Type 4 Ka-Tsu

The special Type 4 Launch Ka-Tsu (特四式内火艇 カツ) was a Japanese amphibious landing craft of World War II.

History
Japan's combat experience in the Solomon Islands in 1942 which revealed the difficulty of resupplying Japanese forces in such situations prompted the IJN to commence an amphibious tractor program in 1943, as the Ka-Tsu, which was designed by Commander Hori Motoyoshi of the Kure Naval Yard.

The Ka-Tsu's primary purpose was to transport cargo ashore and it was unarmored apart from some light shielding for the crew. Its engine compartment and electric final drives were hermetically sealed, as it was intended to be launched from a submarine.

The first prototype was completed in late 1943 and trials were conducted off Kure in March 1944. By the time that development had been completed, it was proposed that the Ka-Tsu be used to attack US battleships anchored in atolls (such as Ulithi) which could not readily be attacked using conventional means. It was proposed that a Ka-Tsu armed with a pair of torpedoes be dropped off by submarine away from the atoll, propel itself to the outer reef using its tracks, and then enter the lagoon on the inside of the reef. Tests were successfully carried out on a modified Ka-Tsu but the war ended before any such mission could be mounted.

Operation Yu-Go
As part of the proposed Operation Yu-Go, a Pearl Harbour–style surprise attack raid on American shipping at Majuro atoll, five IJN submarines - I-36, I-38, I-41, I-44, and I-53 - were to be modified to allow each to carry four torpedo-armed Ka-Tsu vehicles. Initial landing trials, however, revealed many deficiencies of the Ka-Tsu, most of which were caused by its severely underpowered engine. In addition, after the carrying submarine surfaced, it was found that at least 20 minutes were necessary to remove the Ka-Tsu's watertight engine covers and launch the tank.

Full-scale landing trials were carried out with a submarine carrying two Ka-Tsus. These tests revealed that the Ka-Tsus were noisy, very slow in the water, and their tracks tended to slip if they encountered even the slightest obstacles. In addition, it was also discovered that their engine compartments were not completely watertight; the covers tended to leak underwater, eventually leading to the vehicle's entire engine section becoming flooded. In response to these criticisms, the Ka-Tsu's designer pointed out he had been asked to design a "pure” amphibian which was suited to neither underwater transport nor carrying a pair of full size torpedoes (each of which weighed one ton).