The Supersubs of Sasebo
A Short History of the I-400-class Submarines Made in Sasebo, Japan
伊400型 ― 佐世保のスーパー潜水艦
The I-400-class submarines were huge. Construction started in January 1943, and up to that time they were the largest subs ever made in the world... and probably the deadliest. They were called Sensui Kubo -- submarine aircraft carriers -- able to hold three top-secret folding-wing attack bombers, the Aichi M6A Seiran.
Out of a total of eighteen super-subs planned, only five made it into the construction stage, and only four into actual operation. The I-400 (Dec. 1944) and I-404 (July 1944) were built in Kure (near Hiroshima), and the I-405 in Kawasaki, though cancelled during construction. Two of the super-subs were constructed in Sasebo -- the I-401 completed in Jan. 1945 and the I-402 (designed to be a tanker) in July of the same year. Able to cross the Pacific Ocean four times without refueling, armed with five deck guns, fire state-of-the-art torpedoes from eight tubes, and having special radar systems and sound-dampening hull coatings, these subs would be an awesome force facing the Allies.
The super-subs were planned by the same man who planned the Pearl Harbor attack -- Isoroku Yamamoto -- who wanted them to stealthily attack coastal cities in the US, e.g. Los Angeles and New York. As the war situation worsened for Japan, planning became even more ominous -- bombing Panama Canal and dropping infected fleas and rats on West Coast cities. Providentially, those ideas never came to fruition. Nor was Yamamoto ever to see the fruition of his own dream super-submarines -- he was killed during a tour of the South Pacific when his plane was shot down over Bougainville Island on April 18, 1943.
What really made the I-400's formidable was their ability to carry three attack aircraft in a huge cylindrical room in the middle of the deck, the conning tower offset to accomodate the storage area for these planes. The subs could stealthily go anywhere and their planes attack any Allied port city in the world. The special aircraft were called Seiran -- out of the clear blue sky, a storm. Indeed, it was out of the blue, for none of the Allied nations knew about the existence of this top-secret attack bomber.
A total of 28 of these special bombers were produced, though none were actually used for what they were designed -- to attack Panama Canal and demolish the first locks, the Gatun, on the Atlantic side. The special "kamikaze" attack squadron (consisting of the I-400 and I-401) was called Dragon God. Their mission was to head to the Panama Canal and take out the Gatun Locks. The captain of the I-401 was Lt. Com. Nobukiyo Nambu, having arrived earlier in Sasebo on November 1, 1944. He was later joined on the sub by the Commander of First Submarine Fleet, Tatsunosuke Ariizumi.
M6A1 Seiran 晴嵐
US military intelligence got word of this new plane sometime
in the fall of 1944. See this report of Nov. 3, 1944.
Their mission, however, was changed, due to the loss of Okinawa. They were to head to the South Pacific to attack US naval forces staging in Ulithi Atoll for their assault on mainland Japan. However, the war ended while the squadron was enroute, and all armament and planes were thrown into the sea. And for good reason -- the bombers were painted silver with American naval star insignias on them, a clear violation of international law. Interestingly, one of the mechanics on board the I-401 felt it was shameful and just before disposing of the aircraft he was in charge of, painted the red sun over each of the white stars.
But the end was not yet in sight for the I-400's. They were to become centers of attention for the American military. The I-400 and I-401 were captured at the end of the war and ordered to sail to Yokosuka (see this archival document recording the capture of the I-14, I-401 and I-402). They then went to Sasebo in Nov. 1945 to load up supplies, and then sailed for Hawaii a month later for intensive study. In late May and early June 1946, the two I-400's finally met their doom when they were scuttled at sea near Hawaii, being used as target practice (wreckage discovered in 2013). Only the I-402 remained in Japan, having been commandeered in Kure by US Forces where she was in port at the time, then later taken to Sasebo along with a number of other submarines. All these subs, including the I-402, were scuttled on April 1, 1946, in an operation known as "Roads End" (archival video links below). The final resting place of the Sasebo Supersub I-402 is now at the bottom of the sea between southern Goto Island and Nagasaki City. The wreckage was discovered by the Japan Coast Guard on Aug. 7, 2015.
Truly the I-400's were amazing vessels. Much like there is a museum in Kure for the largest battleship created by Japan, the Yamato, perhaps there will one day be a museum in Sasebo detailing the history of Japan's largest submarines.
Japanese Super Sub - Secret Weapon (2009) (dubbed in Japanese, in 8 parts)
Japanese Submarines that Surrendered (April 2011; courtesy of Waller)
Characteristics of Japanese Naval Vessels: Submarines (USN, Jan. 11, 1946)
The capture of the I-14, I-401 and I-402 (Feb. 11, 1946)
Sinking Japanese Subs in Operations (Roads End) 4/1/1946 (9:21)
1) LS Japanese subs being sunk after end of war by units of US fleet. 2) MS Japanese sub exploding and sinking (SV). 3) MS Japanese sub exploding and sinking. 4) LS Sub in water sinking. 5) LS Large, white spray of water high in air as Japanese sub is sinking. 6) LS Japanese sub being sunk by units of US fleet, near Sasebo, Japan. 7) MS Smoke rising from Japanese sub (SV). 8) AtoG MS Japanese subs. 9) MS Japanese crew taken off of sub prior to sinking. 10) AtoG MS Explosions in water throwing up white spray (SV).Sinking Of Captured Japanese Submarines 4/1/1946 (10:50)
Summary: AVs, fleet of Japanese submarines lying in cove at Sasebo Bay. American Navy crews opening boxes of demolition charges aboard Japanese submarine #402. Party consisting of Admiral Robert M Griffen and others go below one of the smaller submarines. Opening door of hangar of the 402 that can carry 4 planes. Admiral Griffen walks inside the hangar. Scenes taken from craft showing different submarines in the cove. Admiral Griffen goes aboard submarine 158 accompanied by party. The 158 is the submarine which sunk the USS Indianapolis. Japanese putting cherry blossoms in the periscope of submarine. CUs, Capt Atogi Nakamura and Lt Tashio Tanaka. The latter is the officer who fired torpedoes which blew up the Indianapolis. LS, crew of the 158. Sequence: aboard destroyer USS Larsen: MCU, Commander F A McKee, on the bridge with Capt Bell, acting commodore of the destroyer squadron. Also in scene is Japanese interpreter giving instruction thru megaphone. Several scenes showing destroyer convoy in bg. From bridge of the Larsen: scenes of the world's largest sub, #402 being sunk by the Larsen's gunfire. LSs, of other subs being sunk by gunfire. MS, naval guns being fired.Damaged Japanese Ships, Sasebo Bay 9/1945 (8:00)
1) LS Rusted Japanese submarine in Sasebo Bay. 2) LS MS <*>Japanese carrier at anchor in Sasebo Bay. 3) CU Starboard side of damaged Japanese carrier, with submarine tied up alongside (SV). 4) MS Various Japanese fishing craft and small tugs tied up at docks (SV). 5) MS <*>Portion of Japanese town and factories in BG (SV). 6) CU <*>RADM. MORTON L. DEYO with staff members, Japanese officers, and interpreters inspecting the damaged destroyer, SUZUTSUKI (SV).<*><*> 7) LS <*>Same in sheltered cove, Sasebo Seaplane Base (SV)Damaged Japanese Ships, Sasebo 9/28/45 (5:39)
LS Japanese drydocks and submarineways, cranes and etc. at Sasebo Naval Base.LS Camouflage Japanese ships, freighter tied up to docks-SV.LS Cruisers, SANTA FE & WICHITAScenes In Sasebo Harbor, Japan 9/24/1945 (7:10)
1) MS APA at anchor shore in BG. 2) MS Several LCT"s at anchor unloading tanks. 3) MS Bulldozer pulls out of LCT. 4) LS PAN Fishing at village waterfront, Sasebo Harbor. 5) LS Japanese worker in small boat. 6) LS Japanese wooden boat tied up in Sasebo Harbor (SV). 7) CU Same.8) LS Crane unloads supplies from AKA in docks at harbor. 9) LS PAN Waterfront on Sasebo showing boats, cranes, buildings, etc. 10) MS Sailors leave LST and go ashore. 11) MS Japanese police directing traffic in Sasebo. 12) MS Hdqts. bldg. of Marine Amphibious Corps. 13) MS Sailor takes still picture of damage in Sasebo. 14) LS PAN Many buildings damaged wreckage. 15) MS Street scene Japanese pulls ricksha.16) LS Custom house and boat landing.Physical Damage, Aerial Coverage Cities Of Japan 4/12~13/1946 (11:19)
Omuta: 19' Flight line flown on a heading of 207 degrees magnetic. 21' Same as above. 17' Same as above. 16' Flight line flown on a heading of 270 degrees magnetic.
Sasebo: 47' Flight line flown on a heading of 340 degrees magnetic. Two passes on Naval Air Station starting on the E side and showing also a portion of the burned-out urban area. Gas dump on fire on a small island of Sasebo Harbor. 13' Flight line flown on a heading of 340 degrees magnetic, over Naval Air Station. 40' Flight line flown on a heading of 332 degrees magnetic showing Sasebo Aircraft Factory. 25' Flight line flown on a heading of 58 degrees magnetic over Sasebo. 12' Same as above. 11' Same as above. 12' Three aircraft carriers beached in a small cove S of Sasebo Harbor.
Omura: 13' Flight line flown on a heading of m magnetic. Nine passes made over factory area and airfield starting along the water. 20' Same as above. 17' Same as above. 17' Same as above. 13' Same as above. 11' Same as above. 21' Passes over Factory area and Airfield starting at edge of water. 27' Same as above. More inland.