I-6 was an Imperial Japanese Navy J2 type submarine commissioned in 1935. She was a large cruiser submarine that served in the Second Sino-Japanese War and World War II. During the latter conflict she operated in support of the attack on Pearl Harbor, torpedoed the aircraft carrier , conducted anti-shipping patrols in the Indian Ocean and South Pacific Ocean, and took part in the Aleutian Islands campaign and New Guinea campaign before she was sunk in June 1944.
Design I-6 was the only Junsen II- (or "J2"-) type submarine. After the four Junsen I-type submarines (, , , and ), the Japanese had built as a modified Junsen I, introducing an aviation capability to the Junsen type with the inclusion of a hangar that allowed I-5 to carry and operate a floatplane. I-6 represented the next step in the evolution of this aviation capability, as she had both a hangar and a catapult for a floatplane. The next and last Junsen-type submarines, and , the only Junsen III-type submarines, also each had a hangar and catapult for a floatplane.
Construction and commissioning Built by Kawasaki at Kobe, Japan, I-6 was laid down on 14 October 1932. She was launched on 31 March 1934 and completed and commissioned on 15 May 1935.
Service history 1935–1937 Upon commissioning, I-6 was attached to the Yokosuka Naval District. On 1 June 1935, she was assigned to Submarine Division 8 in Submarine Squadron 1 in the 1st Fleet, a component of the Combined Fleet. She was taking part in maneuvers off Ise Bay at 14:27 on 1 August 1935 when she collided with the destroyer . She suffered damage to her periscopes and proceeded to Yokosuka, Japan, for repairs. She was reassigned directly to the Yokosuka Naval District on 15 November 1935. In July 1936, she embarked a Watanabe E9W1 (Allied reporting name "Slim") reconnaissance seaplane for testing purposes.
On 1 December 1936, I-6 returned to duty in Submarine Division 8 in Submarine Squadron 1 in the 1st Fleet. On 27 March 1937, she departed Sasebo, Japan, in company with I-1, I-2, I-3, I-4, and I-5 for training in the vicinity of Qingdao, China. The six submarines concluded the training cruise with their arrival at Ariake Bay on 6 April 1937.
Second Sino-Japanese War On 7 July 1937 the first day of the Marco Polo Bridge Incident took place, beginning the Second Sino-Japanese War. In September 1937 Submarine Squadron 1 was reassigned to the 3rd Fleet,Boyd and Yoshida, p. 54. which in turn was subordinated to the China Area Fleet for service in Chinese waters. The squadron, consisting of I-1, I-2, I-3, I-4, I-5 and I-6, deployed to a base at Hong Kong with the submarine tenders and in September 1937. From Hong Kong, the submarines began operations in support of a Japanese blockade of China and patrols of China's central and southern coast. From 20 or 21 (sources disagree) to 23 August 1937, all six submarines of Submarine Squadron 1 operated in the East China Sea as distant cover for an operation in which the battleships , , , and and the light cruiser ferried troops from Tadotsu, Japan, to Shanghai, China.
Submarine Squadron 1 was based at Hong Kong until the autumn of 1938. In an effort to reduce international tensions over the conflict in China, Japan withdrew its submarines from Chinese waters in December 1938.
1938–1941
Remaining a unit of Submarine Division 8, I-6 was placed in Second Reserve in the Yokosuka Naval District on 15 December 1938, then on 1 April 1939 began an assignment at the Torpedo School in the Yokosuka Naval District at Yokosuka. In mid-1940, her hangar and catapult were removed, and thereafter she did not operate aircraft. Submarine Squadron 2, including I-6, was resubordinated to the 6th Fleet on 15 November 1940.
On 10 November 1941 — by which time I-4, I-5, I-6, and the submarine made up Submarine Division 8, with I-7 serving as squadron flagship — the commander-in-chief of the 6th Fleet, Vice Admiral Mitsumi Shimizu, gathered the commanding officers of the fleet's submarines together for a meeting aboard his flagship, the light cruiser , which was anchored in Saeki Bay. His chief of staff briefed them on the upcoming attack on Pearl Harbor, which would bring Japan and the United States into World War II. As the Imperial Japanese Navy began to deploy for the upcoming conflict in the Pacific, I-4, I-5, I-6, and I-7 got underway from Yokosuka at 13:00 on 16 November 1941, bound for the Hawaiian Islands. The submarines 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 First war patrol On 7 December 1941, the submarines of Submarine Squadron 2 took up patrol stations across a stretch of the Pacific Ocean from northeast to northwest of Oahu. The submarines had orders to conduct reconnaissance in the area and attack any ships which sortied from Pearl Harbor during or after the attack, which occurred that morning. I-6′s patrol area was between those of I-4 and I-5 and was located off the northern entrance of Kaiwi Channel between Molokai and Oahu. At 08:40 on 9 December 1941, I-6 sighted the United States Navy aircraft carrier — which she misidentified as a — and two heavy cruisers north of Molokai steaming northeast at . She attempted to attack Enterprise, but was forced to go deep before she could. Several hours later she managed to transmit a sighting report, which resulted in the Japanese ordering nine submarines to attempt to intercept Enterprise, which they assumed was bound for the United States West Coast. Out of position to participate in the pursuit of Enterprise, I-6 was ordered to take up a new patrol area south of Oahu in waters between the Kauai Channel and an area southeast of Pearl Harbor. Later in December, one of her torpedomen was injured during a routine torpedo inspection; he died several days later of sepsis on 27 December 1941 and was buried at sea.
On 9 January 1942, the submarine reported sighting a Lexington-class aircraft carrier northeast of Johnston Island, and the 6th Fleet ordered a number of submarines including I-1 to form a picket line northeast of Johnston Island to find the carrier. When I-1 developed engine trouble, I-6 received orders to replace her in the picket line. During the daylight hours of 10 January 1942, lookouts sighted planes from the carrier on five occasions, and her navigator plotted their courses and used the information to estimate the carrier's position. While on the surface northeast of Johnston Island at 18:41 on 11 January 1942, she sighted a U.S. Navy destroyer and submerged. Not long afterwards, the aircraft hove into view at , steaming southeast at and accompanied by a heavy cruiser and another destroyer. I-6 fired three Type 89 torpedoes at Saratoga at a range of , and at 19:15 one of them hit Saratoga amidships on her port side, killing six firemen and flooding three boiler rooms. Taking on 1,100 tons of water, Saratoga heeled to starboard and then to port and lost headway. I-6 went to a depth of , and when the escorting destroyers began a counterattack at 19:58, they could not locate her. Her sound operator reported hearing two large explosions followed by a number of smaller ones which suggested that Saratoga had sunk and was breaking up on her way to the bottom, and after 22:00 I-6 transmitted a report claiming two hits on, and the probable sinking of, a Lexington-class aircraft carrier. In fact, Saratoga survived, but the damage I-6 inflicted kept her out of combat for the next six months.
On 12 January 1942, I-6 departed her patrol area bound for Kwajalein, which she reached on 22 January 1942 with only of fuel left. She got back underway on 24 January 1942 bound for Yokosuka, which she reached on 2 February 1942. She was drydocked in early February 1942 and began a refit and overhaul.
Second war patrol While I-6 was at Yokosuka, Submarine Squadron 2 — consisting of I-1, I-2, I-3, I-4, I-6, and the squadron flagship, I-7 — was assigned to the Dutch East Indies Invasion Force in the Southeast Area Force on 8 February 1942. On 13 February, the staff of Submarine Division 8 transferred from I-6 to I-4, and I-6 departed Yokosuka on 14 February 1942 bound for the Netherlands East Indies. She arrived at Staring Bay on the Southeast Peninsula of Celebes just southeast of Kendari on 22 February 1942. At 07:00 on 23 February 1942, she got back underway in company with I-4 and I-5 to begin her second war patrol, bound for a patrol area in the Indian Ocean west of Sumatra. While I-5 and I-6 were on the surface west of Timor en route their patrol areas on 25 February 1942, an Imperial Japanese Navy Mitsubishi C5M (Allied reporting name "Babs") reconnaissance aircraft escorted by nine Mitsubishi A6M Zero (Allied reporting name "Zeke") fighters sighted the vessels at 12:30 Japan Standard Time and misidentified them as Royal Netherlands Navy submarines. The Zeroes made repeated strafing attacks that forced I-6 to submerge, but she suffered no damage. Her patrol otherwise was uneventful, and she concluded it with her arrival at Penang in Japanese-occupied British Malaya on 8 March 1942.
Third war patrol Orders arrived from the headquarters of the Combined Fleet for all the submarines of Submarine Squadron 2 except for I-1 to conduct reconnaissance operations along the coast of Ceylon and western coast of India in preparation for Operation C, the upcoming Indian Ocean raid by the aircraft carriers of the Combined Fleet's Mobile Force. Accordingly, I-6 departed Penang on 26 March 1942 to begin her third war patrol, assigned a patrol area in the Indian Ocean west of Bombay, India, and north of the Maldive Islands. On 27 March, the German naval staff in Berlin asked that Japan begin anti-shipping operations against Allied convoys in the Indian Ocean.
At 16:35 on 31 March 1942, I-6 was in the Indian Ocean off Eight Degree Channel when she sighted an Allied steamer. She commenced an approach and was about to fire torpedoes when she identified the ship as a hospital ship — probably the British hospital ship on a voyage from Addu Atoll to the Ceylon area — and called off the attack. On the afternoon of 2 April 1942, I-6 was in the Arabian Sea southwest of Bombay when she sighted the British 5,897-ton steamer , which was on her way from Liverpool, England, to Cochin, India, carrying 3,655 tons of general cargo and 1,027 tons of explosives. She fired two torpedoes at a range of , scoring one hit amidships on Clan Ross′s port side. Clan Ross sank by the stern at 14:14 at with the loss of 11 crew members killed and three injured. I-6 surfaced and approached the survivors. I-6′s crew interrogated the survivors, provided them with fresh water and biscuits, and gave them the bearing to Bombay, then lined up on I-6′s afterdeck, saluted the survivors, and wished them "bon voyage" in broken French.
While on the surface at around 16:00 on 7 April 1942, I-6 encountered the British 5,424-ton merchant ship Bahadur — bound from Bombay to Basra, Iraq, with a cargo of 5,100 tons of government stores and ammunition — in the Arabian Sea northwest of Bombay. I-6 submerged and fired torpedoes, but Bahadur′s crew spotted them and Bahadur made a hard turn to starboard, evaded the torpedoes, and attempted to open the range at top speed. I-6 fired two torpedoes from her stern torpedo tubes, but they also missed. She then surfaced and pursued Bahadur, opening fire with her deck gun at a range of . The gun jammed after firing only one round, and I-6 submerged and gave up the chase. Bahadur suddenly stopped, however, and lowered her lifeboats. I-6 closed and fired two more torpedoes from a position on Bahadur′s port beam. Bahadur sank by the stern at 19:20 at .