L4 (RO-60) class submersible (1922)

Japan 1922-24: RO-60 to RO-68

Here is the next big lineage for 2026-27: IJN submarines. First entry: The L class. The last were still active in WW2: The Type L4 (L4型（呂六十型）, Ro-roku-jū-gata, Ro-60-class)
were a license production model of the British L class, submarine Group 3. The IJN was quite satisfied with them and stopped the development of the alternative Kaichū type. However by concentrating on larger fleet auxiliary types, this left a time gap and more than a decade went on before any new class of medium submarine was started. This was a technical delay that was hard to catch up before the Pacific War, resulting in a critical absence. If a good, affordable medium type such as the German Type VII or Italian Bronzo class could have been available en masse, they would have been a good counter to the fleet of US Gato-type subs as well as to prey on the massive US logistic fleet behind TF38/58.

Development

The L class lineage

Holland 1
Holland N°1

The submarines of the Type L4 subclass were copies of the Group 3 sub-class of the British L-class, based on an Holland design and built under license in Japan. They were slightly larger, with two more torpedo tubes than the L3. First off we need to have a word about the Holland and Japan. Electric Boat became the incontested leader in submarines in 1900, providing nearly all navies but France and Italy. These single-hull types had excellent caracteristics, especially underwater, and initially if the IJN staff was dubious about submarines, the adoption in 1900 of the Holland type by the Royal Navy, that very much shaped the IJN prior to the Russo-Japanese war and on the background of the 1902 alliance, had quite an impact. The technology was not needed but at least many estimated it was useful to test it.

Hence the type developed by Irish immigrant John Holland was the very first purchased and tested by Japan (photo, Holland N°1), albeit late. See the WW1 IJN submersible lineage for more. But the first class was British built in 1904-1905, the “Vickers Kaigun Type”. Next came a US-licenced serie the Kaigun-Holland built by Kawasaki in 1906. There was the Vickers “C” type in 1909, Vickers-Kawasaki Type in 1912, but to sample other designs, Japan also purchased a Schneider-Laubeuf, the Type S1/S2 in 1916-18 as well as the double hulled Laurenti type from Italy in 1919, the F1. This fleet had been quite reduced in size and never intended to fill a well-defined strategic role for the IJN, but withessing the German submarine warfare was an eye-opener and like all other navies, Japan started to take the submarine seriously, finding a way to integrate it for specific roles for the Navy and crate an Japanese designed and built type in 1921, concentrating on two types, a very large ocean-going cruiser submarine and minelayer or a a smaller ocean boat of up to 1000 tons with moderate speed.

For the latter, Japan looked again at Vickers and Holland for inspiration, and purchased updated licences for the British L-type all the way back to 1916. So this class was not the fruit of the 1921 program but a mere confirmation of the class already planned when the first L1 types were laid down at Mitsubishi rather than Kawasaki. The L-gata Sensuikan was this medium-sized submarine the Imperial Japanese Navy (IJN) wanted to develop further. This led to four distinct sub-classes, L1 to L4 that were the bedrock of medium size submarines in the interwar and until WW2 as the L4 took part in the latter, wit the L1 types beng decommissioned in 1932-40, L2 in 1940, and the L3 being active. They were also renamed RO- classes in the interwar. The Type L submarines were built at Mitsubishi Heavy Industries-Kobe Shipyard, but with Vickers naval technical guidance, under contract.

L1 class (Ro-51)

They were copies of the British L-class, the lmargest class of submarines in WW1, rivalling the US Navy S-class. In all 73 were planned, but only 27 were completed. Their trademark was their main gun on the conning tower platform instead of on deck, a peculiarity shared by British submarines until WW2. The L class were made in three groups varying between them, and the Japanese Mitsubishi designs were based on these. For example, Group 1 was 222 ft (67.7 m) long, Group 2 228 ft (69.5 m) long and Group 3 230 ft 6 in (70.3 m) long. They were the largest Electric Boat ever design, so far. In the interwar, these designs will evolve considerably. Thus, Mitsubishi’s L1 class proceeded from the 1916 Mitsubishi Vickers L class patent. This purchase, done on private funds, was a bold one but paid off after loosing to Kawasaki for the Fiat-Larenti Type F design (Ro-1/Ro-3 class).

The IJN as a whole hoped to improveme their submarine fleet and ordered just a pair for testings from Mitsubishi. The latter, to keep delays short, purchased six semi-knock down kits, a technique perfected by Holland for ots export customers enabling faster assembly by the customer, provided advisors from the compant were present in the yard for completion. The Soviet (initially Russian) Amerikanski Goland or AG types were of that breed. The crews reported a good experience with Vickers diesels, very reliable. The IJN however wanted to swap to locally designed diesels, and wanted variants fitrted with Kampon Mk.24 diesels, leading to the Ro-100 class.

The class only comprised Ro-51 (N°25) and Ro-52 (N°26) as 呂号第51潜水艦 or Ro-Gō Dai-51 Sensuikan as standardized in November 1924. They were started on 10 August 1918, launched on 10 October 1919 and 09 March 1920, then completed on 30 June and 30 November 1920 respectively, and versed to training in the late 1920s. The first was decommissioned on 11 April 1940. The second much earlier on 1st april 1932.

L2 class (Ro-53)

They were a slightly modified L type also built by Mitsubishi, Kobe under the the 1918 Programme. The principal difference was omission of the two beam TT and 5t more oil fuel. Particulars as L 1 class. Like the lattr they were used to train new crews until the Pacific war.

No 27 (6.7.20, 10.3.21, Ro 53 1924, stricken 1938)
No 28 (13.10.20, 10.9.21, Ro 54 1924, stricken 1939)
No 29 (10.2.21, 15.11.21, sunk 29.10.23 in accident off Kobe, raised to be Ro 55 1924, stricken 1939)
No 30 (11.5.21, 16.1.22, Ro 56 1924, stricken 1940).

L3 class (Ro-57)

The Ro-57-class (from 1924) were similar to the L2 but with two more bow 21-in TT tubes, an their 3.1 in gun was moved atop their fairwaters instead of the deck, as British ones. A minelaying version with mines carried in tubes (saddle tanks) was planned in 1921 but never built, known as the L(M) Type. Also the Oil reserve went up to 117t. All three still performed inlu the previou boats after 1940: When the Pacific war campaign started they were used as training boats, until the end of the war. No 46 train midget submarine crews at Shodojima in 1945. No 57 trained crews at the Submarine School, Otake. Displacement rose to 897t/1195t, dimensions to 250ft oa , 242ft 9in x 23ft 6in × 13ft 76.2m , 74m x 7.2m x 4.0m. Machinery same except 98t oil tanks capacity for a range of 7000nm/85nm at 10kts/4kts. Since a ww2 career spent training new crews is not matter for career study, that’s way only the L4 class is seen there. If enough info is available, perhaps the L3 and prior will be covered as well, perhaps in 2027.

No 46 (3.12.21, 30.7.22, Ro 57 1924, stricken 1.5.45, BU 1946)
No 47 (2.3.22, 25.11.22, Ro 58 1924, stricken 1.5.45, BU 1946)
No 57 (28.6.22, 20.3.23, Ro 59 1924, stricken 1.5.45, BU)
Nos 48-50 cancelled 1920, replaced by KRS types.

Design of the L4 class

Hull and general design

Compared Overheads of Ro classes (ONI, Naval History & Heritage Cd. src)

The L4 (from 1924 Ro-60 class) displaced 1,004 tonnes (988 long tons) surfaced, 1,322 tonnes (1,301 long tons) submerged. They were the largest Vickers-Mitsubishi types at 78.39 meters (257 ft 2 in) long for a beam of 7.41 meters (24 ft 4 in) and draft of 3.96 meters (13 ft). Their diving depth was 60 meters (197 ft). As ONI noted in 1942 they were “coastal patrol and short-range operation subs from advanced bases, may also be used as transports”. ONI also believed them built at obe, Kawasaki instead of Mitsubishi.

Design-wise they resembled the classic Britsh L-class, albeit wihtout the 2-gun position fore and aft of the conning tower. Instead the single gun was on deck forward, returning to the L2 configuration. The Japanese were not convinced by that British habit. Thus, the gun was on deck, but still a raised bandstand with wave breaker. It was more “wet” than if on top of the sail, but at least the space was not confined and enabled full movements of the gun crew around. This raised platform was in fact part of a larger platform deck around the cnning tower, sponsoned with the upper hull.

L-18-23 class (Royal Navy) giving some idea of the compartments and arrangements.

The aft deck had a slightly raised section, and then went on aft in a classic ogival end, then the rounded part of the presssure hull that ended in a duck tail. The prow was narrow and tall with a sloped section and limited flare. It had a bollarrd and hooks, plus the bracket for the protective anti-net cable (but not cutter) running to the portico up admiship in the sail, and down to a poop deck bracket. It doubled as wireless cable and was supported by deck half-masts. It comprised a small enclosed helmsman post with windows and the long observation platform with fairwaters and a small ladded at its end where was also fixed the flag mast. There was a small attack periscope and larger observation one emerging from the platform, with the CT underneath. There were no forward dive planes. The blister ballasts were filled by a single row of water scoops, so diving was slow to WW2 standards.

Powerplant

For surface running, the L4 class submersibles were powered by two 1,200-brake-horsepower (895 kW) Vickers diesel engines, each driving one propeller shaft. When submerged, each propeller was driven by an 800-shaft-horsepower (597 kW) electric motor. They could reach 15.7 knots (29.1 km/h; 18.1 mph) on the surface and 8.6 knots (15.9 km/h; 9.9 mph) underwater. Whe surfaced, they had a range of 5,500 nautical miles (10,200 km; 6,300 mi) at 10 knots (19 km/h; 12 mph); submerged, they had a range of 80 nmi (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph).

ONI, Ro-60 class, 1942.

Armament

The submarines were armed with six internal 533 mm (21 in) torpedo tubes, all in the bow, and carried a total of twelve 6th Year Type torpedoes. They were also armed with a single 76.2 mm (3 in) deck gun and a 6.5 mm machine gun.

Type 6 533mm Torpedo

The four tubes in the bow and four reloads features the 6th Year Type torpedo, first type using metric measurements. It was designed at the brand new Imperial Japanese Navy Technical Department, favoring a rule era naming scheme. Development started in 1914, completed in 1917 for a service in 1918 on both surface ships and submarines. Originally defined in the Eight-Eight Program replacinf the 18-in by a 21-inch caliber torpedo and planning a 61 cm (24-inch) Type 8. This went with the decision to switch to metric units. In 1922, the Washington Naval Treaty was signed by Japan and forbade any caliber besides 550 mm (French standard) and the internaitonal norm remained at 53 cm.

The 1,432 kg (3,157 lb), 6.84 m (22.4 ft) Type 6 was powered by a Wet heater to bring its 203 kg (448 lb) Shimose warhead at 7,650 m (4.13 nmi) at 36 knots (67 km/h) ot 10,000 m (5.4 nmi) at 32 kn (59 km/h), 15,000 m (8.1 nmi) at 26 knots (48 km/h). These were above average performances already compared to other Navies for that caliber.
It’s possible they tested the Type 89 in the 1930s, however the latter was only carried by the Junsen and Kaidai-type submarines. They could reach 45 knots. They could also have tested or deployed the Type 92 electric torpedo, leaving no trail (more data on latter posts).

8cm/40 Type 88 deck gun

In reality it was of an exact caliber of 76.2 mm (3 in). A copy in 1897 of the British 12 cwt/12 Pdr QF. It was declined as AA Naval gin ad well and dual purpose for smaller units. The 8 cm/40 (3″) Type 88 (Model 1928) Submarine deck gun fired a fixed Quick Fire 76.2 x 405R ammunition weighting 5.7–6 kg (12 lb 9 oz – 13 lb 4 oz).
The Bgun had a sliding block breech and hydro-pneumatic recoil, and was capable of 13–20 rpm at 670–685 m/s (2,200–2,250 ft/s).
Effective firing range was 10,800 m (11,800 yd) at 45°.

Type 3 machine gun

Japan acquired a license and began producing Hotchkiss Mle 1897 machine guns in 6.5mm Arisaka, 87 caliber. During the Russo-Japanese War, each Japanese division had 24 Hotchkiss machine guns. Being lighter than the Russian Maxims, the Hotchkiss performed well. The production evolved to become the Type 3 Heavy Machine Gun in 1914.

Sensors

These submersibles had a pair of hydrophones, K type echo-sounder vertically only. Other equipments included a ratio transmitter special 4 and 2 receiver RDF-T4. They also had degaussine equipment with three 3/8 inch coils directly below deck and two fitted on the sail.

⚙ RO-60 specifications
Displacement	988 long tons (1,004 t) surfaced, 1,301 tons (1,322 t) submerged
Dimensions	78.39 x 7.41 x 3.96m (257 ft 2 in x 24 ft 4 in x 13 ft)
Propulsion	2 shaft Vickers diesels 2,400 bhp (1,790 kW) surfaced. 1,600 shp (1,193 kW) submerged
Speed	15.7 knots (29.1 km/h) surfaced, 8.6 knots (15.9 km/h) submerged
Range	75 tons fuel, 5,500 nm/10 kts surfaced, 80 nm/5.7 kts submerged
Armament	6× 533 mm TTs bow) (12), 76.2 mm L/40 deck gun, 6.5 mm LMG
Test depth	60 m (200 ft)
Sensors	Hydrophones
Crew	48

Career of the L4 class

Ro-60

Ro-60, credits Naval History & Heritage Command

ex-No. 59, renamed Ro-60 Nv. 1924. Ro-60 was initially laid down as No.59 on 5 December 1921 at Mitsubishi, Kobe, and Launched on 22 December 1922, completed and commissioned on 17 September 1923, assigned to the Sasebo Naval District and from 9 February 1924 to Subdiv 26 until the end of her career, Sasebo Defense Division. On 1 April 1924 her unit integrated SuRon 1, 1st Fleet. On 1 December 1925 her unit fell to SubRon 2, 2nd Fleet (Combined Fleet) and on 1 March 1926 her unit composed also of Ro-57, Ro-58, Ro-59, Ro-61, Ro-62, Ro-63, Ro-64, and Ro-68 departed Sasebo for Okinawa and started on 30 March 1926 a training cruise in Chinese waters, off Shanghai and Amoy, then making it to Mako in the Pescadores on 5 April 1926. On 20 April 1926 she returned off China, Chusan Island, and back to Sasebo on 26 April 1926. On 27 March 1927, Ro-60 to 64, and Ro-68 departed Saeki Bay for a training cruise off Qingdao an back to Sasebo on 16 May 1927. She was decommissioned on 10 February 1928, 3rd Reserve but recommissioned on 20 September 1928.

On 10 December 1928 her unit returned in SubRon 1, 1st Fleet and on 1 December 1930, she returned in the Sasebo Defense Division from 15 November 1933. On 1 June 1934 she returned in reserve, decommissioned, Sasebo Guard Squadron until 15 November 1935, then 3rd Reserve on 15 December 1938, 4rth Reserve on 15 November 1939. Fully recommissioned on 15 October 1940 she sas her unit reassigned amongh later to SubRon 7, 4th Fleet (Combined Fleet). She was first deploted at Kwajalein in the Marshall Islands when receiving the message “Climb Mount Niitaka 1208” on 2 December 1941 and with Ro-61 and Ro-62 she was sitll at Kwajalein on 8 December, on “standby alert”, with the objective of venturing to Wake Island after a thwarted invasion. On 12 December 1941 they were deployed in support of the second Wake Island invasion on 14 December. Ro-60 was surfaced, 25 nm southwest of Wake at 16:00 pm on 21 December when spotted by a USMC F4F Wildcat from VMF-211. She was strafed and two 100-pound (45.4 kg) bombs fell near so she crash-dived with her periscopes damaged as some diving tanks. She resurfaced that night to inspect her damage and as the battle ended she was ordered back to Kwajalein. But while approaching in predawn darkness of 29 December she ran hard aground on a reef north, her pressure hull damaged, starboard diving tanks pierced. At 13:00, SubRon 7 commander arrived to inspect the damage on the Submarine tender Jingei, HQ of the squadron. High surf was threatening to break Ro-60 and she took such as heavy list her crew prepared to evacuate, destroying secret documents. All 66 members were rerscued. She was stricken on 15 February 1942. Her wreck was later in the war finished off by an aircraft, strafing her, detonating her torpedoes. She lays there since.

Ro-61

ex-No.72 in 1922-1923, renamed Ro-61 Nov. 1924. She attached to the Sasebo Naval District, SubDiv 26, Sasebo Defense Division like her sister. So her interwar cruise are the same, at first in Subron 1, 1st Fleet, SubRon 2, 2nd Fleet (Combined Fleet). She accompanied the eight subs to Okinawa in 1926. By June she took part in underwater habitability tests, when a dummy torpedo exploded, poisoning 20 sailors, receiving treatment aboard IJN Jingei. She was part of the training cruise off Qingdao and was ultimately decommissioned on 1 November 1929, 3rd Reserve at Sasebo, recommissioned 1 December 1930, decommissioned 4 November 1931, recommissioned 16 July 1934, in reserve from December 1934, 2nd Reserve until November 1935, 3rd Reserve on 15 December 1938, 4th on 15 November 1939 and recommissioned on 15 November 1940 with her unit assigned to SubRon 7, 4th Fleet (Combined Fleet). The Day of Pearl Harbour she was at Kwajalein, Marshall on “standby alert”. Soon she was deployed off Wake Island from 14 December 1941, but the island fell on 23 December and she saw no action, being back to Kwajalein on 27 December, Marshalls Area Guard Unit from 5 January 1942, patrolling the island’s waters until the US carrier raid of 1 February 1942. The 6th Fleet wanted Sibton (Ro-61, Ro-62, I-9, I-15, I-17, I-19, I-23, I-25, I-26) to intercept the carrier task force, however none made contact.

In March 1942, Ro-61 and Ro-62 returned to Japan via Truk (Caroline) from 9 to 19 March, Saipan (Mariana) by 22 March and Sasebo to resupply, refit and R&R from 30 March to 31 May, to Saipan 6-7 June, Truk on 10 June 1942. Back to Japan from Truk on 27 June she headed for Yokosuka (5 July). On 14 July her unit was reassigned to the 5th Fleet, Aleutian Islands, Attu and Kiska. Ro-61 and Ro-62 departed on 24 July for Paramushiro, northern Kurile (30 July), underway on 1 August 1942 for Kiska, arriving on 5 August 1942, based there with I-6, Ro-63, Ro-64, Ro-65, Ro-67, and Ro-68. On 7 August 1942 they were surprised by an the US bombardment over Kiska and I-6, Ro-61, Ro-64, Ro-68 anchored in the harbor had to crash-dived to avoid damage. On 8 August Ro-61 sortied but failed to find targets. She had another go on 11 August 1942 and on 15 August. On 28 August 1942 she was radioed by a Kiska-based Aichi E13A1 “Jake” after spotting the seaplane tender USS Casco (AVP-12), mistaken by a light cruiser and escort destroyer in Nazan Bay, Atka. Ro-61, Ro-62, and Ro-64 sailed to intercept them and arrived on 29 August, ordered to penetrate Nazan Bay, to lure out US ships for an ambush. This was done at sundown on 30 August. Ro-61 approached Casco (misidentified as a Northampton-class cruiser) which fired three torpedoe from 875 yards (800 m). Only the second torpedo hit her, forward engine room. The tender was beached to prevent sinking. It was the first attack by any Japanese Ro-type.

On 31 August she was spotted while underway surfaced by a PBY-5A Catalina from VP-42 in the Bering Sea, off Atka. The flying boat had bombs and attacked Ro-61, also strafing her. Two depth charges fell right in target, causing heavy damage. Another PBY from VP-43 rakllied and also dropped depth charges in turn. The sub crash-dived, but left a large oil slick behind so she was easy to track. Indeed soon USS Reid (DD-369) was signalled her last position and rushed to the scene, soon spotting the oil slick, dotted with smoke floats by the Catalinas. Ro-61 was now down to 130 feet (40 m) when Reid dropped 13 depth charges, so she dived to 200 feet (61 m). The destroyer cornered her with 13 depth charges in a second pass. Ro-61′s central switchboard had a short that started fires in her control room and shell magazine, fed soon by leaks in the engine room and battery room. Not only she took an unctrollable stern trim, with the crew trying to correct it by throwing bags of rice and canned food in her bow, but soon seawater in her batteries reupted in a deadly cloud of chlorine gas, killing one petty officer. After taking a forward trim, her crew transferred 3-in shells to her aft compartment, but her CO decided to save his crew and ultimately forced her to surface. Some men rushed, arms in hands and fired at Reid with their stock Navy Arisaka rifles whil her 76.2 mm deck gun was manned, but had no time to fire as Reid depressed her 20 mm Oerlikon enough to swept clear her decks. She then turned her main 5-in/38 and started to score hits on her hull, breaching the inner hull until she capsized and sank by the stern, with almst all hands but 5 survivors helping identifying the sub as Ro-61. Presumed lost on 1 September 1942, she was stricken on 20 October 1942.

Ro-62

ex-No.73 she was renamed Ro-62 in November 1924. Ro-62 was laid down on 08 Sept. 1922, launched on 29 Sept. 1923 and completed on 24 July 1924. Upon commissioning, she joined the Maizuru Naval District, subdiv 33, Sasebo Defense Division. On 1 September 1924, she joined SubDiv 26, SubRon 1 with most of her sisters (see above for interwar records). On 15 November 1939, her unit joined SubRon 7, 4th Fleet (Combined Fleet) and when the Pacific campaign started she was in Kwajalein. On 12 December 1941 with her sisters Ro-60 and Ro-61 she was assigned to support the second attempt to invade Wake and followed her sisters on 14 December. She was surfaced, 25 nm (46 km; 29 mi) southwest of Wake to recharge her batteries in a heavy squall, predawn on 17 December when her lookouts sighted Ro-66 surfacing to do the same, on a collision course. She attempted to back off, but too late: Ro-62 rammed Ro-66 at 20:20 with shich force the latter sank with all but three hands, pressure hull cut in two, rescued when thrown overboard. She was back to Kwajalein on 28 December, reassigned to the Marshalls Area Guard Unit on 5 January 1942. On 1 February the 6th Fleet asked SubRon 1 to intercept the US carriers, but nothing was found. By March 1942, Ro-61 and Ro-62 returned to Japan via Truk and Saipan. Theyt returned to Saipan in June and Truk before being recalled to Japan, Yokosuka, on 5 July and on the 14th her unit was reassigned to the 5th Fleet, bound for the Aleutian Islands. She arrived at Paramushiro, northern Kurile Islands on 30 July, then Kiska reached on 5 August but on 7 August 1942, the task force bombardment forced her underwater. The next day she was asked to sortie and find the fleet, but it was gone.

On 28 August 1942 she was radioed by a “Jake” the location of USS Casco (AVP-12) and later was ordered to attack Atka on 29 August but Ro-62 found no targets and was back to Kiska on 5 September. On the 14th she was strafed by the 11th Air Force, but she escaped damage. In September 1942 she made two patrols off Kiska (19-21, 29-30th). On 1 October she departed for Crook Bay, northern Adak coast from 3 to 5 October 1942 and back on the 8th. She made more short sorties in October but reported an air pump failure and she had to go back to Japan on 18 October, via Paramushiro, Ōminato, and Yokosuka on 5 November, the Kure for refit and repair. SubDiv 26 was the reassigned to training at the Kure Guard Unit. But on 1 December her unit was disbanded and she was reassigned to SubDiv 33, Kure Submarine Squadron, on 10 October 1944 for training and by 12 April 1945 at the sub school at Ōtake. She was in Maizuru when hostilities ended on 15 August 1945. She departed on 21 August for Kure, and there had a skeleton crew until she was decommissioned on 28 August and surrendered to the Allies in September, struck on 20 November, scuttled at Iyo Nada (Seto Inland Sea) by May 1946.

Ro-63

ex- No.84 she was laid down on 02 April 1923, launched on 24 January 1924 and completed on 20 Dec. the same year, renamed Ro-63. She was attached to the Maizuru Naval District, SubDiv 33 and from 1 June 1925, transferred to Sasebo and SubDiv 24 until 1939. It was attached to SubRon 1, 1st Fleet from December 1925 and she was part of a cruise to Okinawa in 1926, patrolling off Shanghai, Amoy, Mako, Chusan Island, Qingdao and back to Sasebo in May 1927. See above for other details. She was decommissioned and recommissioned but while training on 13 November 1929 she collided off Sasebo with the small Japanese motor vessel Segawa Maru. She needed drydocing to repair her bow. In 1934 she was at the Sasebo Guard Squadron and then Naval District until 1936 and 1938, back to Subdiv 33 at the submarine school, Kure and then SubRon 7, 4th Fleet deployed to Kwajalein. On 8 December she was ordered a reconnaissance of Howland Island and spotted a plane 85 nm (157 km; 98 mi) northwest of Howland on 12 December, then sent a party and took photos on 14-15 December, back to Kwajalein on the 19th. On 7 January 1942 she was assigned to Truk. She made later a sortie with Ro-64 and Ro-68 to Rabaul, New Britain, Cape St. George (New Ireland) and back on 29 January.

With her sisters Ro-64, and Ro-68 sje left again on 18 February for Ponape (23-24) and the Marshall Islands but reported an horizontal rudder failure on 27 February so she was detached to Bikini Atoll for repairs with Ro-64. She was at sea on 1 March 1942, but her rudder broke again so she was back. Ro-64 proceeded independently. On 2 March she departed for Japan and permanent repairs via Ponape, Saipan, to Maizuru on 3 April. She was at sea on 5 June for Saipan and then Truk but recalled to Japan to Yokosuka on 4 July. Reassigned to the 5th Fleetshe was to take part in the Aleutian Islands campaign. She headed for Paramushiro when reporting food poisoning intoxication of her crew. She was forced back to Yokosuka via Paramushiro on 29 July. On 31 July she head for Kiska, arrived on 4 August. She crash dived on 7 August, sortied on 8-10 August byt found nil. 11-13 August with Ro-64 and Ro-68 she searched for a ditched reconnaissance plane. She then multiplied unevetful sorties in the Aleutians from Kiska in August, September, escaping another air raod on 14 September in Kiska. Eventually an 11th AF B-24 Liberator dropped bombs nearby, a shrapnel damaged one periscope while underwater. Her unit SubDiv 33 was attached to the 51st Base Unit at Kiska from 15 September and she had another patrol on 22-24 September. She departed on the 26th for Maizuru and was againat sea on 6 November for Kure, assigned to training duties later with the Kure Naval District, Kure Submarine Squadron on 1 December 1943.

On 28 March 1945 she was selected for conversion to a tender for Type D (Kōryū) midget submarines at Amami Ōshima. After the conversion at Kure with a Type 96 25 mm AA gun and radar detector, she sailed on 8 April, stopped at Sasebo, and by 10 April sailed for Amami Ōshima, rerached on the 13th to unload her cargo of torpedoes and spare parts. Air attacks forced her submerged on the harbor at Koniya by day, surfacing at night for tender duties. On 29 April she was ordered back to Sasebo with a cargo of sugar from Kasari Bay (Amami Ōshima) and arrived on 3 May, drydocked until mid-May 1945, then sent to Ōtake to resume training at the submarine school. She was caught at Maizuru on 15 August 194 and surrendered to Allied forces, struck on 20 November and scuttled at Iyo Nada by May 1946.

Ro-64

ex- No.79 she was namred than way when laid down on 15 Oct. 1923, launched 19 Aug. 1924 and completed 30 April 1925, renamed Ro-64 since November the year before. Atached to the Maizuru Naval District she joined SubDiv 33 (see above). By 1 May 1939 she was part of the Chinkai Defense Division, Chōsen, then SubDiv 24, Sasebo Naval District from July and on 15 November back to SubDiv 33, Kure, submarine school until decommissioned from 20 March 1940 to 26 July. Like her sisters she was in Kwajalein by December 1941. But on 4 December already she was sen with Ro-68 for a reconnaissance of the Phoenix Islands, Howland Island. On 5 December, same for Majuro and on the 7th they were off Phoenix Islands. She learned about the raid on Howland that day. On 10 December she had order to shell facilities on Howland. That day her CO wanted to send a landing party ashore but decided against due to possible defenders ashore sighting her, and high sea state. Her 76.2mm (3 in) deck gun barked at 02:00 JST on 11 December, destroying the wireless and weather station, barracks, even the lighthouse. Next she joined Ro-68 off Baker Island, for another bombardment at 15:00-15:20 JST and back to Kwajalein on 15 December.

She departed Kwajalein on 24 December for Wake after its fall a day before, arrived on the 27th, patrolled east of the atoll until the 30th and entered Wake port. She left on 1 January 1942 for Truk with Ro-63 and Ro-68 and led a reconnaissance of Rabaul, patrolled south of Cape St. George and back to Truk on 29 January. On 18 February 1942 via Ponape she returned to the Marshall Islands but underway her sister Ro-63 reported horizontal rudder issues. On 27 February it broke down. She had repairs at Bikini Atoll from 28 February to early March, and after a second incident Ro-64 proceeded independently to Kwajalein and then via Ponape on 19-20 March to Truk, Saipan and Maizuru on 7 April. Her unit was reassigned to the 5th Fleet for the Aleutian Islands campaign (see above details). She departed Maizuru on 6 October 1942 to Kure on 8 October and started training with SubDiv 33, part of Kure Submarine Squadron from December 1943. On 12 April 1945 she was submerged underway in Hiroshima Bay when she detonated a magnetic mine laid by a USN aicraft. The detonation reuptured her tanks and she sank quickly with all 81 men, crew and commander of SibDiv 33 plus 30 trainees. She was struck on 10 August.

Ro-65

Ro-65 ws laid down on 15 November 1924, launched on 19 September 1925 and completed on 30 June 1926. Attached to the Maizuru Naval District, SubDiv 33. On 15 December 1926, Sasebo Naval District, SubDiv 27 until 1942, reassigned to SubRon 1, 1st Fleet, Combined Fleet from 15 January 1927. Placed in reserve at Sasebo on 1 April 1931. Recommissioned and decommissioned in Second, tird and fourth Reserve in 1938. Like her sister she was in Kwajalein, Marshalls with SubDiv 27, and on 5 December she proceeded to Roiand then Wake Island, supporting Japanese forces attempting to seize the atoll. Her first war patrol ladted until 13 December and back to Kwajalein. On the 24th she was sent to the Howland Island and Phoenix Islands and back to Kwajalein on 2 January 1942. On 16 January 1942 she took part in the invasion of Rabaul on New Britain and patrolled south of Cape St. George, then with SubDiv 33 in St. George’s Channel and back to Truk on 29 January 1942. On 10 February, SubDiv 27 was disbanded and she ended in SubDiv 26 operating from Truk from February, reconnoitering the Butaritari area (Gilberts) via Ponape until 3 March. Next she headed back to Japan via Jaluit Atoll, Saipan, to Sasebo on 2 April. Next she was assigned to the Aleutian Islands, steaming there via Paramushiro in the northern Kuriles to Kiska on 26 September. US aircraft attacked and damaged her, so she was back to Ōminato for repairs and back on 28 September (minor damage to her CT in another another air raid). She later left Ro-62 for a reconnaissance of Cold Bay, Alaska and entered it on 10 October and back 5 days later.

On 17 October she rescued survivors of the destroyer Oboro which was sunk in the Bering Sea and then the crew of Hatsuharu after heavy damage in the same attack. She resumed patrol on 19 October 1942 and on 21 October received orders to conduct a reconnaissance of Holtz Bay, northeast Attu, arriving on 23 October and back to Kiska on 31 October. On 3 November she was there when her anchorage was raided by B-24 Liberator bombers from the 11th AF. She crash-dived, albeit with a main induction valve and CT hatch still open. Heavy flooding drowned 19 of her crew and she sank with a 30° down angle by the stern to the bottom. 45 survivors escaped through the torpedo tubes until the bow section lost buoyancy and she settled in the bottom. Lacking salvage vessels she stayed there, struck on 1 August 1943.

Ro-66

Ro-66 was laid down on 01 Dec. 1925, launched on 25 Oct. 1926 and completed on 28 July 1927. At commissioning she was at the Sasebo Naval District, SubDiv 27, SubRon 1, 1st Fleet (Combined Fleet) and in attached to SubRon 2, 2nd Fleet and then decommissioned and recommissioned at several occasions in the interwar.Howevber she was firmly recommissioned and versed in SubRon 7, 4th Fleet by 15 November 1940. She was based in Kwajalein at the time of the Pearl Harbor attack. On 6 Decembe she was sent for a reconnaissance of Wake Island for the planned attack on the 8th. She was still there for the second and larger invasion until ordered back to Kwajalein, albeit she stayed behind due to a radio failure (no order received) and remained off Wake Island until relieved by SubDiv 26. While surfaced 25 nm (46 km; 29 mi) southwest of Wake to recharge her batteries in a heavy squall predawn on 17 December her lookouts suddenly sighted Ro-62 for the same, arriving opposite at speed. Ro-62 rammed her at 20:20 and she sank, pressure hull ruptured and practically cut in half with the loss of 63, inc. SubDiv 27 Commander but Ro-62 rescued three survivors, thrown overboard from the bridge. She was truck on 15 January 1942.

Ro-67

Ro 67 was laid down on 05 March 1925, launched on 18 March 1926 and completed on 15 Dec. 1926, attached to the Sasebo Naval District, SubDiv 27. Same as above, she was decommissioned and recommissioned at several occasions in the interwar, making training patrols all along without notable incident. She may have been recommissioned on 1 September or 15 November 1939, 2nd Reserve and in SubRon 7, 4th Fleet by 15 November 1940. At the time of Pearl Harbor on 6 December she left Kwajalein for a reconnaissance of Wake Island. On the 8th until the 13th her patrol ended with a return to Kwajalein on 17 December after a radio recall. On 24 December she had a second patrol but engine trouble forced her back to Kwajalein. On 16 January 1942 with Ro-65 she supported the Japanese invasion of Rabaul (New Britain, Admiralty Islands), sent south of Cape St. George, New Ireland and on the 21st she joined SunDiv 33 (Ro-63, Ro-64, Ro-68) in St. George’s Channel dueing the landings at Rabaul. She was back to Truk on 29 January. On 10 February she was assigned to SubDiv 26 and departed Truk on 18 February for a recce of the Butaritari area (Gilbert Is.) via Ponape (Carolines) and afterwards, sailed on orders for Japan via Jaluit, Saipan, reaching Sasebo on 2 April. She had a refit and supplies there.

On 14 July she was reassigned to the 5th Fleet for the Aleutian Islands campaign (see above). She reached Kiska on 26 September and was strafed, had several near-misses in an air raid. Both periscopes and electric motors were damaged, so she departed to Ōminato for repairs, arrived on 4 October, departed on the 9th to Yokosuka to complete repairs. On 15 November her unit was reassigned to the Kure Submarine Squadron and she became a training submarine, reassigned to SubDiv 33 from 1 December 1943. She was caught in drydock for repairs on 19 March 1945 when an air raid of TF58, the first massive strike on Kure happens. 240 from USS Essex, Intrepid, Hornet, Wasp, Hancock, Bennington, Belleau Wood rampaged the area, sunk many ships and strafed Ro-67, killing 13 men. Damage prompted a decommission on 20 July and she was stricken. She remained used as a floating jetty at Sasebo until surrendered to the Allies after 15 August, scrapped in 1946.

Ro-68

Ro-68 was laid down on 06 February 1924, launched on 23 Feb. 1925 and completed on 29 Oct. 1925, attached to the Maizuru Naval District, SubDiv 33. On 1 December 1925 she was in SubDiv 24, SubRon 1, 1st Fleet and in March 1926 she was part of a large training cruise to Okinawa (see above) and along the coast of China, ending in Mako on 5 April, followed by a second cruise ending at Sasebo on 16 May 1927 and she was decommissioned in reserve at Maizuru. She later recommissioned and decommissioned at several occasions from the Sasebo Naval District or at Maizuru like in 1937. By 15 November 1939, she was at SubDiv 33, submarine school at Kure, then assigned to SubRon 7, 4th Fleet on 15 November 1940. She was at Kwajalein as flagship of SubDiv 33 when ordered “Climb Mount Niitaka 1208” and on 4 December 1941 was sent off Howland Island and on 10 December, Baker Island to shell a suspected US flying boat base. This was done with her deck gun from 03:45 to 04:10, destroying the wireless and weather stations, and back to Kwajalein with Ro-64 on 15 December. On the 24th she departed to patorl off Wake, until the 29th and entered its port then departed for Truk, stayed until 15 January and with Ro-63 and Ro-68, was sent to Rabaul, patrolled south of Cape St. George and back to Truk on 29 January.

Ro-63, Ro-64, and Ro-68 left Truk on 18 February via Ponape and Ro-68 arrived at Kwajalein on 11 March while Ro-63 abandoned the trip due to rudder issues, sent to Japan while Ro-64 joined Ro-68 there at Kawajalein on 12 March. On the 16th they were recalled to Japan via Ponape but Ro-64 headed for Truk and then Japan, Ro-68 stopped at Saipan and arrived at Maizuru on 3 April for a refit.
On 5 June 1942, she sailed for Saipan and Truk but was recalled to Japan, Yokosuka on 4 July and on the 14th reassigned to the 5th Fleet for the Aleutian Islands campaign. She was at Kiska on 4 August but attacked by an air raid on 7 August, crash-dived. On 8-10 August she was sent in interception, found nil. On 11-13 August she lloked for the crew of a ditched reconnaissance plane. She stayed on patrol around Kiska on 17-20 August, 28-30 August, 3-5 September, 6-11 September and still anchored on the 14th during a massive air raid by B-24 completeled by low-flying strafing from P-39 Airacobra. She had her periscopes crippled. Her final patrol was on 22-24 September south of Kiska. On the 25th she was reassigned to the Kure Naval District and arrived at Maizuru on 5 October, departed on 6 November for Kure and assigned to training duties, at the Kure Submarine Squadron from 1 December 1943. On 15 August 1944, this was the Kure Guard Force, with Ro-62 and Ro-500. On 15 May 1945, she was in SubDiv 51, SubRon 7, Maizuru Naval District. After 15 August 1945 she surrendered to the Allies, struck on 30 November 1945. She was scuttled along with Ro-500 and I-121 in Wakasa Bay, Kanmurijima, on 30 April 1946.