I-121, laid down in 1924 as Submarine No. 48 and known as I-21 from November 1924 to June 1938, was an of the Imperial Japanese Navy that served during the Second Sino-Japanese War and World War II. During the latter conflict, she conducted operations in support of the Japanese invasion of Malaya, the Japanese invasion of the Philippines, the bombing of Darwin, the Battle of Midway, the Guadalcanal campaign, the Battle of the Eastern Solomons, and the New Guinea campaign. She surrendered at the end of the war in 1945 and was scuttled in 1946.
After she was renumbered I-121 in 1938, the number I-21 was assigned to a later submarine which also served during World War II.
Design
I-121 and her three sister ships — I-22 (later renumbered ), I-23 (later renumbered ), and I-24 (later renumbered ) — were the Imperial Japanese Navy's only submarine minelayers.Boyd and Yoshida, p. 18. They were known in Japan by the type name , commonly shortened to .
The Kiraisen-type design was based on that of the Imperial German Navy minelaying submarine , a Type UB III submarine which was the largest of seven German submarines transferred to Japan as a war reparation after World War I and served in the Imperial Japanese Navy as O-6 from 1920 to 1921. Like UB-125, the Kiraisen-type submarines had two diesel engines producing a combined , could carry 42 mines, and had four torpedo tubes and a single deck gun — a gun on the Japanese submarines in contrast to a gun on UB-125. Compared to the German submarine, they were larger — longer, and displacing 220 more tons on the surface and 300 more tons submerged — and had a longer range both on the surface — farther at — and submerged — farther at . They were slower than UB-125 both surfaced and submerged, carried two fewer torpedoes, and could dive to only compared to for UB-125.
Construction and commissioning Built by Kawasaki at Kobe, Japan, I-121 was laid down on 20 October 1924 with the name Submarine No. 48. While she was on the building ways, she was renamed I-21 on 1 November 1924. She was launched on 30 March 1926 and was completed and commissioned on 31 March 1927.
Service history 1927–1937 Upon commissioning, I-21 was assigned to either the Kure Naval District or the Yokosuka Naval District, according to different sources. On 1 November 1927, she was assigned to Submarine Division 9 in the Yokosuka Defense Division in the Yokosuka Naval District. On 9 March 1928 she was conducting speed trials off Yokosuka Bay when she collided with the destroyer , which was underway to conduct torpedo launch trials. There were no casualties aboard either ship, but both vessels suffered light damage. I-21′s bow was bent 60 degrees to starboard. She proceeded to Yokosuka, where she underwent repairs.
I-21′s tour of duty in Submarine Division 9 came to an end on 1 December 1930, when she was assigned directly to the Kure Naval District. She was reassigned to Submarine Division 13 in the Kure Naval District on 15 October 1931, and on 1 October 1932 her division became part of the Kure Defense Division in the district.
On 15 March 1933, I-21 was decommissioned and placed in reserve. On 15 November 1933, Submarine Division 13's assignment to the Kure Defense Division ended and it was reassigned directly to the Kure Naval District, and that day I-21 was recommissioned and returned to active service in the division. After her sister ship I-24 suffered damage to her main ballast tanks while I-23 and I-24 were conducting deep diving trials on 25 May 1935, Submarine Division 13 was assigned to the Kure Defense Squadron in the Kure Naval District on 15 November 1935, and that day I-21 was decommissioned so that her main ballast tanks could be reinforced. In 1936 all four submarines of her class had their designed diving depth limited to . With the ballast tank work completed, I-21 was recommissioned on 20 March 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 Division 13, consisting of I-21 and her sister ship I-22, moved to a base at Qingdao, China, and began operations in northern Chinese waters as part of a Japanese blockade of China.Boyd and Yoshida, p. 54. On 1 December 1937, Submarine Division 13 was assigned to Submarine Squadron 3 in the 4th Fleet, a component of the Combined Fleet, and in December 1937 the light cruiser arrived at Qingdao to serve as flagship of Submarine Squadron 3, which consisted of I-21, I-22, I-23, and I-24.
I-21 and I-22 received orders to provide distant cover for Kuma while she put a Special Naval Landing Force (SNLF) unit ashore off Chefoo, China, on 3 February 1938, but both submarines experienced engine trouble that prevented them from getting underway to support Kuma. The two submarines had greater success on 22 March 1938, when they departed Port Arthur, Manchukuo, and each of them landed 15 Shanghai-based SNLF troops on Liukung Island in Weihai Bay before they returned to Port Arthur.
I-21 was renumbered I-121 on 1 June 1938, freeing up her previous number for the new submarine , whose keel was laid that year. On 20 June 1938, I-121 was placed in the Second Reserve in the Kure Naval District. In an effort to reduce international tensions over the conflict in China, Japan withdrew its submarines from Chinese waters in December 1938.
1938–1941 While in reserve, I-121 and all three of her sister ships — which, like her, had been renumbered on 1 June 1938, I-22 becoming I-122, I-23 becoming I-123, and I-24 becoming I-124 — underwent conversion into submarine tankers. Retaining their minelaying and torpedo capabilities, they were modified so that each of them could carry 15 tons of aviation gasoline with which to refuel flying boats, allowing the flying boats to extend their range during reconnaissance and bombing missions by meeting the submarines in harbors and lagoons for more fuel.
On 1 May 1940, I-121 returned to active service in Submarine Division 13, which was assigned that day to Submarine Squadron 5 in the 4th Fleet. She soon began a lengthy training cruise in the Pacific in company with I-121, I-123, and I-124: The four submarines departed Sasebo, Japan, on 16 May 1940 and visited the waters of the Caroline Islands, Marshall Islands, and Mariana Islands before concluding their cruise with their arrival at Yokosuka, Japan, on 22 September 1940. On 11 October 1940, I-121 was one of 98 Imperial Japanese Navy ships that gathered along with more than 500 aircraft on the Japanese coast at Yokohama Bay for an Imperial fleet review — the largest fleet review in Japanese history — in honor of the 2,600th anniversary of the enthronement of the Emperor Jimmu, Japan's legendary first emperor.
Submarine Division 13 was reassigned directly to the Kure Naval District on 15 November 1940, and from 30 January to 4 February 1941, I-121 temporarily substituted for I-122 as flagship of the division. On 1 May 1941, Submarine Division 13 was assigned to Submarine Squadron 6 in the 3rd Fleet, a component of the Combined Fleet.
As the Imperial Japanese Navy began to deploy in preparation for the impending conflict in the Pacific, I-121 departed Yokosuka in November 1941 bound for Samah on Hainan Island in China. She arrived at Samah on 27 November 1941. On 1 December 1941, she departed Samah to begin what would become her first war patrol. She 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 (7 December 1941 on the other side of the International Date Line in Hawaii, where the war would begin with Japan's attack on Pearl Harbor). On 7 December 1941, the last day in East Asia before the attack, I-121 laid forty-two Type 88 Mark 1 mines northeast of Singapore in the vicinity of . She then took up her patrol station off the eastern entrance of Johore Strait.
World War II First war patrol
In East Asia, the Pacific campaign of World War II began on 8 December 1941, and Japanese forces invaded British Malaya to the north of Singapore that day. Reassigned to the Southern Submarine Force, I-121 called at Cam Ranh Bay in Japanese-occupied French Indochina on 12 December 1941, then got back underway either later the same day or on 18 December 1941 (according to different sources) to support the Japanese invasion of the Philippines, bound for the vicinity of Davao on Mindanao. Patrolling in the Sulu Sea, she supported the Japanese landings at Davao on 19–20 December 1941 and at Jolo on 24 December 1941. Reassigned to Submarine Group "A" along with I-122, I-123, and I-124 on 26 December 1941, she concluded her patrol on either 27 or 31 December 1941 (according to different sources) with her arrival at newly captured Davao.
Second war patrol On 5 January 1942, I-121 set out from Davao to begin her second war patrol, tasked with laying mines at the entrance to Port Darwin, Australia, which the United States Asiatic Fleet had made its main logistics base after retreating from its bases in the Philippines. At 05:30 on 11 January 1942, she unsuccessfully attacked an unidentified Allied destroyer in the Timor Sea off the coast of Australia's Northern Territory southwest of the Tiwi Islands. By 04:55 on 12 January 1942, she had laid 39 mines in the Clarence Strait between Melville Island and the mainland of Australia, after which she returned to her patrol area in the Timor Sea. She pursued an unidentified Allied merchant ship escorted by a patrol vessel in the Timor Sea west of Wetar on 18 January 1942 and fired three torpedoes at it. She then returned to the Clarence Strait, where on 21 January 1942 three Allied patrol vessels attacked her, dropped 42 depth charges, and damaged two of her fuel tanks.
Not knowing that Royal Australian Navy ships had sunk I-124 on 20 January 1942, I-121 departed her patrol area on 25 January 1942 to rendezvous with I-124. She reached the rendezvous point on 28 January 1942 at 04:00 and loitered in the area until 23:00 awaiting I-124. When I-124 failed to arrive, I-121 set a course for Davao, where she and I-122 arrived on 30 January 1942. At Davao, she took on fuel, ammunition, and supplies from the submarine tender and had her damaged fuel tanks repaired.
Third war patrol In company with I-122, I-121 departed Davao on 9 February 1942 to begin her third war patrol, with a primary mission of supporting an impending air attack on Port Darwin by planes from the aircraft carriers of Vice Admiral Chuichi Nagumo′s Mobile Force and land-based aircraft from both Ambon and Kendari, Celebes. Her specific tasks included a reconnaissance of Port Darwin prior to the attack and the transmission from the Arafura Sea off northern Australia of weather reports to Nagumo's ships, which would be approaching from Palau. After patrolling northeast of Morotai, she parted company with I-122 — which independently headed southeast — on 13 February 1942 and proceeded south on her own. By sunset on 16 February 1942, I-121 had reached her patrol area northwest of Darwin, Australia.
On 18 February 1942, I-121 surfaced before sunset to recharge her batteries, but had to crash-dive when an Australian patrol plane strafed her. After dark, she surfaced again to finish recharging her batteries, and at 20:30 sent a weather report to Nagumo's ships, which were underway from Palau. She sent Nagumo another weather report at 02:30 on 19 February 1942. At 09:57 that morning, 188 aircraft from Nagumo's carriers began an attack on Darwin's harbor, airfields, and urban center, followed by a raid by 55 Japanese land-based bombers. The attack sank eight ships, damaged nine ships, destroyed 15 aircraft, and damaged structures in the city itself.