At the end of 1942, the Japanese strategists faced the need to urgently respond to the American unlimited submarine war in the Pacific. A particular case of its consequences was that the Japanese fleet could not ensure the transition of supply transports to the island Japanese garrisons. American submarines and partly aviation made this either very difficult or impossible. This problem manifested itself especially clearly during the battles for the Solomon Islands.
The Japanese intended to solve this problem through technical innovations. Separately rational, they eventually led to a weapon system that could only be described as a technical curiosity. She, nevertheless, was quite "working", and only the negative course of the war for Japan did not allow to show this.
Formulation of the problem
The Japanese acted rationally. What threats are there for transport ships? The main one is submarines, and the second most important (which turned into the first in places of intense battles) is aviation. What means of sea transport is in itself or generally invulnerable in relation to submarines and aircraft, or is it hardly vulnerable? The answer is their submarines. And this is so, in those years the capabilities of aviation to defeat them were limited, submarines could also hit them only when the targets were on the surface.
The Japanese had their own submarines, and they had them in considerable numbers. Therefore, the decision was immediately obvious - to use the submarine as a transport, and not a combat weapon. In principle, not only Japan did this, there was nothing special in this approach.
There was, however, another problem - the time under unloading. The sub is quite vulnerable when surfaced and drifting. And it takes a lot of time to unload the delivered property - the submarine is not a steamer, everything must be carried with your hands through the hatches.
This was especially evident on Guadalcanal, where a lot of equipment and military equipment were destroyed by the Americans on the shore.
At that moment, somewhere in Japan, someone again showed the ability to simple logical thinking. Since the boat is vulnerable near the coast during loading, then it is necessary to carry out the loading either somewhere in the sea, where the enemy does not wait, or near the coast, but not where he will look for transport ships. The second option logically required the presence of a floating craft on board the boat, on which it was possible to reach the shore.
The next logical step is that on many islands the boat is unable to land on the coast due to the combination of terrain and currents. And the coast is vulnerable too. The cargo should not be unloaded on the shore, but not stopping to be transported deep into the territory. And also - the task is to build supply chains not according to the "ship - island" scheme, but "island - island". All of this taken together excludes boats and boats. What's left?
What remains is a tracked vehicle of high cross-country ability, capable of getting ashore on soft ground or through deposits of sand, small heaps of stones, steep ascents and immediately leave the open shore with a load. This solution was also suitable for moving from island to island. We just need to make sure that this floating vehicle can be carried on submarines!
This is how a somewhat unique example of military equipment was born - a large-capacity tracked conveyor delivered under water to deliver cargo from a submarine to the shore. True, this exotic does not describe what tasks these machines were supposed to solve at the end of the war. But first things first.
Ka-Tsu
The development of a new transporter began in 1943 by Mitsubishi, and preparation for serial production was under the leadership of the naval officer Hori Motoyoshi at the Kure naval base. By the fall of 1943, the car was tested and, in principle, confirmed the characteristics laid down in it. The vehicle was put into service under the name "Type 4" Ka-Tsu ".
The car turned out to be large - length 11 meters, width 3, 3 and height 4, 06. The curb weight of the car was 16 tons. The armament consisted of a pair of 13 mm machine guns on rotary mounts, which, at the same time, between the machine guns there was a "standing" cockpit for machine gunners. In total, the crew consisted of five people - a commander, a driver, two gunners and a loader. The engine from the amphibious tank "Type 2" Ka-Mi ", a 6-cylinder air-cooled diesel engine" Mitsubishi "A6120VDe, 115 hp was taken as a power plant. The total carrying capacity of the vehicle was 4 tons. The power-to-weight ratio was thus approximately 5.75 hp. per ton, which was very little. Instead of cargo, the car could carry up to twenty soldiers with weapons.
The speed of the car on land could only reach 20 kilometers per hour, and on water up to 5 knots. In order to provide the necessary stability and weight distribution, and due to the low-power engine, the Japanese engineers had to abandon the car's reservation - a certain amount of armor plates 10 mm thick were used to protect the cockpit, but in general the car was unarmored.
On the water, the car was driven by a pair of propellers. "Ka-Tsu" was equipped with a special device that allows the crew to switch the drive from tracks to propellers and vice versa.
The most specific feature of the machine was its ability to be transported, being attached to the hull of the submarine from the outside, and after surfacing, it was brought into working condition. For this, the engine was enclosed in a hermetically sealed capsule, was equipped with devices for sealing the intake tract and the exhaust system.
The electrical wiring was sealed and insulated in the same way.
The vehicle's suspension was also assembled from the components of the Type 95 serial tank. It was the use of standard components that made it possible to develop, test and launch this machine into production in almost a year.
In March 1944, tests of the first three prototypes were completed.
According to the test results, which turned out to be quite successful, the Navy planned to build 400 of these machines.
However, much to the disappointment of the Japanese, the Americans rather quickly took by storm from the sea those islands that the Japanese needed to supply. The concept of a self-propelled and floating supply vessel has sharply lost its sharpness - the US Navy took those islands for work on which the "Kat-Tsu" was originally intended.
But by that time another job had been found for them.
Atolls
As the war drew near to the Japanese islands, the issue of naval basing arose for the Americans. The answer was the atoll lagoons turned into docks. Some were large enough to house hundreds of ships. So, for example, the lagoon of Ulithi atoll made it possible to place up to 800 warships. The Americans immediately began using these islands to avoid having to drive ships to Pearl Harbor for repairs. All the necessary materials were delivered there, floating docks and ships of the floating rear were transferred.
Defensive positions were also equipped, primarily obstacles of various types, in order to exclude the actions of Japanese submarines. Coastal artillery was also deployed. The Japanese tried to attack such places, but they had nothing to do with it - they could not talk about a breakthrough of aviation to such a number of fighter carriers, the fleet was badly battered, and the passages to the lagoons themselves were guarded.
And then some of the Japanese commanders had an original idea.
The submarine cannot enter the lagoon. But you can always find a place that, due to its unsuitability for mooring to the shore, is not kept under continuous surveillance. And there it is necessary to launch some kind of striking agent from the boat. Since this percussion agent does not pass through the channels into the lagoon, it must pass overland. So it must be an amphibious vehicle on tracks. But how to hit surface ships? Torpedoes are needed for their guaranteed defeat!
Conclusion - a tracked amphibious vehicle, which will pass into the lagoon with American ships on the ground, must be armed with torpedoes.
"Ka-Tsu" was the only option suitable in terms of carrying capacity. Thus began a project that occupies a unique place in the history of military technology - a floating combat tracked vehicle designed to carry out sabotage against surface ships, regularly delivered to the target under water, attached to the submarine's hull and armed with torpedoes.
The Ka-Tsu received 45-cm Type 91 torpedoes as the "main caliber".
Tests carried out in the first half of 1944 showed that although the vehicle with torpedoes on board has poor stability and speed, their launch on target is not difficult. After that, the "Ka-Tsu" for some time became part of military planning.
For the delivery of tracked torpedo bombers, the Japanese adapted five submarines - I-36, I-38, I-41, I-44 and I-53. The first combat debut of combat vehicles was to be Operation Yu-Go - an attack on American ships in the lagoon of Majuro Atoll, Marshall Islands.
When planning the operation, fears were expressed that tracked vehicles afloat could perform worse than expected, and the Japanese were also worried about the time to bring the engines to readiness for launch - the realities of 1944 were very different from the first stage of the war and the time factor was very critical. At the same time, it was quite possible to go to the shore of the atoll on tracks, unlike other options.
Operation "Yu-Go", as we know today, did not take place. "Ka-Tsu" did not prove themselves as torpedo bombers. Their release was stopped on the 49th car out of 400 planned. At the very end of the war, the Japanese command was considering the option of somehow using them in kamikaze attacks if the Americans would land in the metropolis, but Japan surrendered earlier. As a result, the abandoned Ka-Tsu went to the Americans in the port of Kure without a fight.
These machines were of no interest to them.
To date, there is only one surviving copy of the "Ka-Tsu", of those machines that did not have time to convert into torpedo bombers. For a long time, it was stored in the open air at the US Marine Corps depot in Barstow, California. Today, this vehicle, still in poor condition, is on display at an amphibious armored vehicle exhibition at the US ILC Camp Pendleton, California.
Despite the very unusual idea of combat use, "Ka-Tsu" cannot be considered a delusional project. This is an example of how extreme circumstances force a person to resort to extremely non-standard, unusual solutions. And an example of the fact that, no matter how unusual these solutions may be, they may well turn out to be “working” if they are brought to life on time.
