Built by Mitsubishi Shipbuilding in Kobe. Laid down November 16, 1937. Launched January 25, 1939. Commissioned on September 26, 1940 into the Imperial Japanese Navy (IJN) with commander Yamada Takashi in command based in the Yokosuka Naval District.
Assigned to SubRon 1's SubDiv 2, Sixth Fleet on January 31, 1941.
On November 17, 1941 at the Kure Naval Club, officers are briefed on "Operation Z" the Hawaii Operation. I-20 is assigned to the Special Attack Unit along with I-16, I-18, I-24 and flagship I-22.
On November 18, 1941 departed Kure as part of the Special Attack Unit for
Kamegakubi Naval Proving Ground where each submarines embarks a Type A midget submarine code named "Mato", I-20 embarks her first HA-20 Type A Midget Submarine (Midget A). On November 19, 1941 at 2:15am departs Kamegakubi across the Pacific Ocean passing Midway bound for Oahu. On December 2, 1941 the submarine receives the coded signal to proceed with the attack.
On December 7, 1941 at 2:57am I-20 launches HA-20 Type A Midget Submarine (Midget A) roughly 5.3 miles off the entrance to Pearl
Harbor on Oahu. Afterwards, I-20 proceeds to the recovery area south of Lanai. None of the midget submarines arrive and all are presumed lost. On December 8, 1941 in the morning departs for Kwajalein arriving with I-16 on December 20, 1942.
On January 4, 1942 departs Kwajalein to conduct a war patrol in the Samoa-Fiji area. On January 11, 1942 surfaces and shells Pago Pago.
On January 16, 1942 off Suva fires two torpedoes at HMNZS Monowai but both explode prematurely. As I-20 surfaces, she is fired on by the vessel's 6" guns but her deck gun fires and claims a hit on the bridge before the sumarine crash dives when straddled by shells. Both are undamaged,
HMNZS Monowai sends a submarine altert signal reporting the contact. On January 24, 1942 returns to Kwajalein then departs for Yokosuka arriving February 1, 1942.
16 April 1942:
The I-16 is in Captain (later Rear Admiral) Ishizaki Noboru's (former CO of HYUGA) SubRon 8, in the KO ("A") detachment with the I-10, I-18, I-20, I-30 and their support ships, the auxiliary cruisers/supply ships Aikoku Maru and the HOKOKU MARU.
Hashirajima, Hiroshima Bay. Vice Admiral Komatsu, Captain Ishizaki their staffs and midget submarine crews pay a courtesy call on the CINC, Combined Fleet, Admiral Yamamoto aboard his flagship, the new battleship YAMATO. At 1100, the A detachment departs for Penang, Occupied British Malaya.
On Aprul 27, 1942 I-20 arrives at Penang as part of "A detachment" including I-16, I-18 and I-20 with seaplane tender Nisshin transporting Type A midget submarines. At Penang, each "mother" submarines is loaded with a midget submarine, the second transported aboard I-20.
On April 30, 1942 departs Penang with "A detachment" bound for the Indian Ocean. On May 5-15, 1942 refueled at sea by Aikoku Maru and Hokoku Maru.
17 May 1942:
While proceeding surfaced, I-20's engine room is flooded in heavy seas through the main induction valve. Just after all damage is repaired, the same compartment is flooded the second time.
20 May 1942:
The I-10 catapults her Yokosuka E14Y1 "Glen" floatplane to reconnoiter Durban, South Africa.
24 May 1942:
Captain Imaizumi reports to Captain Ishizaki about the heavy traffic encountered en route to the target area.
On May 29, 1942 I-10 surfaces and her floatplane reconnoiters Diego Suarez on Madagascar. The plane sights the HMS Ramillies, an old 29,150-ton ROYAL SOVEREIGN-class battleship, at anchor in the bay. Also in the harbor are the destroyers HMS DUNCAN and ACTIVE, corvettes HMS GENISTA and THYME, troopship HMS KARANJA, hospital ship ATLANTIS, MV British Loyalty, merchant LLANDAFF CASTLE and an ammunition ship.
Captain Ishizaki orders a midget submarine attack for the next night.
On May 30, 1942 I-20 launches HA-20b Type A Midget Submarine. Aboard is commander Lt Saburo Akieda with crew member PO1C Masami Takemoto. At 10:25pm, their midget submarine fires her first torpedo at HMS Ramillies that hits and damages it. Afterwards, the midget submarine was depth charged but fires her second torpedo at MV British Loyalty and sinks her in shallow water. Ten days later, HMS Ramillies departs for Durban on Union of South Africa.
Lt Akieda and PO1C Takemoto's craft is beached. Aided by natives, both sailors reach the shore and head for the recovery area. Afterwards, I-10's floatplane again reconnoiters Diego-Suarez Bay twice to assess the results. The missing Ramillies is erroneously considered as sunk by the Japanese.
1 June 1942:
Around 1100, both Japanese are spotted off Anijabe village. The natives report their sighting to the British.
2 June 1942:
Amponkarana Bay (12-00S, 49-12E). Lt Akieda and PO1C Takemoto are intercepted by Royal Marines' Commando No. 5. During the ensuing gunfight both Japanese and one Marine are killed.
3 June 1942:
After the other "mother" submarines depart the recovery area, the I-20 surfaces and unsuccessfully tries to contact the midgets by firing flares and sending radio signals.
5 June 1942: Commerce Raiding in the Indian Ocean:
The I-20 torpedoes and sinks the 5,086-ton Panamanian-flagged merchant JOHNSTOWN at 13-12S, 42-06E.
8 June 1942:
Indian Ocean. The I-20 torpedoes and sinks the 5,209-ton Greek merchant CHRISTOS MARKETTOS at 05-05S, 40-53E.
11 June 1942:
Indian Ocean. The I-20 torpedoes and sinks the 7,926-ton British merchant MAHRONDA at 14-37S, 40-58E.
12 June 1942:
The I-20 shells and sinks the 2,052-ton Panamanian-flagged merchant HELLENIC TRADER at 14-40S, 40-53E.
Later that day, the I-20 torpedoes and sinks the 5,063-ton British merchant CLIFTON HALL at 16-25S, 40-10E.
19 June 1942:
Refuels from the AIKOKU MARU.
29 June 1942:
Indian Ocean. The I-20 torpedoes the 5,063-ton Norwegian merchant GOVIKEN that is en route from Aden to Lourenço Marques, Mozambique. She sinks within 20 minutes at 13-25S, 41-53E.
30 June 1942:
Indian Ocean. The I-20 attacks the 5,311-ton British tanker STEAUA ROMANA at 09S, 42E. First she fires 15 shells from her deck gun, achieving one hit. The STEAUA ROMANA returns fire. Yamada dives and fires a torpedo, that explodes prematurely. The tanker drops a smoke float and tries to open the range but is sunk by a second torpedo.
Later that day, using Allied radio communication intercepts, LtCdr Yamada ascertains his victim's name.
5 July 1942:
In order to discover the cause of his frequent torpedo prematures, Cdr Yamada orders one Type 95 to be dismantled aboard.
21 July 1942:
The I-20 extends her patrol to the Gulf of Aden, then returns to Penang.
5 August 1942:
Arrives at Penang.
14 August 1942:
The I-20 is in Vice Admiral Komatsu Teruhisa's Sixth Fleet (Submarines) at Truk in Subron 1 with the I-15, -16, -17, -18, -19, -21, -22, -24, -25 and the I-26.
On August 23, 1942 arrives at Yokosuka for repairs and overhaul. On September 15, 1942
assigned to commander LtCdr Yoshimura Iwao. On October 24, 1942 departs Yokosuka bound for Shortland.
On November 2, 1942 at Shortland I-20 joins I-16 and I-24 to form an attack group under the command of Captain Ota Nobunosuke that will transport a midget submarine from Chiyoda to operate off Guadalcanal. On November 5, 1942 embarks her third midget HA-11 Type A midget submarine and departs Shortland that evening bound for Guadalcanal.
On November 7, 1942 at 5:20am launches HA-11 Type A Midget Submarine four miles north of Cape Esperence off Guadalcanal. Aboard is commander Lt(jg) Nobuharu Kunihiro and and crew member PO1C Goro Inoue. Afterwards, I-20 returns to Truk.
On November 13, 1942 at Truk embarks her fourth midget HA-37 Type A Midget Submarine and departs bound for Guadalcanal. On November 18, 1942 I-20 arrives at her assigned launch area off Guadalcanal.
On November 19, 1942 at 3:00am launches HA-37 Type A Midget Submarine roughly six miles off Cape Esperence on Guadalcanal. Aboard is commander Lt(jg) Miyoshi Yoshiaki with crew member PO1C Umeda Kiyoshi.
On November 26, 1942 at Truk embarks her fifth midget HA-8 Type A Midget Submarine and departs bound for Guadalcanal. On December 1, 1942 I-20 arrives in her launch area off Guadalcanal.
On December 2, 1942 at 11:08pm launched HA-8 Type A Midget Submarine roughly 10.8 miles off Cape Esperence on Guadalcanal. The midget submarine is commanded by Lt(jg) Tanaka with crew member Mitani.
On December 18, 1942 assigned to Lt. Cdr Kudo Kaneo.
On December 31, 1942 transports 25 tons of supplies in rubber containers to Cape Esperence on Guadalcanal.
On January 1, 1943 assigned to SubDiv 1, SubRon 8 along with I-16, I-18, I-21 and I-24.
Damaged HMS Ramillies on 09-05-1942 by Kō-hyōteki.
Damaged RMS British Loyalty on 09-05-1942 by Kō-hyōteki.
Sank Panamanian merchant ship Johnstown on 05-06-1942.
Sank Greek merchant vessel Christos Markettos on 08-06-1942.
Sank RMS Mahronda 11-06-1942.
Sank Panamanian merchantman Hellenic Trader on 12-06-1942.
Sank RMS Clifton Hall on 12-06-1942.
Sank Norwegian merchant ship Goviken on 30-06-1942.
Sank RMS Steaua Romana on 30-06-1942.
Damaged unknown merchant ship on 03-12-1942 by Kō-hyōteki.
On November 18, 1943 presumed lost operating off Espiritu Santo. On December 1, 1943 officially removed from the Navy List.
Fates of the Crew
The entire crew is presumed lost with the submarine.
Other sources state this submarine was sunk by USS Ellet off Vella Lavella on October 1, 1943.
Combined Fleet - HIJMS Submarine I-20: Tabular Record of Movement
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February 3, 2017