IJN I-400 & I-401

Sen-togu - class

As seen in 1945 Attack on Ulithi atoll

Pit-Road SkyWave W48

Scale 1:700 Water Line Series

Click on the pictures to enlarge.



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Box art

IJN I-400 & I-401 Sen-togu - class


The Kit

This model kit from Pit Road /Sky wave contain the seas "Jumbo"  The Sen togu I-400 class - not one vessel, but two! And again it's with the choice of building the model with a full hull or a waterline model. This is a very nice feature as this gives the modeller many display opportunities. The box art of Pit Road is always a pleasure to experience - Pit Road really has a skilled artist (Mr. Y. Takani) in their marketing department.

The overall quality of this kit is good. The only "missing parts”, was the hinge for the aircraft hanger door and finer details on the periscope suite and 2 extra Serian aircrafts. And if I have to be nitpicky - an in-mould door aft. on the deck house. The kit comes with a set of 2 aircraft handling cranes, you can attach in upright position, but the lowered crane is moulded in the deck too! Just as with the I-13 class. It had been better the crane on the deck was a part by its own.

 But never the less to my opinion one of the best IJN sub's on the marked, even today when the Pit Road age is over 10 years.



5 stars out of 6 for: Good fit, fine deck details, Full hull/water line, exciting ship-class.

Building the model

It was very straightforward as the build process, was with no hidden surprises along the way. You have to track down the PE-set from Tom's Model Work's IJN Subs. enough to build 8 Pit Road sub! Also a Lion Roar IJN 3 bar railing cut to fit, was used as well as IJN ladders Lion Roar and IJN Water tight doors from Voyager. Again I used the new "Nano Dred" 25mm AA guns from Fine Molds - they are a fine substitute of ordinary PE-set, but a bit pricy!

The scratch building was; the binoculars, rear rudder guards on the deck, periscopes and mast etc. The only "major" error missing in the kit was the swan neck on the snorkel + the small hinges on the hangar door. -Easily corrected with styrene plastic. Invisible tread was used for the antenna lines. I have seen some PE-set's have the "starfish" of the inside of the hangar door included. But Tom's PE-set missing this part. I made mine of two each aircraft propeller!

I choose to show the boats as seen in 1945, when the attack on the US anchor place of Ulithi atoll (Not carried out-please read the history chapter below). This was a special attack mission (suicide Kamikaze mission) Therefore the Serian-aircraft is without the floats - no need!

 The aircraft is painted in US markings to lure the enemy - I used the markings from another surplus Sky Wave kit (IJN Torpedo boats) the decals are intended for the B-25, so they are a bit over scaled. Note also the 800 Kg bomb under the fuselage (PE-set from Lion Roar IJN Aircraft details) The big banner flying from I-400 is what I must call "artistic impressions" (surplus from Musashi kit)

History of the ship-class

The Sen Toku I-400-class (伊四〇〇型潜水艦) Imperial Japanese Navy submarines were the largest submarines of World War II and remained the largest ever built until the construction of nuclear ballistic missile submarines in the 1960s. They were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface, launch the planes then dive again quickly before they were discovered. They also carried torpedoes for close-range combat.

The I-400-class was designed with the range to travel anywhere in the world and return. A fleet of 18 boats was planned in 1942, and work started on the first in January 1943 at the Kure, Hiroshima arsenal. Within a year the plan was scaled back to five, of which only three (I-400 at Kure, and I-401 and I-402 at Sasebo) were completed.


The I-400 class submarine was the brainchild of Admiral Isoruku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet. Shortly after the attack on Pearl Harbor in December 1941, he conceived the idea of taking the war to the United States mainland by making aerial attacks against cities along the US 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 a fleet of 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 had also 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 during an inspection tour of the Solomon Islands 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, three weeks before the end of the war, but never made it to sea.


Each submarine had four 3,000-horsepower (2.2 MW) 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 400 ft (120 m) long overall, they displaced 6,500 tons, more than double their typical American contemporaries. The pressure hull had a unique figure-of-eight section which afforded the necessary strength and stability to handle the weight of a large on-deck aircraft hangar. To allow aircraft stowage 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 opened from the outside by turning a large hand-wheel connected to a rack and spur gear. The door was made waterproof with a two-inch thick rubber gasket.

The hangar of the I-400s was originally designed to hold two aircraft. In 1943, however, Commander Yasuo Fujimori, Submarine Staff Officer of the Naval General Staff, requested it be enlarged. This was deemed feasible and, as remodeled, I-400s could stow up to three Aichi M6A Seiran aircraft.

The Seiran was specifically designed for use aboard the subs and could carry an 800 kilogram (1,764 lb) bomb 650 miles (1000 km) at 295 miles per hour (474 km/h). To fit inside the narrow confines of the hangar, the wings rotated 90 degrees and folded back hydraulically against the fuselage, the horizontal stabilizers folded down and the top of the vertical stabilizer folded over so the overall forward profile of the aircraft was within the diameter of its propeller. When deployed for flight, the aircraft had a wing span of 40 feet (12 m) and a length of 38 feet (11.6 m). A crew of four could prepare and launch all three in 45 minutes (or 15 minutes if the planes' pontoons were not attached). As the Seiran would normally be launched at night, parts and areas of the plane were coated with luminescent paint in order to ease assembly in the dark.

The Seirans were launched from a 26 m (85 ft) Type 4 No. 2 Model 10 compressed-air catapult on the forward deck of the submarine. Underneath the catapult track were four high-pressure air flasks connected in parallel to a piston. The aircraft, mounted atop collapsible carriages via catapult attachment points along their fuselages, would be slung 70-75 feet along the track, though the piston itself only moved between eight and ten feet during operation.

Two sets of pontoons for the Seirans were stored in special watertight compartments located just below the main deck on either side of the catapult track. From there they could be quickly slid forward on ramps and attached to the plane's wings. A third set of pontoons and additional spares were kept inside the hangar.

The existence of the Seiran was not known to Allied intelligence during the war.

Sited atop the hangar were three water-proofed Type 96 triple-mount 25 mm (1.0 in) machine guns for AA defense, two abaft and one forward of the conning tower. A single 25mm gun on a pedestal mount was also located just behind the bridge. One Type 11 140 mm (5.5 in) deck gun was positioned aft of the hangar. It had a range of 15,000 m (49,000 ft).

Eight torpedo tubes were mounted at 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 sub's Seiran floatplanes. The crane had an electrically operated hoist and was capable of lifting approximately 5 tons. It was raised mechanically to a height of 8 m (26 ft) via a motor inside the sub. The boom extended out to a length of 11.8 m (39 ft).

A special trim system was fitted to the subs, allowing them to loiter submerged and stationary while awaiting the return of their aircraft.

Strung along the submarine's gunwales were two parallel sets of demagnetization cables, running from the stern to the bow planes. These were intended to dissipate the static charge that normally builds up when a ship's hull slices through the water, causing the steel in the hull to deteriorate over time.

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 43 nm (80 km), though Japanese operators later admitted that planes flying below the radar horizon could escape detection altogether. The subs also had Mark 2 Model 2 air/surface radar sets with distinctive horn-shaped antennas. Each sub 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 subs 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, based on German technology, was applied to the hulls from the waterline to the bilge keel. This was intended to absorb or diffuse enemy sonar pulses and dampen reverberations from the subs' internal machinery, making detection while submerged more difficult.

In May 1945, I-401 was fitted with a German-supplied snorkel, a hydraulically-raised air intake device allowing the sub to run its diesel engines and recharge its batteries while remaining at periscope depth. This retrofit occurred while the sub was laid up at Kure for repairs after being damaged by an American mine in April.

I-402 was completed immediately before the war ended, but had been converted during building to a tanker and was never equipped with aircraft.

Operational history

Panama Canal strike

As the war turned against the Japanese and their fleet no longer had free rein in the Pacific, the Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto, devised a daring plan to attack the cities of New York, Washington D.C., and other large American cities.

Following an inspection of Rabaul in August 1943, Capt. 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 this proposed target began later that year.

The Japanese were well aware that American fortifications existed on both sides of the Canal. On the Atlantic side the large coastal artillery batteries of Fort Sherman had a range of 30,000 yards, 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.

However, as the war continued and Japan's fortunes declined, security around the Canal grew increasingly lax. In January 1944 Commander Fujimori personally interviewed an American POW who had done guard duty there. He told Fujimori that defensive air patrols had virtually ceased as it

was considered increasingly unlikely the Axis powers would ever attempt an attack on 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 these documents and concluded that, although the locks at Mira Flores on the Pacific side were the most vulnerable to aerial bombing, 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.

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. It was originally planned that two of the Seirans would carry torpedoes and the other eight would carry 800 kilogram (1,764 lb) bombs. They were to make a combined torpedo and glide-bombing attack against the Gatun Locks. Eventually though, torpedo-bombing was dispensed with, as 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 100 nm (190 km) off the coast of Ecuador, the submarines would launch their Seiran aircraft at 03.00hrs 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 southwest 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 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 practicing 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 from the Japanese naval base at Maizuru, Okinawa fell, and word reached Japan that the Allies were preparing an assault on the Japanese home islands; the Japanese Naval General Staff concluded that 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 Ulithi atoll, preparatory to making a series of raids against the home islands; the Japanese mission was changed to attack 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-engine 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 this 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 one-way kamikaze mission against the American carriers. The Seirans, each with an 800 kilogram (1,764 lb) bomb bolted to its fuselage, were to fly less than 50 m (160 ft) above the water in order to avoid radar detection and the American fighters expected to be patrolling 4,000 m (13,000 ft) above.

In addition, just before departing Maizuru Naval Station, the Seirans were completely over painted in silver with American stars and bars insignias covering up the red Hinomarus. This was an attempt to further confuse recognition should the aircraft be prematurely spotted but it was not a popular decision with the aircrew. Some felt it was not only unnecessary but also a personal insult to fly under American naval markings and dishonorable to the Imperial Navy.

Following the attack on Ulithi, I-400 and I-401 would head for Hong Kong where they would embark six new Seirans. From there they were to sail for Singapore where fuel oil was more readily available. They would then join up with 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 the sub with depth-charges.

However, Japan surrendered before the attack was launched, and on August 22, 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 an American destroyer, the U.S. crew was astounded at its size.

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 40 miles (60 km) west of Nagasaki and off the Gotō Islands, packed with charges of C-2 explosive and destroyed; they sank to a depth of 200 meters.

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 (SS-425) on June 4, 1946, apparently because Soviet scientists were again demanding access to them.


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 meters.

A restored Seiran airplane is displayed at the National Air and Space Museum in Washington, D.C.. It is the only surviving example of this aircraft, and was found at the Aichi Aircraft Factory following the end of the war in August 1945. Shipped to the Naval Air Station at Alameda California, it was left on outdoor display until 1962 when it was transferred to the Paul E. Garber Facility in Silver Hill, Maryland. There it remained in storage until 1989 when a comprehensive restoration effort was mounted. 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.

Technical data:


5,223 tons surfaced
6560 tons submerged


122 m


12 m


7,02 m


4 diesels: 7,700 hp
electric motors: 2400 hp


18.7 knots surfaced
6.5 knots submerged


37,500 nm (69,500 km)  at 14 knots

Test depth:

100 m (330 ft)


144 officers and men


8 × 533 mm forward torpedo tubes (20 torpedoes carried)
1 × 140 mm (5.51 in) 40 caliber gun
10 x 25 mm AA guns, (3x3) + (1x1)

Aircraft carried:

3 Aichi M6A1 Seiran seaplanes + parts for a forth


I - 400 class

I - 400

 I - 401

 I - 402

Builder Kure Sasebo Sasebo
Laid down

Jan 18 1943

Mar 26 1943 Oct 20 1943
Launch  Jan 18 1944 Mar 11 1944 Sep 5 1944
Completed Dec 30 1944 Jan 8 1945 Jul 24 1945
Surrendered Aug 27 1945 Aug 25 1945 Aug  1945
Scuttled May 28 1946 May 28 1946 Apr 1 1946


Books form my own library :  

Gakken 45  by: Gakken


Reference: Pictures, drawings, history


Japanese Warship at War, Vol. 1  by: Trojca, Waldemar, Lengerer / Hans


Reference: Pictures


Submarine I-400: Tabular Record of Movement:


Submarine I-401: Tabular Record of Movement:


The building time was 10 days. 

Aug. 2010

If you have any questions, remarks or things you will share, please don't hesitate to contact me.



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