Of all the pre-war and wartime secrets concealed by the Japanese Admiralty, none were so assiduously guarded nor as diligently protected as those pertaining to submarine development and construction. So carefully conserved were these facts that even today much of interest relating to Japanese submarines is unknown to the general public.
It would of course be impossible to examine completely and thoroughly each and every detail regarding Japanese pre-war and wartime submarines in a paper of this size, for, to do it proper justice the subject would, Perhaps, require the scope of a small book. Specifically three salient features are to be reviewed here: first, the midget submarine Program, including a complete description of those vessels which participated in the Pearl Harbor attack; second, a description of the I-201 class of submarines whose characteristic features made them the immediate Predecessors of the United States “Guppy” design submersibles; and third, a description of the I-400 class submarines—the possible forerunners, insofar as size is concerned, of the submarines of the future.
Midget Submarines
On December 7, 1951, ten years to the day of the assault on Pearl Harbor, there appeared in several publications reviews of this attack, which reviews laid particular emphasis on the role played by the midget submarines. All without exception expressed regret at their being unable to provide anything more than the most superfluous description of these small vessels.
The midget submarine employed in the Pearl Harbor attack were units of the first of five classes of such submersibles built by Japan during and just prior to World War II. These classes, known as A, B, C, D and Kairyu, were built in almost complete secrecy and in very considerable numbers, although it must be admitted that substantially more were projected than were ever completed. In all, 1,278 midget submarines were projected, of which 438 were actually completed. Our main interest is centered however in the class A submersibles, of which a total of sixty-two were completed.
The story of midget submarine development in Japan begins in 1934 when two of these vessels were constructed at the Kure Naval Dockyard for experimental purposes. The underlying purpose behind these experiments was to develop an auxiliary weapon which would take part in decisive fleet actions, based in the hangars of the Chitose class seaplane tenders. These boats had a cigar shape and were without a conning tower of any description whatsoever. Officially they were known as “A-targets” (A-Hyoteki in Japanese).
Throughout 1934 and 1935 extensive tests were undertaken and carried out with the net result that in 1936 two additional units were constructed, again at Kure. These vessels had small conning towers but were in all other respects identical to their predecessors. Tests with these vessels were held in 1937 and complete success was achieved. Included in these exercises were several “push-off” experiments from the Chitose, which proved the feasibility of their utilization in long range operations. As a result of these highly successful experiments, the Japanese Admiralty decided to develop their midget submarine program, and, to facilitate this project, constructed a special factory at Ohurazaki, near Kure, especially for this purpose. This building, a branch of the Kure Navy Yard, was completed in 1940, and its existence and location was intended and chosen to promote the highest possible degree of secrecy. Factory personnel were especially selected for the task of producing these craft. Fearful of the instant application of the penalties prescribed for violations by the Military Security Law, they kept the secret well.
In the summer of 1941 the seaplane tender Chiyoda, a sister to the Chitose, was converted to a midget submarine carrier at Kure. As reconstructed she had a capacity of twelve boats—all stored in her hangar and launched through hinged doors at the stern. Tests run with this vessel determined that under ideal conditions—in a calm sea and using highly trained personnel—all twelve boats could be launched within seventeen minutes. At this time, too, several of the I-16 class fleet type submarines were refitted as “mother ships” for midget submarines.
Five of these class A boats participated in the Pearl Harbor attack, being transported to the point of attack by I-16 class fleet submarines. Boats of this class, using the same source of transportation, also made up the groups which assaulted Diego Suarez and Sydney. Likewise they constituted important segments of the local defenses in the Aleutian and Solomon Islands. Their unsuitability to the latter type of operations was directly responsible for the later and larger types of these submarines.
Their specifications reveal several interesting facts. Each class A boat had a submerged displacement of forty-six tons. Dimensions were 78 1/3 X 6 X 6 feet. For propulsion equipment, these boats were fitted solely with electric batteries and motors but with no recharging facilities; thus, the initial charge being exhausted, these vessels became helpless and powerless. Despite this shortcoming, however, their motors were capable of propelling them at a submerged speed of nineteen knots for a maximum period of fifty minutes. Actually the first two boats built in 1934 were capable of twenty-four knots maximum speed submerged, and the two 1936 vessels with the conning tower were capable of twenty-three knots speed. Additional hull fittings, such as propeller guards and “jumping wires,” prevented the others from making better than nineteen knots.
All vessels were armed with two 45 cm. torpedo tubes and carried two torpedoes. Originally, the torpedo tubes were of the floodable type, but later on, most boats, including those participating in the Pearl Harbor attack, were fitted with caps to their tubes. Each boat carried a crew of two men. (Later vessels of other classes carried crews as large as five men. Because of torpedo shortage, some of these later vessels were equipped instead with 600 kg. of explosives fitted in the bow.)
Though over 400 vessels of this type were constructed, the Japanese Admiralty never acknowledged that it possessed midget submarines and never designated them by that name in their despatches. The Pearl Harbor assault craft were referred to in the war communiques as “Special Submarine Boats.” Other designations foisted upon the uninformed in order to preserve the most intense secrecy were “A-Target”, “H-Metal fittings”, “M-Metal fittings”, “Anti-submarine bombing target”, Model T.B.”, “Model B”, and finally Koryu (dragon with scales). Despite this secrecy, however, the results achieved by these submersibles were meager, and they will be chiefly remembered if at all, for their constructional characteristics.
I-201 Class
The I-201 class of fleet type submarines were perhaps the most important underseas vessels constructed by the Japanese during the war, since, besides being a thoroughly feasible operational type for the immediate present, they were also forerunners of future submarine types and represented a radical departure in design and in theory from conventional submarines.
In the planning and formulation of this design, which was intended to provide the Imperial Navy with a fleet of high underwater-speed submarines, there is some evidence of cooperation and exchange of information between the two Axis’ partners, Japan and Germany. This is evident since it is inconceivable that both nations should seize upon the same theory regarding submarines at practically the same instant. The extent of this exchange of plans and data is not at all clear and, in fact, may have been limited to the bare exchange of ideas, but it is most probable that there was some degree of kinship between this Japanese type and the late German designs.
Once the basic and novel theory that a submersible’s maximum performance should be attainable while submerged has been accepted, the major problem of how to achieve this desirable end remained to be solved. In its solution two principal theories were employed—the increasing of the power of the underwater propelling machinery and the reducing of underwater resistance by radically fairing the hull and superstructure. Thus contrived, the new submarines had a designed submerged speed of twenty knots.
Insofar as particulars are concerned, the I-201 class submarines had a standard surface displacement of 1,291 tons and possessed the following dimensions: 259 1/6 X 19 X 15. It will be observed that the beam is extraordinarily small for such a length. This was a further device employed to reduce underwater resistance. Propulsion was provided by two Diesels which produced 2,750 S.H.P. for surface power and 5,000 S.H.P. for submerged power. To facilitate this largest ever submerged power supply, large motors and powerful batteries were fitted in each vessel. Also new lightweight Man diesels were employed. The designed speed for this class was fifteen knots surface speed and, as before mentioned, twenty knots submerged speed. The latter was not realized on trials, however, nineteen knots being the maximum obtained. Maximum underwater speed could be maintained for a period of fifty-five minutes after which the submarine was capable of further movement for twelve hours at three knots. Maximum surface radius was 5,800 miles at fourteen knots, while the maximum submerged radius was 135 miles at three knots. All boats were capable of diving to 360 feet.
Despite the desire for a high underwater speed and the careful minimizing of underwater resistance through the elongated and faired hull and conning tower, there was still some surface armament carried. Each boat was equipped with two 25 mm. AA guns mounted on the deck just before and just aft of the conning tower. Perhaps had these mounts been eliminated these submarines would have achieved their designed submerged speed or even exceeded it. Torpedo armament consisted of four 53 cm. bow tubes. Each boat had a capacity of ten torpedoes.
Faith in this new design was considerable, and as a result the Japanese Admirality projected a total of 100 of these novel craft. Due, however, to the adverse progress of the war, material shortages, and the lateness of the program, only eight were actually laid down, and of these only three were completed —I-201, I-202, and I-203. All of the eight laid down were built at Kure. The program got under way on March 1, 1944 when the keel of the I-201 was laid down. Electric welding was widely employed in their hull construction, and for the most part these vessels were built in shops with the completed section being assembled on the stocks. As a result of this the elapsed construction time on these vessels was comparatively short, with one boat, the I-202, being completed in ten months.
No operational statistics are available on any of this class, and it is doubtful if any of them ever made a war patrol. It is certain, however, that if operations were conducted they failed to achieve any conspicuous success. The five incomplete vessels, as well as the I-202, were destroyed by Allied forces following the surrender, while the remaining two boats were handed over to the United States Navy by whom, it is presumed, they were likewise despatched. What, if anything, was learned from these prizes has not been revealed.
Completed too late in the war to have their potentialities adequately tested and evaluated under the stress of actual combat, the I-201 class were nevertheless an extremely interesting group of underseas craft and representive of one of the most significant recent developments in the field of submarine construction.
I-400 Class
Even as the Japanese Yamato and Musashi were the largest warships of their type ever constructed, so were the Japanese I-400 class submarines the largest underseas craft ever built. Their existence, which first became known to the world in September 1945, following the surrender, caused a considerable stir in naval circles throughout the world, and the boats themselves received a certain amount of publicity as a result of this attention. It is not believed, however, that the complete story regarding their construction had been revealed.
In 1942, under the Fifth Fleet Replenishment Program, there were authorized a total of eighteen super-submarines designed to be the largest undersea craft in the world. Work on these vessels commenced immediately, and the I-400 was laid down at Kure on January 18, 1943.
As first designed this new class was to have a standard surface displacement of 4,550 tons and would be able to carry two aircraft. During the process of construction, however, for some reason the plan was altered and the design was considerably enlarged. At that time the surface displacement was set at 5,220 tons standard and the aircraft capacity was increased to a total of three. So altered, the vessels proceeded to completion.
Although the original program called for eighteen boats, only a total of five were ever laid down. The I-400 and I-404 were constructed at Kure; the I-401 and I-402 at Sasebo; the I-405 at Kawasaki, Kobe. Of these only the I-400, I-401 and I-402 were fully completed. The first two vessels were handed over to the United States Navy following the surrender, while the I-402 was despatched by Allied Forces outside of Sasebo Harbor on May 1, 1946. Of the remaining two submersibles, one, the I-400, never reached the launching stage, while the other was sunk by U. S. Third Fleet carrier aircraft at Kure on July 28, 1945, when ninety-five percent complete. At the end of the war the operational boats were enroute to attack the Ulithi area. So far as is known, no attacks were ever carried out by any of these submarines, though one, and perhaps several, sorties were made by them during the course of the war.
Aside from the particulars relating to tonnage and aircraft capacity which have been mentioned before, there were several other features of these monsters which deserve attention. First and perhaps foremost were their dimensions: 400 1/4 X 39 1/3 X 23. Following this, the most spectacular figures related to the propulsion machinery which consisted of four diesel engines. There were also two sets of Vulcan Hydraulic couplings employed. These engines produced 7,700 S.H.P. on the surface and 2,400 S.H.P. submerged, which power was capable of producing a surface speed of 18.7 knots and a submerged speed of 6.5 knots. Radius was phenomonal, at least as far as surface performance was concerned, for these submarines possessed a radius of 37,500 miles at 14 knots, and one of 30,000 miles at 16 knots. Submerged radius was something else again, the figures being a modest 60 miles at 3 knots.
Armament as might be expected was fairly heavy. This class mounted one 14 cm. AA gun located aft as well as three 25 mm. AA machine guns in one mount and two twin 25 mm. AA machine gun mounts. Also three aircraft were carried in hangars fitted amid-ships. The one and only catapult, a long one, was fitted forward of the bridge and reached to the bow. The bridge and conning tower were placed portside so that the space on the starboard side could be used as hangar space. Torpedo armament consisted of eight 53 cm. bow tubes. Twenty torpedoes could be stowed aboard. In some boats dummy funnels were fitted behind the bridge for camouflage purposes. Complement was 144 officers and men.
Here again, as was the case with the I-201 class vessels, the monster I-400 class boats will be principally remembered for what they were and not for their accomplishments. What they were, however—and the shadow cast by their immensity may well project itself on future submarine designs— is sufficiently notable to warrant due attention and notice.
One of the most encouraging aspects of the histories related here is the fact that none of these submarines fell into Soviet hands. What there was to be learned from these classes, then, is the sole property of the Western Allies. That there should be something of value seems indicated by the preceding pages, for, regardless of operational results obtained, these submarines were functionally superior to all but the latest German models. In short, these classes were significant contributions toward the progress of submarine development.