Aircraft carrier submarines "Sentoku". Reasons for failure

Submarine I-400 shortly after delivery. Photo Ww2db.com

The peak of the development of Japanese submarine shipbuilding during the Second World War was the project of the aircraft-carrying submarine "Sentoku". Such ships were supposed to operate at a great distance from the bases and ensure the delivery of airstrikes against enemy targets. However, all efforts to build these submarines were not justified - they have never been able to complete a combat mission.

Special tasks

At the turn of 1941-42. Japanese command fleet began to study the issue of striking the continental United States. The use of aircraft carriers or surface ships was overly dangerous, and therefore the idea of ​​a heavy submarine carrying seaplanes-bombers appeared. The development of the preliminary and technical design continued until the spring of 1942, after which construction began. The project received the designation "Tokugata Sensuikan" (abbreviated "Sentoku") - "Special submarine".

General view of the "Sentoku" type boats. Graphics Wiki.wargaming.net

The original plans called for the construction of 18 ships. However, in 1943 the construction plan was cut in half. Then several more orders were canceled. As a result, the fleet hoped to receive only five submarines. Only three of them were completed and handed over - the other two, for various reasons, did not reach service and were dismantled.

The lead boat I-400 was laid down on January 18, 1943 at the Kure shipyard. Construction of the next I-401 began in April, and three more buildings were laid in the fall. Exactly one year after the laying of the I-400 was launched, and during 1944 three more boats followed. The lead ship was handed over on the eve of the new 1945, and I-401 and I-402 began service in January and July. Curiously, at the final stage of construction, I-402 was converted from an aircraft carrier to a tanker. Thus, in the end, the fleet received only two heavy aircraft-carrying boats.

Submarine diagram. Below are the sections of the sturdy case. Graphics Wikimedia Commons

Features and Benefits

"Sentoku" was a diesel-electric boat with a length of 122 meters and a total displacement of 6,7 thousand tons. The ships of this series remained the largest in the world before the advent of nuclear submarines. A sturdy casing with a cross-section in the form of intersecting circles, divided by a transverse and longitudinal bulkhead, was used. Due to this, it was possible to obtain a large width of the boat, necessary to accommodate the hangar-superstructure and the catapult.

An American officer examines an open hangar. Photo by US Navy

The crew consisted of one and a half hundred people, incl. two dozen officers. Autonomy - 90 days, but the service conditions left much to be desired.

The large submarine received a developed complex of torpedo and artillery weapons. On the two decks of the bow compartment were placed four torpedo tubes of 533 mm caliber. Ammunition - 20 torpedoes. On the deck, behind the superstructure, was a 140-mm rifled gun. Anti-aircraft armament included 10 25 mm caliber machine guns on one single and three triple mountings.

Hangar interior on I-400. Photo Ww2db.com

The main strike means of the I-400 and its sisterships were float bombers "Aichi" M6A "Seiran". They developed speeds of up to 480 km / h and could deliver an 800-kg bomb or a comparable load at a range of 1,2 thousand km.

The superstructure of the Sentoku submarine was made in the form of a cylindrical sealed hangar, capable of accommodating 3 aircraft, as well as containers with fuel and ammunition. The exit from the hangar was carried out through the bow hatch; in front of him there was a catapult rail guide. It was proposed to land on the water, after which the plane climbed onto the deck using a crane. The possibility of flying without returning to the boat was also considered.

The winners examine the 140-mm gun of the submarine I-400, autumn 1945. Photo by US Navy

Combat service

By the time the construction of Sentoku was completed, it became clear that a successful attack on the continental United States was simply impossible. If the submarine-aircraft carrier and managed to approach the line of launching the aircraft, then the air defense would not let them pass to important targets. In this regard, an alternative plan appeared - to attack the structures of the Panama Canal from the Atlantic side.

Planning and preparation was seriously delayed, and the operation could only begin in June 1945. Submarines I-400, I-401, as well as I-13 and I-14 of another project had to covertly round South America and approach the entrance to the Panama Canal. Then ten aircraft with suicide pilots were to attack the gates of the first airlock.

Bomber M6A "Sayran" with float landing gear. Photo by Wikimedia Commonc

However, at the end of June, a new order followed. Aircraft carriers "Sentoku" decided to transfer to Ulichi Atoll to attack American surface ships. Preparations again took a lot of time, and the submarines went on a campaign only in early August. Not reaching their target, the submariners received a message of surrender. A few days later, all participants in the operation met the US Navy ships and surrendered.

At this time, preparations were being made for another operation. At the end of September, the Seirans from the Sentoku were to drop bombs with infected insects on US soil. However, the defeat of Japan canceled this bombing.

Submarine I-401 at sea. Photo Wikimedia Commons

The winners studied the captured submarines, but did not save them. From April to June 1946 the ships I-400, I-401 and I-402 were used as targets for firing. As a result of these exercises, three unique ships went to the bottom. Two unfinished boats were dismantled.

Reasons for failure

The Sentoku-class submarines were developed and built longer than they served. In addition, for several months of service, they never made full-fledged campaigns - and did not participate in battles. Thus, a complex and ambitious project did not give any results, except for demonstrating the fundamental capabilities of shipbuilding.

Submarine I-400 on board the American floating base USS Proteus (AS-19), August 29, 1945 Photo Ww2db.com

The main problem of the project, with which other shortcomings and difficulties were directly related, can be considered a mistaken concept. Placing strike aircraft on a submarine can provide some advantages, but it introduces many limitations and difficulties. It is because of this that "Sentoku" turned out to be too large and heavy, as well as difficult to manufacture and operate. In addition, the hypothetical potential was reduced due to the small number of aircraft and ammunition on board, as well as due to the specifics of their use.

The start of the construction of aircraft carrier submarines coincided with the period when Japan first faced serious resource and industrial capacity shortages. For this reason, the series of 18 boats was reduced several times, and in the end it was possible to build and commission only two aircraft-carrying submarines and one underwater tanker. The combat value of such a "powerful" group was dubious.

The Proteus floating base (only part of the superstructure is visible on the left), submarines I-400 (left) and I-14 (right). Photo Ww2db.com

Finally, in the last months of the war, the Japanese command found itself in an extremely difficult situation. Not having the desired grouping of ships, it tried to carry out decisive and even adventurous operations. However, throwing between different plans led to the fact that a number of operations did not have time to prepare and carry out in time - and the surrender put an end to all plans.

Place in history

Thus, the Sentoku submarines were built on the basis of a dubious concept, were too complex and few in number, and were not used correctly. All this did not allow them to become full-fledged combat units and cause at least some damage to the enemy. On the contrary, I-400 and I-401 helped the American sailors to practice the issues of taking and studying trophies, and also provided shooting training.

However, "Sentoku" found their place in stories - not least because of their failures. They turned out to be the largest, heaviest and most useless submarines of the Second World War.

Comments

[The leader of the Redskins]

I read a monograph about "seiran". Translated as "mountain fog".

[NDR-791]

The start of the construction of aircraft carrier submarines coincided with the period when Japan first faced a serious shortage of resources and industrial capabilities.
Maybe it's all a "small complex"? Struggling to build the biggest boat, the biggest battleship, the biggest aircraft carrier (sunk unfinished).

[igordok]

Tell me how the plane took off? Like a seaplane or on "rails" on deck?

[Macsen_wledig]

Quote: igordok

Like a seaplane or on "rails" on deck?

"Rails" is a catapult

[beeper]

Quote: igordok

Tell me how the plane took off? Like a seaplane or on "rails" on deck?

From the submarine, the japanese bomber took off on rails with a ramp, along an aircraft carrier (the mother submarine was in full swing, heading against the wind), and landed on the water, upon returning, on floats. Then he climbed aboard by crane.

All three aircraft-carrying submarines of the United States were hidden from the Soviet allies and, after a short study, was hastily drowned in order to avoid legitimate allied demands to share trophies with the USSR, since they did not plan post-war cooperation and considered it as their next goal (which should not be given advanced developments and submarine technologies! )!

[Avior]

By agreement, all Japanese submarines, without exception, were to be sunk, and were not included in reparations.
so there could be no demands on Japanese submarines from the USSR in terms of reparations.
According to the German submarines, all were subject to sinking, except 30, which were divided equally between the three sides.
Reparations in the Japanese fleet included 134 ships, divided into 4, including China, of which 34 were received by the USSR.
Here is more
https://cyberleninka.ru/article/n/yaponskie-i-germanskie-voennye-korabli-peredannye-sovetskomu-soyuzu/viewer

[beeper]

Avior, at one time in sufficient detail (after reading the memoirs of a Japanese submarine commander published in the USSR and knowing about "kaiten" and their tactics from other publications and stories of my father), he was interested in and studied everything related to the actions, design and weapons of Japanese submarines (in including the modern then-class "Yusio").
At the same time, I came across a description of the I-400 aircraft carriers, and in the Soviet magazine "Tekhnika-Molodyozhi" they were published about them.
In the same way that the Soviet command informed the allies in the anti-Hitler coalition about the captured enemy weapons and facilitated their study (for example, they handed over to the British and Americans the latest German homing torpedoes found on the U-250 submarine!), The Americans were obliged to inform the USSR about the unique captured Japanese submarine aircraft carriers and provide the opportunity to explore them!
And there was something to look at - the Japanese designers worked real miracles, cleverly solving the most difficult technical problems (not to mention their 650-mm torpedoes!)!
Suffice it to say that, first used by the Japanese on the I-400, the innovative twin strong hull was repeated several decades later by Soviet designers in the world's gigantic Typhoon submarine missile carriers!
And in the post-war projects of Soviet diesel amphibious and aircraft-carrying submarines, knowledge about the design of Japanese super-large submarines of the same type would be very useful!
Therefore, in order not to contribute to the progress of the Soviet submarine shipbuilding, the American "allies" concealed the fact of the capture of the sisterships of the "I-400" project and quickly destroyed them!
IMHO

[Avior]

They had information on electric torpedoes without it, they made a trap.
and our transfer of the sample was delayed almost until the end of the war. And it was during the war, not after, which are completely different things.
But seriously, there is an agreement on the sinking of Japanese submarines - the Americans sunk them.
If there was an agreement that should have been transferred to the USSR for study, I will read it with interest, let's link.

[Macsen_wledig]

Quote: pishchak

From the submarine, the Japanese bomber took off on rails with a ramp, along an aircraft carrier (the mother submarine was in full swing, heading against the wind),

The rails with the ramp were called "Catapult Type 4 Mod. 10".
The length of the launch track is 27 m. The aircraft speed at the start is 34 m / s with an acceleration of 2,5 g.

[beeper]

Quote: Macsen_Wledig

Quote: pishchak

From the submarine, the Japanese bomber took off on rails with a ramp, along an aircraft carrier (the mother submarine was in full swing, heading against the wind),

The rails with the ramp were called "Catapult Type 4 Mod. 10".
The length of the launch track is 27 m. The aircraft speed at the start is 34 m / s with an acceleration of 2,5 g.

Massen_Wledig, but do I somehow object that the rails with the ramp are part of the boat catapult, I just wrote "japanabomber took off on rails with a ramp-on an aircraft carrier"??!
After all, so that after such a short deck launch (this is not a lightened wooden "whatnot" by the American Eugene Eli, if someone did not know and in childhood did not read "Model-Constructor" - a brave pilot who for the first time in the world took off from 25 m of wooden flooring on ship and which laid the foundation for aircraft carrier aviation) piston combat seaplane due to one motor accelerate and take off (with full tanks and bomb load!) It is impossible to do without a catapult (or jet powder boosters) - it's a no brainer!
When I wrote my comment, I haven't seen yours yet, it happens, you write, you paint, then you edit, you send, and someone has matured earlier and already, albeit briefly, conveyed the essence (as you are in this case)!
And when I updated the page and read your comment that appeared, I realized that I would also need to clarify exactly about the catapult (although its two drive pulleys are already very clearly visible in the photo of the sub's deck!).
But the editing time has expired, so I did not consider it necessary to add anything, tk. our two comments gave quite an adequate answer to the question aka Igordok.
In general, I am very lazy to "google" and usually use only my memory, because in my life I have read a lot and learned personally. I know that there will always be more meticulous commentators who will write more accurately and in more detail in the details.
Thank you for clarification!

[Macsen_wledig]

Quote: pishchak

Thank you for clarification!

Understood, and well ...

[petrol cutter]

Well, what to say ...
As for shipbuilding, the topic is interesting and the real experience of building vessels.
For shipbuildings, in general, someone's practical experience is always interesting. Bo theory is one thing, on iron usually another picture comes out ...
The military has long been without interest ...
Although, how to say ...

[Avior]

The allies were not interested in the technology of the Japanese submarine fleet, all Japanese submarines had decided in advance to sink by agreement, so it is not surprising that the Americans drowned them.
German submarines are another matter, some of them were divided among themselves.

[petrol cutter]

Probably.
But nevertheless, it cannot be that the drawings and documentation are not studied. I will never believe it.
Considering that the Japanese are far from the worst designers and technologists.

[Kostadinov]

By the time the construction of Sentoku was completed, it became clear that a successful attack on the continental United States was simply impossible. If the submarine-aircraft carrier and managed to approach the line of launching the aircraft, then the air defense would not let them pass to important targets.

This is not entirely true or not at all. If it launches aircraft about 100-150 kilometers from the target at low altitude, their interception is very unlikely in 1945.
And the targets for which these planes were created and beat the bases of the US fleet. A new Pearl Harbor was being prepared with the participation of several such submarines.

[deddem]

No.
Once again, it slipped past the author's attention that the I400 was the limit of technology development, and before it the Japanese had as many as three series of aircraft-carrying submarines, which were actively used in the database and even for strikes on the territory of the United States (albeit anecdotal).
But, as is usually the case with translated articles from American popmehs, these episodes dry up to the hapless I-14, "from a different project."