First the I-400-class submarines.
The I-400 class was the brainchild of Admiral Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet. Shortly after the attack on Pearl Harbor, he conceived the idea of taking the war to the United States mainland by making aerial attacks against cities along the U.S. western and eastern seaboards using submarine-launched naval aircraft. He commissioned Captain Kameto Kuroshima to make a feasibility study.

Yamamoto submitted the resulting proposal to Fleet Headquarters on 13 January 1942. It called for 18 large submarines capable of making three round-trips to the west coast of the United States without refueling or one round-trip to any point on the globe. They also had to be able to store and launch at least two attack aircraft armed with one torpedo or 800 kg (1,800 lb) bomb. By 17 March, general design plans for the submarines were finalized. Construction of I-400 commenced at Kure Dock Yards on 18 January 1943, and four more boats followed: I-401 (April 1943) and I-402 (Oct 1943) at Sasebo; I-403 (Sept 1943) at Kobe and I-404 (February 1944) at Kure. Only three were completed.

Following Yamamoto's death in April 1943, the number of aircraft-carrying submarines to be built was reduced from eighteen to nine, then five and finally three. Only I-400 and I-401 actually entered service; I-402 was completed on 24 July 1945, five weeks before the end of the war, but never made it to sea.
Design features and equipment
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. Measuring more than 120 m (390 ft) long overall, they displaced 5,900 t (6,500 short tons), more than double their typical American contemporaries. 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. To allow stowage of three aircraft along the vessel's centerline, the conning tower was offset to port.

Located approximately amidships on the top deck was a cylindrical watertight aircraft hangar, 31 m (102 ft) long and 3.5 m (11 ft) in diameter. 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. The door was made waterproof with a 51-millimetre-thick (2.0 in) rubber gasket.

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. A single 25 mm (1 in) autocannon on a pedestal mount was also located just aft the bridge. One Type 11, 140 mm (5.5 in) deck gun was positioned aft of the hangar. It had a range of 15 km (9.3 mi).

Eight torpedo tubes were mounted in the bow, four above and four below. There were no aft tubes.

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. The crane had an electrically operated hoist and was capable of lifting approximately 4.5 t (5.0 short tons). It was raised mechanically to a height of 8 m (26 ft) via a motor inside the boat. The boom extended out to a length of 11.8 m (39 ft).[9]

A special trim system was fitted to the boats, allowing them to loiter submerged and stationary while awaiting the return of their aircraft. However, operation of this system was noisy and its usefulness was in doubt.

Strung along the submarine's gunwales were two parallel sets of demagnetization cables, running from the stern to the bow planes. They were meant to protect against magnetic mines, by nullifying the magnetic field which normally triggers the fusing system of the mines. A similar demagnetizing system was carried on many Japanese surface ships during the first part of the war, until they were later removed during refitting.

Electronics on board the I-400s included a Mark 3 Model 1 air search radar equipped with two separate antennas. This unit was capable of detecting aircraft out to a range of 80 km (43 nmi). The boats were also equipped with Mark 2 Model 2 air/surface radar sets with distinctive horn-shaped antennas. Each boat carried an E27 radar warning receiver, connected to both a trainable dipole antenna and a fixed non-directional antenna made up of a wire mesh basket and two metal rods.

The submarines were equipped with two periscopes of German manufacture, about 12.2 m (40 ft) long, one for use during daylight and the other at night.

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. This coating was apparently based on German research, though completely different in composition from German anechoic tiles such as Alberich or Tarnmatte.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.

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. This retrofit occurred while the boat was laid up at Kure for repairs after being damaged by an American mine in April.

I-402 was completed shortly before the war ended, but had been converted during building to a tanker and was never equipped with aircraft.
Characteristics
The I-400-class subs were unwieldy and relatively difficult to maneuver while surfaced owing to their small rudders. The large superstructure also caused the sub to veer off course during any strong wind.The maximum safe diving depth of the I-400-class submarine was only 82% of its overall length, which presented problems if the submarine dived at too steep an angle in an emergency.Because of their large aircraft hangars and conning tower, all I-400-class boats had significant visual and radar signatures on the surface, and could be detected by aircraft relatively easily. 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.

When submerged and traveling at a slow speed of two knots, the offset superstructure forced the helmsman to steer seven degrees starboard in order to steer a straight course.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. Like other Japanese submarines, crew members in I-400 subs had no air conditioning to control temperatures in tropical waters and no flush toilets. Lack of cold storage greatly limited the crew's diet, while inadequate sleeping quarters forced some of the crew to sleep on the decks or in passageways.
Operational history
Panama Canal strike
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. Intelligence gathering on the proposed target began later that year.

The Japanese were well aware that American fortifications existed on both sides of the Canal. 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. In August 1942, the 88th Coast (Anti-Aircraft) Artillery unit was added to help defend against aerial attacks.

As the war continued and Japan's fortunes declined, however, security around the Canal grew increasingly lax. In January 1944 Commander Fujimori personally interviewed an American prisoner-of-war who had done guard duty there. He told Fujimori that defensive air patrols had virtually ceased, since it was considered increasingly unlikely the Axis powers would ever attack the locks. This further convinced Fujimori of his plan's feasibility.

A Japanese engineer who had worked on the Canal during its construction handed over hundreds of documents to the Naval General Staff, including blueprints of the Canal structures and construction methods. A team of three shipping engineers studied the documents and concluded that the locks at Miraflores on the Pacific side were the most vulnerable to aerial bombing, but the Gatun locks on the Atlantic side offered a chance of causing greater damage, since it would be harder to halt any outflow of water. They estimated the Canal would be unusable for at least six months following a successful attack on the locks.

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.[20] It was originally planned that two of the Seirans would carry torpedoes and the other eight would carry 800 kg (1,800 lb) bombs. They were to make a combined torpedo and glide-bombing attack against the Gatun Locks. Eventually though, torpedo-bombing was dispensed with, because only one Seiran pilot had mastered the technique.

The Panama Canal strike plan called for four aircraft-carrying submarines (I-400, I-401, I-13 and I-14) to sail eastward across the Pacific to the Gulf of Panama, a journey expected to take two months. At a point 185 km (100 nmi) off the coast of Ecuador, the submarines would launch their Seiran aircraft at 0300hrs on a moonlit night. The Seirans, without floats, would fly at an altitude of 4,000 m (13,000 ft) across the northern coast of Colombia to the vicinity of Colón. 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.

Around April 1945, Captain Ariizumi, the man appointed to carry out the attack, decided the Seiran pilots would make kamikaze ramming attacks against the gates, rather than conventional bombing runs, a tactic becoming increasingly common as the war went against the Japanese. The Seiran squadron leader had already suggested as much to Ariizumi earlier that month, though for a time this was kept secret from the other pilots. At the end of May, however, one pilot happened to observe a Seiran having its bomb-release mechanism removed and replaced with a fixed mount. 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."

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. There was also rudimentary formation flying. From 15 June the Seiran pilots made practice daylight bombing runs against the wooden gate mock-up. By 20 June, all training ended and the operation was set to proceed.
Ulithi atoll
Before the attack could commence, Okinawa fell, and word reached Japan that the Allies were preparing an assault on the Japanese home islands. 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.

Fifteen American aircraft carriers had assembled at the Ulithi atoll, preparatory to making a series of raids against the home islands.The Japanese mission was changed to an attack on the Ulithi base.

The attack on Ulithi Atoll was to take place in two phases. The first, codenamed Hikari (light), involved transporting four C6N Saiun (Myrt) single-engined high-speed reconnaissance planes to Truk Island. They were to be disassembled, crated and loaded into the water-tight hangars of submarines I-13 and I-14. Upon reaching Truk, the Saiuns would be unloaded, reassembled and then flown over Ulithi to confirm the presence of American carriers anchored there. Following the delivery, I-13 and I-14 were to sail for Hong Kong, where they would embark four Seiran attack planes. They would then head to Singapore and join I-400 and I-401 for further operations.

The second phase of the Ulithi attack was codenamed Arashi (storm). I-400 and I-401 were to rendezvous at a predetermined point on the night of 14/15 August. On 17 August they would launch their six Seirans before daybreak on a kamikaze mission against the American carriers. The Seirans, each with an 800 kg (1,800 lb) bomb bolted to its fuselage, were to fly less than 50 m (160 ft) above the water to avoid radar detection and the American fighters expected to be patrolling 4,000 m (13,000 ft) above.

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. This was an attempt to further confuse recognition if the aircraft were prematurely spotted, but it was not well received by the pilots. Some felt it was both unnecessary and a personal insult to fly under American markings, as well as dishonorable to the Imperial Navy.

Following the attack on Ulithi, I-400 and I-401 would sail for Hong Kong. There they would take on six more Seirans and sail for Singapore, where fuel oil was more readily available. They would then join I-13 and I-14 and stage further attacks with a combined force of ten Seiran aircraft.

On 22 June, I-13 and I-14 arrived at Maizuru Harbor to take on fuel. They reached Ominato on 4 July to pick up their Saiun reconnaissance aircraft. I-13 departed for Truk on 11 July but never reached her destination. She was detected running on the surface, attacked, and damaged by radar-equipped TBM Avengers on 16 July. An American destroyer escort later arrived and sank her with depth charges.

Japan surrendered before the Ulithi attack was launched, and on 22 August 1945, the crews of the submarines were ordered to destroy all their weapons. The torpedoes were fired without arming and the aircraft were launched without unfolding the wings and stabilizers. When I-400 surrendered to the American destroyer, Blue, the U.S. crew was astounded at her size, nearly 24 ft (7.3 m) longer than the USS Blue and just as wide – considerably longer and wider than the largest American fleet submarine of the day.
American inspections
The U.S. Navy boarded and recovered 24 submarines, including the three I-400 submarines, taking them to Sasebo Bay to study them. While there, they received a message that the Soviets were sending an inspection team to examine the submarines. To prevent this, Operation Road's End was instituted. Most of the submarines were taken to a position designated as Point Deep Six, about 35 km (19 nmi) southeast of Fukue Island, packed with charges of C-3 explosive and destroyed; they sank to a depth of 200 m (660 ft).


Four remaining submarines, I-400, I-401, I-201 and I-203, were sailed to Hawaii by U.S. Navy technicians for further inspection. Upon completion of the inspections, the submarines were scuttled in the waters off Kalaeloa near Oahu in Hawaii by torpedoes from US submarine USS Trumpetfish on June 4, 1946, to prevent the technology from being made available to the Soviets who were demanding access to them. 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.
Below Japanese Under Guard. I-400 Japanese Submarine


Below Japanese Submarine I-400 with Japanese crew aboard.

Below Hangar Water tight Door I-400 Japanese Submarine


Below View from Inside of Hangar on I-400 Japanese Submarine. Japanese Crew unloading cargo.

Below Hangar Door and Hangar I-400. Men pass cargo down the line on Japanese Submarine.

Below I-400, or I-401 (Japanese Submarine 1944) Interior of the aircraft hangar, showing tracks for rolling planes out. Taken at Yokosuka, Japan. October 14, 1945

Below 25mm Guns on Deck of I-400 Japanese Submarine. On Deck looking aft.

Below I-400. View of the after 5.5 deck gun, with U.S. Navymen S1c Rudolph Massengill and Torp. 1c Willis Clement. October 14, 1945

Below Crew's Living Quarters aft on I-400 Japanese Submarine.

Below Forward Upper Torpedo Room in the I-400 Japanese Submarine

Below I-400 inside Maneuvering Room looking forward.

Below Main Control Room on the I-400 Japanese Submarine


Main Control Room on the I-400 Japanese Submarine, looking aft

Below Main Starboard Engines on the I-400 Japanese Submarine

Below Port Engine room I-400 Japanese Submarine

Below Man stands with Radar Gear on Japanese Submarine I-400.

Below Officer's Quarters Amidship Port I-400 Japanese Submarine.

Below Outboard profile and deck plan prepared by U.S. Navy after World War II

Aichi M6A Seiran
Design and development
From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets.

In December 1941, Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto, proposed constructing a large fleet of submarine aircraft carriers (also designated STo or sen-toku—special submarine) whose purpose was to mount aerial attacks against American coastal cities. The submarines would surface to launch their aircraft by catapult, submerge to avoid detection, then surface again to retrieve the aircrews, who would ditch their planes nearby. By June 1942, the plan was to build a fleet of eighteen such submarines. This was later cut to nine, then five, and finally just three, as Japan's wartime fortunes declined.

To equip the submarine aircraft carriers, the Imperial Japanese Navy Air Service requested that Aichi design a folding attack aircraft with a range of 1,500 km (810 nmi) and a speed of 555 km/h (300 kn). Aichi was already manufacturing under license the D4Y1 Suisei (Judy), a relatively small single-engined carrier dive bomber with exceptionally clean lines and high performance. Detailed engineering studies commenced in an effort to modify the Suisei for use aboard the I-400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated.

Aichi's final design, designated AM-24 by Aichi and given the military designation M6A1, was a two-seat, low-winged monoplane powered by a 1,050 kW (1,410 hp) Aichi AE1P Atsuta 30 engine (a licence-built copy of the Daimler-Benz DB 601 liquid-cooled V12 engine). The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility. If conditions permitted, these would allow the aircraft to land next to the submarine, be recovered by crane and then re-used. The floats could be jettisoned in flight to increase performance or left off altogether for one-way missions. The Seiran's wings rotated 90 degrees and folded hydraulically against the aircraft's fuselage (with the tail also folding down) to allow for storage within the submarine's 3.5 m (11 ft) diameter cylindrical hangar. Armament was a single 850 kg (1,870 lb) torpedo or an equivalent weight in bombs. One 13 mm (0.51 in) Type 2 machine gun was mounted on a flexible mounting for use by the observer.

As finalized, each I-400 class submarine had an enlarged watertight hangar capable of accommodating up to three M6A1s. The Seirans were to be launched from a 26 m (85 ft) compressed-air catapult mounted on the forward deck. A well-trained crew of four men could roll a Seiran out of its hangar on a collapsible catapult carriage, attach the plane's pontoons and have it readied for flight in approximately 7 minutes.

In order to shorten the launching process and eliminate the need for time-consuming engine warm-ups, the Seirans were to be catapulted from a cold start. This necessitated heating the engine oil for each plane to approximately 60 °C (140 °F) in a separate chamber and pumping it, as well as hot water, through the engine just prior to launch while the planes were still in the hangar. In this way, the aircraft's engine would be at or near normal operating temperature immediately upon getting airborne. The idea was borrowed from the Germans who planned on using a similar launch method for the aircraft of their unfinished carrier Graf Zeppelin.

The first of eight prototype Seirans was completed in October 1943, commencing flight testing in November that year. A problem with overbalance of the auxiliary wings was eventually solved by raising the height of the tail fin. Further testing was sufficiently successful for production to start in early 1944. In order to aid pilot conversion to the Seiran, two examples of a land based trainer version fitted with a retractable undercarriage were built. These were given the designation M6A1-K Nanzan (南山, Southern Mountain). Besides the difference in landing gear, the vertical stabilizer's top portion, which was foldable on the Seiran, was removed.
Operational history
The first production examples of the Seiran were completed in October 1944. Deliveries were slowed by an earthquake near Nagoya on 7 December 1944, and by an American air-raid on 12 March 1945.Construction of the STo submarines was stopped in March 1945, after two submarine aircraft carriers had been completed and a third finished as a fuel tanker. These were supplemented by two smaller Type AM submarines, originally designed as command submarines carrying reconnaissance floatplanes, but capable of carrying two Seirans. Owing to the reduced carrier submarine force, production of the Seiran was halted, with a total of 28 completed (including the prototypes and the M6A1-Ks).

The new submarines and aircraft were assigned to the 1st Submarine Flotilla, comprising the two STo submarines, the I-400 and the flagship I-401, each carrying three Seirans together with two type AMs, the I-13 and I-14. The 1st Submarine Flotilla commenced training with the Seirans in January 1945, and the crews gradually learned how to handle the submarines and aircraft. Launching all three Seirans took longer than expected: 30 minutes if floats were fitted, although this could be reduced to 14.5 minutes if the floats were not used.

The first mission of the Seiran squadron, which was named the Shinryuu Tokubetsukougeki-tai (神龍特別攻撃隊; meaning "God-Dragon Special Attack Squad") was to be a surprise air strike on the Gatun locks of the Panama Canal, to cut the main supply line for US forces in the Pacific. When the force was finally ready to set off on their mission against Panama, Japan's increasingly desperate situation led to a change in plan, with the target for the attack, called Operation Hikari (Splendour), being switched to the American base at Ulithi Atoll where forces, including aircraft carriers, were massing in preparation for attacks on the Japanese Home Islands. The flotilla departed Japan on 23 July 1945 and proceeded towards Ulithi. On 16 August, the flagship I-401 received a radio message from headquarters, informing them of Japan's surrender and ordering them to return to Japan. All six Seirans on board the two submarines, having been disguised for the operation as American planes in violation of the laws of war, were catapulted into the sea with their wings and stabilizers folded (for the I-401) or pushed overboard (for the I-400) to prevent capture.
Variants
M6A1
Prototypes powered by Atsuta 30 or 31 1,044 kW (1,400 hp) engine and removable floats, 8 built.
M6A1 Seiran (Shisei-Seiran)
Special Attack Bomber, 18 built.





M6A1-K Nanzan (Shisei-Seiran Kai)
Prototypes of training version, retractable wheeled landing gear, 2 built.





M6A2
Prototype modification, powered by Mitsubishi Kinsei MK8P 62 1,163 kW (1,560 hp) engine, 1 built
Surviving aircraft
A single M6A1 has been preserved and resides in the Udvar-Hazy Center of the Smithsonian's National Air and Space Museum. It is located in the Washington, DC suburb of Chantilly, Virginia near Dulles International Airport. The Seiran was surrendered to an American occupation contingent by Lt. Kazuo Akatsuka of the Imperial Japanese Navy, who ferried it from Fukuyama to Yokosuka. The US Navy donated it to the Smithsonian Institution in November 1962. Restoration work on the Seiran began in June 1989 and was completed in February 2000.


Specifications (M6A1)
General characteristics
Crew: 2
Length: 11.64 m (38 ft 2 in)
Wingspan: 12.262 m (40 ft 3 in)
Height: 4.58 m (15 ft 0 in)
Wing area: 27 m2 (290 sq ft)
Empty weight: 3,301 kg (7,277 lb)
Gross weight: 4,040 kg (8,907 lb)
Max takeoff weight: 4,445 kg (9,800 lb)
Powerplant: 1 × Aichi AE1P Atsuta 30 or Atsuta 31 V-12 inverted liquid-cooled piston engine, 1,000 kW (1,400 hp) for take-off

1,000 kW (1,340 hp) at 1,700 m (5,600 ft)
960 kW (1,290 hp) at 5,000 m (16,000 ft)

Propellers: 3-bladed constant-speed propeller
Performance
Maximum speed: 474 km/h (295 mph, 256 kn) at 5,200 m (17,100 ft)
Cruise speed: 296 km/h (184 mph, 160 kn) at 3,000 m (9,800 ft)
Range: 1,188 km (738 mi, 641 nmi)
Service ceiling: 9,900 m (32,500 ft)
Time to altitude: 3,000 m (9,800 ft) in 5 minutes 48 seconds
Wing loading: 149.6 kg/m2 (30.6 lb/sq ft)
Power/mass: 0.3574 kW/kg (0.2174 hp/lb)
Armament
Guns: 1× 13 mm cabin-mounted Type 2 machine gun
Bombs:
1× Type 91 torpedo or
2× 250 kg (551 lb) or 1× 850 kg (1,874 lb) bombs
(Text from Wikki)