was an Imperial Japanese Navy Sentoku-type (or I-400-class) submarine commissioned in 1944 for service in World War II. Capable of carrying three two-seat Aichi M6A1 "Seiran" (Mountain Haze) float-equipped torpedo bombers, the Sentoku-class submarines were built to launch a surprise air strike against the Panama Canal. Until 1965, the Sentaku-type submarines—I-400 and her sister ships and —were the largest submarines ever commissioned.
Design and description
The I-400-class submarines had four diesel engines and carried enough fuel to circumnavigate the world one-and-a-half times. Measuring long overall, they displaced , more than double their typical American contemporaries. Until the commissioning of the United States Navy ballistic missile submarine in 1965, the I-400-class were the largest submarines ever commissioned.
thumb|U.S. Navy personnel inspect I-400′s aircraft hangar after World War II.
thumb|U.S. Navy personnel examine I-400′s deck gun at Yokosuka, Japan, on 14 October 1945.
The cross-section of the pressure hull had a unique figure-of-eight shape which afforded the strength and stability to support the weight of a large, cylindrical, watertight aircraft hangar, long and in diameter, located approximately amidships on the top deck. The conning tower was offset to port to allow the stowage of three Aichi M6A1 Seiran ("Clear Sky Storm") float-equipped torpedo bombers along the centerline. Aircraft were launched from a catapult on the forward deck forward of the hangar. A collapsible crane allowed the submarine to retrieve her floatplanes from the water.
In addition to the three floatplanes, each I-400-class submarine was armed with eight torpedo tubes, all in the bow, with 20 Type 95 torpedoes, a Type 11 deck gun aft of the hangar, three waterproofed Type 96 triple-mount antiaircraft guns mounted atop the hangar—one forward and two aft of the conning tower—and a single Type 96 25 mm antiaircraft gun mounted just aft of the bridge.
I-400-class submarines had a rather noisy special trim system that allowed them to loiter submerged and stationary while awaiting the return of their aircraft; demagnetization cables meant to protect against magnetic mines by nullifying the submarine's magnetic field; an air search radar, two air/surface-search radar sets, and a radar warning receiver; and an anechoic coating intended to make detection of the submarine while submerged more difficult by absorbing or diffusing sonar pulses and dampening reverberations from the submarine's internal machinery.
Construction and commissioning
Ordered as Submarine No. 5231, I-400 was laid down on 18 January 1943 by the Kure Naval Arsenal at Kure, Japan. After the United States Army Air Forces conducted a major fire-bombing raid on Tokyo on the night of 9–10 March 1945, the 6th Fleet proposed to the Imperial Japanese Navy General Staff a retaliatory raid on San Francisco, California, by Aichi M6A1 Seiran floatplane bombers launched by the submarines of Submarine Division 1, but by April 1945 the staff's vice chief, Vice Admiral Jisaburō Ozawa, had rejected the proposal.
was in drydock at Kure, Japan, on 19 March 1945 when the United States Navy's Task Force 58 launched the first Allied air strike against the Kure Naval Arsenal.