 convoy, which ended as a disaster on 19–24 February 1944. All six oilers under her watch were destroyer by a sub pack. Nampo Maru under her direct escort, closest to her, was torpedoed and sunk by USS Grayback and Jack. After that, the Naval General staff discontinued the practive of having a single escort for convoy work and doubled extra ships with aircraft, better able to located submerged submarines and organized larger convoys with more escorts.

shimushu 1942

Shimushu was part of TA No. 2 and 3 convoys towards Leyte island. In one attack, she claimed shooting down one of five B-25 Mitchell medium bombers, lost in the first wave. On 25 November, Shimushu was torpedoed by the submarine USS Haddo. She lost her bow but her bulkhead held on and was maintained so by the crew for her to limp back to Japan; She was taken in hands to be repaired from 20 January 1945 and was captured as such in September. After full repairs she resumed service as a repatriation ship in 1946-47, until placed on dispsoal. After negiociaitons she was ceded to the Soviet Union as war reparation on 5 July 1947. She joined the Pacific Fleet as “patrol ship EK-31”. The next year she became the “dispatch ship PS-25”. In 1957, no longer serviceable, she became the repair ship PM-74. She was decommissioned to years later by 16 May 1959 and BU.

IJN Kunashiri 国後 (1940)

IJN Kunashiri was ordered from NKK-Tsurumi Shipyards, laid down on 1 March 1939, launched on 6 May 1940 and completed on next 3 October. No data for her early career (yet), but by July 1943 she took part in the Kiska evacuation, Operation Ke. Thanks to her initial ice-friendly design the was assigned to the Kuriles and Hokkaido area for escort work. On 28 July 1944. she was reported “damaged” by unknown cause, but likely by USS Tambor. After the end of World War II she was back to Sasebo, and due to the lack of fuel, stranded there until August 1945. She was undaaged by summer air attacks and discovered intact by US occupation forces in September. She was then assigned to the Allied Repatriation Service and started to sail in the South Pacific and China to recover Japanese discharged troops. On 4 June 1946, while en route to Uraga she ran aground and was later abandoned. Kamikaze tried to pull her free only to also ran aground, while also repatriating Japanese troops from Singapore. She was left there and broken up later, stricken.

IJN Ishigaki 石垣 (1940)

Ishigaki was ordered from Mitsui-Tamano Shipyards, laid down on 15 August 1939, launched on 14 September 1940 and commissioned on 15 February 1941. Based in the Kuriles, she patrolled and escorted convoys to this area, but by 7 October 1943, she detected and sank the old US submarine USS S-44, after spotting her by radar. Initially she thought her to be a small freighter and rushed to investigate. S-44 opened fire with her 4-inch deck gun at the time, attacking the refrigerator ship Koko Maru. Ishigaki spotted the submarine thanks to her fire flashes, and opened fore below 3,300 yards (3,000 m) with her bow gun. Captain Francis Brown ordered a crash dive, but Ishigaki managed to hit her conning tower before, jamming open the hatch. Unable to dive, S.44 resolved to an unequal gun duel to the death.

Ishigaki’s skipper took advantage of darkness to point at her and blinde her gunners with the ship’s 75-cem (30 in) searchlight. S.44 was unable to land any hits whereas Ishigaki scored a second hit admiship, close to her battery section. She then made a turn to present her broadside, firing all three guns began at closer rang, and scoring more hits, until she ceased fire to asses the results. Burning, S-44 was heeling and started to sink rapidly. Just eight men hat best escaped before she went down, but Ishikagi only picked up Chief Torpedoman’s Mate Ernest A. Duva and Radioman Third Class William F. Whitemore. They were interrogated and likely tortured, and later landed in a POW camp. However the escort ship would not survived S.44 for long, as on 31 May 1944, she was avenged by USS Herring. Her bow was severed by a torpedo hit, and albeit her bulkhead resisted enough for her to drop depth charge on site, she sank with 167 sailors.

IJN Hachijō 八丈 (1940)

IJN Hachijo was ordered from Sasebo Naval Arsenal (the only proper navy yard in her class), laid down on 3 August 1939, launched on 10 April 1940 and completed on 31 March 1941. Like her sister ship Ishigaki, Hachijō also remained more her career in the cold water Kuriles in escort missions. On 19 February 1943, Hachijō was detached from Akagane Maru to reach the Island of Attu in the morning for support, but in the evening, the ship she left behund was spotted and destroyed by the heavy cruiser USS Indianapolis escorted with two destroyers. On 7 July 1944 Hachijō was found in the middle of an air attack, and crippled. Her auxiliary engine room was flooded, she lost personnel and had fires that needed time to extinguish. Shrapnel also holed her hull and needed emergency patchup. In the end after repairs, she was still in Japan when captured by the allies past September 1945. She survived to be part of evacuations and scrapped on 30 April 1948.

IJN Hachijo’s bridge in 1945

Escort ships in Maizuru 1947, formerly used as repatriation ships and waiting for their fate