[4] Each submarine had four 1,680 kW (2,250 hp) engines and carried enough fuel to go around the world one-and-a-half times—more than enough to reach the United States traveling east or west.

The cross-section of its pressure hull had a unique figure-of-eight shape which afforded the necessary strength and stability to handle the weight of a large on-deck aircraft hangar.

The outer access door could be opened hydraulically from within or manually from the outside by turning a large hand-wheel connected to a rack and spur gear.

[5][6] Situated atop the hangar were three waterproofed Type 96 triple-mount 25 mm (1 in) autocannon for AA defence, two aft and one forward of the conning tower.

[5][8] Stowed in an open recessed compartment on the forward port side, just below top deck, was a collapsible crane used to retrieve the submarine's Seiran floatplanes.

[13] A special anechoic coating made from a mixture of gum, asbestos, and adhesives was applied to the hulls from the waterline to the bilge keel.

[14] This was intended to absorb or diffuse enemy sonar pulses and dampen reverberations from the boat's internal machinery, theoretically making detection while submerged more difficult, though its effectiveness was never conclusively established.

[14][15][16] In May 1945, I-401 was fitted with a German-supplied snorkel, a hydraulically raised air intake device allowing the boat to run its diesel engines and recharge its batteries while remaining at periscope depth.

Dive time was 56 seconds, nearly double that of U.S. fleet subs, which made the boats easier to destroy from the air when caught on the surface.

[19] When conducting a torpedo attack the captain had to take into account his larger turning circle to starboard than to port, again because of the offset design.

[20] The Seiran was specifically designed for use aboard the submarines and could carry an 800 kg (1,800 lb) bomb, with a range of 1,000 km (620 mi) at 475 km/h (295 mph).

(For a similar defensive measure involving catapult-launched, disposable aircraft used by Allied convoy groups in the Battle of the Atlantic, see CAM ship.)

[citation needed] Following an inspection of Rabaul in August 1943, Captain Chikao Yamamoto and Commander Yasuo Fujimori conceived the idea of using the sen toku (secret submarine attack) to destroy the locks of the Panama Canal in an attempt to cut American supply lines to the Pacific Ocean and hamper the transfer of U.S. ships.

On the Atlantic, the large coastal artillery batteries of Fort Sherman had a range of 30,000 yards (17 miles (27 km)), preventing enemy ships from getting near enough to shell the locks.

In the months following the attack on Pearl Harbor, air and sea patrols had been strengthened around both entrances, and barrage balloons and anti-submarine nets erected.

[26] To increase the size of the airborne attack force, Commander Fujimori requested that two additional fleet submarines still under construction at Kobe, I-13 and I-14, be modified to house two Seirans each, bringing the total number of planes available to ten.

Now on the Caribbean side of the isthmus, they would turn westward on a heading of 270 degrees, then angle south-west and make their final approach to the Canal locks at dawn.

After completing their bombing runs, the Seirans were to return to a designated rendezvous point and ditch alongside the waiting submarines where the aircrews would be picked up.

Realizing the implications of this change, he angrily confronted the executive officer of the squadron, who explained that the decision to withhold this intention from the other men was made to "avoid mental pressures on the aircrews.

"[29] By 5 June 1945, all four aircraft-carrying submarines had arrived at Nanao Wan where a full-scale wooden model of the Gatun Locks gate had been built by the Maizuru Naval Arsenal, placed on a raft and towed into the bay.

The following night, formal training commenced with the Seiran flight crews practising rapid assembly, catapult launch and recovery of their aircraft.

The Japanese Naval General Staff concluded the Panama Canal attack would have little impact on the war's outcome, and more direct and immediate action was necessary to stem the American advance.

[32] Just before departing Maizuru Naval Station, the Seirans were completely over-painted in silver with American stars and bars insignias covering the red Hinomarus, a direct violation of the rules of war.

The Japanese conceived of an attack on the United States through the use of biological weapons specifically directed at the civilian population in San Diego, California.

Most of the submarines were taken to a position designated as Point Deep Six, about 35 km (19 nmi) southeast of Fukue Island,[37] packed with charges of C-3 explosive and destroyed; they sank to a depth of 200 m (660 ft).

Dr. James P. (Jim) Delgado of the National Oceanic and Atmospheric Administration's maritime heritage program reported that the official government position that the exact location of the sinking was unknown, but has been confirmed by declassified US Navy documents.

[38] The wreckage of I-401 was discovered by the Pisces deep-sea submarines of the Hawaii Undersea Research Laboratory in March 2005 at a depth of 820 metres (2,690 ft).

[44] NOAA researcher Jim Delgado, working aboard Pisces V, told the Chicago Tribune "It was torpedoed, partially collapsed and had sunk at a steep angle.

Shipped to Naval Air Station Alameda, it was left on outdoor display until 1962, when it was transferred to the Paul E. Garber Preservation, Restoration, and Storage Facility in Silver Hill, Maryland.

Though the plane had been ravaged by weather and souvenir collectors, and original factory drawings were lacking, the restoration team was able to reconstruct it accurately, and by February 2000 it was ready for display.