Since the early thirties, Japanese shipbuilders were engaged in the creation of submarines capable of carrying light aircraft. It was assumed that the aircraft transported by boat would perform reconnaissance functions. In the future, the strike role of the aircraft was not excluded, but a number of features of aircraft carrying submarines and aircraft imposed certain restrictions on the combat load for them. However, reconnaissance aircraft were also sufficiently useful for submarines. In particular, after the attack on Pearl Harbor, a photographic and filming of the results of the raid was carried out by a reconnaissance aircraft brought to the Hawaiian shores by the A9 I-1 submarine.
At the beginning of 1942, on the initiative of Admiral I. Yamamoto, the creation of a new submarine project began, capable of carrying several bomber aircraft at once. At the same time, the development of a specialized seaplane, suitable for transportation on a promising submarine and capable of carrying bomb weapons, was launched. It was assumed that the new submarines will be able to secretly deliver strike aircraft to the coastal targets or enemy ship groups. A secretive approach and a surprise attack, as expected, were to ensure the high combat effectiveness of the new submarines and their aircraft. For a promising submarine high demands. She was supposed to carry four aircraft and have a cruising range of 65-70 thousand kilometers. The range of the underwater course was to exceed 100 km. The result of such requirements were the size and displacement of a promising submarine. Underwater displacement, according to calculations, was to exceed 6,5 thousand tons.
The specific purpose of the perspective submarine determined its architecture. The Sentok submarine was proposed to be double-hulled, with a strong hull of complex shape. To transport a sufficient amount of fuel, as well as to accommodate an airtight sealed hangar, it was necessary to develop an original, robust, variable section housing. In the nose part it consisted of two large "pipes" and in the section resembled the figure eight. To the middle part, the form of a strong case smoothly changed and became similar to the infinity sign. This was due to the placement of the cylindrical hangar. The stern section of the strong hull was made in the form of the 8 digit and also smoothly matched the central one. Sturdy case was proposed to close a streamlined light.
A cylindrical (diameter about 3,5 meter) aircraft hangar was placed above the middle part of the hull, similar to the “∞” sign. The submarines of the Sentok project were supposed to carry three Aichi M6A Seiran seaplanes with fuel and weapons. The large dimensions of the submarine allowed to some extent simplify the procedure for takeoff. After the ascent, the submariners had to open the front hatch of the hangar, roll out the plane to the deck and prepare it for departure. Next, the M6A seaplane needed to be mounted on the catapult that provided the takeoff. The landing was proposed to be made, as before, on the water with the subsequent lifting of the aircraft on board with the help of a crane.
The most noticeable consequence of the high demands placed on the Sentok project was the large dimensions of the submarine. With an underwater displacement of 6560 tons (surface - no more than 5200 tons), new submarines should have a total length of 122 meters and a maximum hull width of 12 meters. The average draft was 7 meters. Thus, the Japanese submarines of the I-400 series became the largest representatives of their class during the Second World War. Moreover, their record in size and displacement lasted until the appearance of atomic submarines. Another record of the largest Japanese submarines concerned the cruising range. The estimated value of this parameter exceeded 69 thousand kilometers. This meant that the Sentoku boat could reach any part of the planet without needing refueling.
To bring such a large submarine in motion had to create a power plant of the appropriate power. In the surface position, the submarines "Sentoku" had to move using four diesel engines with an 2250 horsepower. each. To move under water, the boats were supposed to be equipped with two electric motors with an 2100 horsepower. With such a power plant, promising aircraft-carrying submarines in the surface position could reach speeds of at least 18 nodes, in submarines, near 12 nodes. The characteristics of the robust hull and hangar allowed to dive to a depth of 100 meters.
The crew of the submarines of the project "Sentoku", according to various sources, consisted of 144 people (21 officer) or more. The supply of food and fresh water was designed for swimming lasting up to 90 days.
Like other submarines of the time, "Sentoku" proposed to equip torpedo and artillery weapons. In the bow compartment of the submarines provided eight torpedo tubes caliber 533 mm. Ammunition consisted of 20 torpedoes. In addition, the submarines had to carry powerful anti-aircraft weapons. It consisted of three three-barreled artillery installations based on an automatic gun "Type 96" caliber 25 mm, as well as one such gun mounted separately. 140-mm gun "Type 11", installed behind the hangar, was intended to attack both air and surface targets.
Despite the presence of powerful torpedo and artillery weapons, Aichi M6A Seiran bomber aircraft were the main means of combat for the Sentoku submarines. Three of these aircraft were transported in a large and long hangar submarine. Designers of the company “Aichi” under the leadership of N. Ozaki managed to create a relatively compact aircraft suitable for transportation in a hangar of limited size, as well as capable of carrying bomb weapons. The Seyran bomber had a wingspan of an 12,26 meter, the total length was 11,64 m and was equipped with an Aichi Atsuta 32 liquid-cooled 1200 liquid-cooled engine. Normal take-off weight was equal to 4050 kg, the maximum reached 4450 kg.
"Seyran" was made by the aerodynamic scheme "nizkoplan." A characteristic feature of it are two large floats intended for landing on water. These elements of the design seriously limited the flight characteristics of the bomber, which could affect its combat capabilities. However, the specific requirements of the customer forced aircraft designers to look for compromises, sacrificing certain characteristics. Therefore, the maximum speed of the M6A aircraft (at an altitude of 5000 meters) was equal to 475 km / h, and cruising did not exceed 300 km / h. The crew of the bomber, consisting of a pilot and an arrow, could defend against enemy fighters using a 2 Type 13 mm machine gun located in the rear cockpit. Depending on the task, the Seyran aircraft could carry two 250 kg bombs or one 800 kg bombs. In addition, it was possible to mount a single torpedo weighing no more than 850 kg.
To accommodate a submarine in a small hangar on M6A Seiran bombers, wing folding and tail units were provided. The floats were dismantled. After preparation for transportation in the hangar, the transverse dimensions of the aircraft were determined by the diameter of the propeller. The size of the hangar allowed to transport three aircraft in the folded form, the stock of fuel and ammunition. In addition, in the remaining volume it was possible to place another bomber in a disassembled form. On its assembly would have to spend much more time than on preparing for the flight of others.
According to the original plans, the shipbuilding industry of Japan was to transfer submarines of the type Sen Toku to the navy 18. However, the industry was busy with the fulfillment of other orders, which affected the number of built aircraft carrier submarines. The lead I-400 submarine entered the Japanese fleet at the very end of the 1944 of the year. A few days later the second submarine I-401 was handed over to the navy, and in the last days of July 1945, the sailors received the third submarine. Until the end of hostilities in the Pacific, Japan managed to begin construction of six submarines "Sentoku". In addition, due to the difficult economic situation, the required series of submarines were reduced from the originally planned 18 units to 12. However, these plans remained on paper.
At the beginning of 1945, the first two submarines of the “Sentoku” type tested their main armament in the conditions of the landfill, after which their service began. The possibility of bombing targets, as well as the highest cruising range, determined the first combat mission of the new submarines. Their target was the Panama Canal locks. The Japanese command considered that the destruction or damage of these objects would seriously affect the supply of American troops in the Pacific. Submarines I-400 and I-401 went to the shores of Central America, but soon received an order to return. The command understood the senselessness of such an attack and the risks associated with it, because of which it ordered the submariners to return to the base. In early August, the second march of the Sentok submarines began, which also ended in failure. Two submarines were to go to the attic of the Uliti and attack the American ships. A few days after the I-400 submarine went out to sea, a fire occurred and she was forced to return to the base for repair. I-401, in turn, could not independently perform the combat mission and also returned home.
The second trip to the Uliti Atoll was first planned for August 17, but later it was transferred to the 25. However, the submarines "Sentoku" never got to their goal. The war in the Pacific was coming to an end, and on August 20, the crews of aircraft carrier submarines were ordered in accordance with which they were to destroy all weapons and aircraft. The bombers with ammunition were dropped overboard, and the torpedoes were launched into the sea. Realizing his powerlessness at the critical moment of the war, the commander of the I-401 submarine, Aridzumi, shot himself to death.
After the end of the Second World War, all three built submarines of the type Sen Toku were transferred to Pearl Harbor, where American specialists carefully studied them. In the spring of 1946, the Soviet Union decided to exercise its rights previously agreed with the Allies, and demanded access to the captured Japanese ships. Fearing possible consequences, the American side quickly got rid of trophies. On April 1, the submarine I-402 was shot by torpedoes, and on May 31 I-400 and I-401 set off to the bottom. The exact location of the I-401 submerged boat was determined in 2005. The lead submarine of the project was discovered later, in August of the 2013.
Airborne submarines "Sentoku" appeared too late and could not have any impact on the course of the Second World War. Perhaps the unique ships were able to influence the course of sea battles, but there is reason to doubt their high combat effectiveness. The fact is that in the case of submersible torpedoes, the Sentoku submarine had almost no advantages over other projects. As for the Aichi M6A Seiran bombers, their capabilities look ambiguous. Low flight speed, combined with relatively low maneuverability and weak defensive armament, could offset all the advantages associated with hidden transportation on board a submarine. Thus, under the conditions of a real battle, the submarine I-400 or its “sistershipy” could be left without an air group in a short time.
Despite the dubious combat qualities and the short combat path that was traveled without any success, the project of aircraft carrying submarines Sen Toku is of particular interest. Japanese designers managed to solve the problem that their colleagues from many countries set for themselves. In addition, Japan vividly showed the real capabilities of the “aircraft-carrying submarine-aircraft” complexes, which probably affected the further development of such military equipment. Submarines "Sentoku" were the last representatives of their class, built serially. In the future, aircraft carrying submarines did not go beyond the project stage.
On the materials of the sites: