was the third Type C cruiser submarines built for the Imperial Japanese Navy. During World War II, she operated as the mother ship for a midget submarine during the attack on Pearl Harbor and the attack of Diego-Suarez, conducted war patrols in the Pacific Ocean and Indian Ocean, and served in the Guadalcanal campaign and New Guinea campaign. She was last heard from on 31 August 1943.
Design and description The Type C submarines were derived from the earlier Kaidai-type VI with a heavier torpedo armament for long-range attacks. They displaced surfaced and submerged. The submarines were long, had a beam of and a draft of . They had a diving depth of .Bagnasco, p. 192
For surface running, the boats were powered by two diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a electric motor. They could reach on the surface and underwater.Chesneau, p. 201 On the surface, the C1s had a range of at ; submerged, they had a range of at .
The boats were armed with eight internal bow torpedo tubes and carried a total of 20 torpedoes. They were also armed with a single /40 deck gun and two single or twin mounts for Type 96 anti-aircraft guns. They were equipped to carry one Type A midget submarine aft of the conning tower.Carpenter & Dorr, p. 104
Construction and commissioning
Ordered under the 3rd Naval Armaments Supplement Programme and built by Mitsubishi at Kobe, Japan, I-20 was laid down on 16 November 1937 with the name Submarine No. 46. Renamed I-20 by the time she was launched on 25 January 1939, she was completed and commissioned on 26 September 1940.
Service history Pre-World War II Upon commissioning, I-20 was attached to the Yokosuka Naval District. During the autumn of 1940, she took part in Japanese tests of the German-made Atlas Werke Periphon A passive sonar. She was assigned to Submarine Division 2 in Submarine Squadron 1 in the 6th Fleet along with the submarines and . In the autumn of 1941, she underwent conversion into a mother ship for a Type A midget submarine. The submarines , , , and also underwent the conversion.
At the Kure Navy Club in Kure, Japan, on 17 November 1941, the commander of Submarine Division 3 briefed the commanding officers of the five converted submarines on the upcoming attack on Pearl Harbor and on the role of their submarines in it. He had been designated the commander of the Special Attack Unit, made up of all five submarines, each of which was to launch a Type A midget submarine off Pearl Harbor so that the midget submarines could participate in the attack. I-22 was to serve as flagship of the Special Attack unit.
On 18 November 1941, the five submarines moved from Kure to the Kamegakubi Naval Proving Ground, where each embarked a Type A midget submarine. At 02:15 on 19 November 1941, the five submarines got underway from Kamegakubi bound for the Hawaiian Islands, taking a direct route that took them south of Midway Atoll. While at sea, they received the message "Climb Mount Niitaka 1208" () from the Combined Fleet on 2 December 1941, indicating that war with the Allies would commence on 8 December 1941 Japan time, which was on 7 December 1941 on the other side of the International Date Line in Hawaii.
World War II Pearl Harbor At 02:57 on 7 December 1941, I-20 launched her midget submarine, No. 17, south of Oahu from the entrance to Pearl Harbor. After the United States Navy coastal minesweeper reported an unidentified submarine in a submarine exclusion zone off Pearl Harbor, the destroyer began a search for the submarine at 04:08, finding nothing. At 06:30, however, Ward sighted No. 17′s conning tower in the wake of the general stores ship , which was approaching the harbor′s outer gate with a target barge in tow. A PBY Catalina flying boat of Patrol Squadron 14 (VP-14) dropped smoke markers to indicate the midget submarine′s position. Ward opened gunfire on the submarine at 06:45 at a range of only , firing the first shot of World War II by the American armed forces. She closed the range to and hit No. 17′s conning tower with a shell. Ward steamed past the midget submarine and dropped four depth charges as it wallowed in her wake, and the PBY then dropped more depth charges. No. 17 sank outside the harbor′s entrance with the loss of her two-man crew. On 28 August 2002, the Hawaii Undersea Research Laboratory deep submergence vehicles Pisces IV and Pisces V found a midget submarine resting almost upright on the bottom in of water off the entrance to Pearl Harbor, prompting speculation among historians and maritime archaeologists that it was I-20′s midget.
I-20 and the other four "mother" submarines proceeded to the planned recovery area for their midget submarines west of Lanai, where they spent the night of 7–8 December 1941. None of the midget submarines returned. Early on 9 December 1941, I-18, I-20, and I-24 received orders to leave the recovery area. I-20 departed the Hawaiian Islands on 12 December 1941 and arrived at Kwajalein with I-16 on 22 December 1941.
First war patrol
On 4 January 1942, I-20 departed Kwajalein to begin her first war patrol, assigned a patrol area in the vicinity of Fiji and the Samoan Islands. She surfaced off the harbor at Pago-Pago on Tutuila in American Samoa before dawn on 11 January 1942 and fired twelve rounds from her deck gun at the naval station. Most of her shells missed, and the only casualties were a United States Marine Corps officer and a Samoan Marine Reserve Battalion member who suffered wounds.
Moving to Fijian waters, I-20 attacked the Royal New Zealand Navy armed merchant cruiser on 16 January 1942 just after Monowai departed the harbor at Suva. Her torpedoes exploded prematurely at 16:03. Believing that she was attacking a merchant ship, I-20 surfaced from Monowai at 16:08 to attack her with gunfire. Monowai opened a heavy volume of fire with her portside guns. I-20 traded fire with Monowai, claiming a hit on Monowai′s bridge, but submerged at 16:14 after Monowai straddled her. Neither vessel suffered damage, and Monowai transmitted a submarine contact signal and steamed out of the area at high speed. I-20 returned to Kwajalein on 24 January 1942, then proceeded to Japan, where she arrived at Yokosuka on 1 February 1942.
February–April 1942
During I-20′s stay in Japan, the German naval staff in Berlin formally requested on 27 March 1942 that Japan begin attacks on Allied convoys in the Indian Ocean. On 8 April 1942, the Japanese formally agreed to meet this request by dispatching submarines to operate off the coast of East Africa, and that day they withdrew Submarine Division 1 of Submarine Squadron 8 from its base at Kwajalein to Japan. By 16 April 1942 they had created the "A" detachment within Submarine Squadron 8, consisting of I-20 and the submarines , I-16, I-18, and , as well as midget submarines and the auxiliary cruisers and , which were to operate as supply ships for the submarines. That morning, the commander of the 6th Fleet, Vice Admiral Teruhisa Komatsu, the commander of Submarine Squadron 8, their staffs, and the midget submarine crews paid a courtesy call on the commander-in-chief of the Combined Fleet, Admiral Isoroku Yamamoto, aboard his flagship, the battleship , at Hashirajima anchorage. After the visit with Yamamoto, the detachment got underway at 11:00, bound for Penang in Japanese-occupied British Malaya.
During the detachment′s voyage, 16 United States Army Air Forces B-25 Mitchell bombers launched by the aircraft carrier struck targets on Honshu in the Doolittle Raid on 18 April 1942. The detachment received orders from the 6th Fleet that day to divert from its voyage and head northeast, passing north of the Bonin Islands, to intercept the U.S. Navy task force that had launched the strike. The detachment failed to find the U.S. ships and soon resumed its voyage.
I-30 and Aikoku Maru called at Penang from 20 April to 22 April 1942 before heading into the Indian Ocean to conduct an advance reconnaissance of the "A" Detachment′s planned operating area. The rest of the "A" Detachment reached Penang on 27 April 1942, where the seaplane carrier — which had undergone modifications allowing her to carry Type A midget submarines — rendezvoused with it. I-16, I-18, and I-20 each embarked a midget submarine at Penang.
Indian Ocean operation
I-20 and the other "A" detachment units got underway from Penang on 30 April 1942, headed westward into the Indian Ocean with I-10 serving as the detachment′s flagship. The submarines refueled at sea from Aikoku Maru and Hōkoku Maru on 5, 10, and 15 May 1942. I-20 suffered a mishap on 17 May when seawater entered through her main induction valve and flooded her engine room in heavy seas. Her crew repaired the damage, only to have the engine room flood a second time, necessitating further repairs.
I-10′s Yokosuka E14Y1 (Allied reporting name "Glen") floatplane began reconnaissance flights over ports in South Africa by reconnoitering Durban on 20 May 1942, followed by flights over East London, Port Elizabeth, and Simon's Town over the next week, and by 24 May the "A" detachment submarines were encountering heavy Allied shipping traffic as they approached East Africa. On the night of 29 May, I-10′s floatplane flew over Diego-Suarez, Madagascar, sighting the British battleship among the ships anchored there. The "A" detachment commander selected Diego-Suarez as the target for a midget submarine attack, scheduled for 30 May 1942.
On 30 May 1942, I-18′s midget submarine suffered engine failure and could not launch, but I-16 and I-20 launched their midget submarines off Diego-Suarez. I-20′s midget torpedoed Ramillies at 20:25. At 21:20, while British corvettes dropped depth charges, I-20′s midget torpedoed and sank the 6,993-ton tanker British Loyalty in shallow water. After the midget ran aground, its two-man crew reached shore and attempted to make their way overland to the designated recovery area, but they were reported to the British as having been seen outside Anjiabe at around 11:00 on 1 June and died in a gunfight with Royal Marine Commando No. 5 on 2 June 1942 at Amponkarana Bay (). One Royal Marine also died in the exchange of gunfire. Ramillies survived the attack and departed for Durban ten days later, although the Japanese assessed her as sunk after I-10′s floatplane noted her missing from Diego-Saurez during a reconnaissance flight. British Loyalty later was refloated, towed to Addu Atoll, and scuttled.
On 3 June 1942, after I-16 and I-18 had departed the recovery area, I-20 arrived and made an unsuccessful attempt to contact the midget submarines and their crews. She then joined the rest of the "A" detachment in anti-shipping operations. She torpedoed and sank the Panamanian 5,086-ton armed merchant ship Johnstown at on 5 June, the Greek 5,209-ton merchant ship Christos Markettos at on 8 June, and the British 7,926-ton armed merchant ship Mahronda at on 11 June 1942. On 12 June, she sank the Panamanian 2,052-ton merchant ship Hellenic Trader at with gunfire and torpedoed and sank the British 5,063-ton merchant ship Clifton Hall at .
After refueling from Aikoku Maru on 19 June 1942, I-20 torpedoed the Norwegian 5,063-ton armed merchant ship Goviken, which was on a voyage from Aden to Lourenço Marques in Portuguese East Africa, on 29 June 1942. Goviken sank less than 20 minutes later at . On 30 June, I-20 attacked the British 5,311-ton armed steam tanker with gunfire at . She fired fifteen rounds and scored one hit. After Steaua Romana returned fire, I-20 submerged and fired a torpedo, but it exploded prematurely. Steaua Romana dropped a smoke float in attempt to conceal herself and tried to escape, but I-20 fired a second torpedo which sank her. That evening, I-20 was able to identify her victim as Steaua Romana by intercepting Allied radio communications.