The name ‘Kaiten’ comes from the Japanese ‘kaiten igyo’ which means ‘a great undertaking’. Given the desperate situation of Japan in 1944–45, faced by the overwhelming strength of the US Navy and threatened with invasion of the home islands, the Imperial Japanese navy (IJN) turned to extreme measures. The airborne kamikaze pilots are well known, but if there were not enough aircraft and not sufficient trainers, the young Japanese recruits were drafted into other branches of the suicide corps.
The lethal performance of the 24in Type 93 torpedo was legend. The problem now was how to bring sufficient Type 93s to bear to have a significant effect. Traditional Japanese submarine training had stressed the need to attack enemy warships, in order to whittle away the severe numerical disadvantage under which Japan fought. Now the stress was to be on sinking attack transports headed for invasion beaches. But Japanese submarines were all too easily detected and sunk by the Americans whenever they put in an attack. The answer seemed to be to put a pilot in the Type 93 torpedo, to guide it to its target. The pilot was initially given the option of baling out of his craft when its fuel ran out, but since the thought of being captured was felt to bring dishonour, many Japanese chose suicide when approached by would-be American rescuers. In later sorties, the entry hatch was bolted shut from the outside.
Many casualties also occurred during training. The trainees would be carried in a motorboat at speed, having to acclimatise themselves by peering through the type of periscope which they would use in their Kaiten. Once proficient in navigating using this primitive aid, they would begin training on a Kaiten fitted with an exercise head. This had an automatic blowing device if the trainee dived below a preset depth. The final part of their course involved ramming the Kaiten into a target vessel. Unfortunately, all too often the shock of impact not only badly damaged the Kaiten but also seriously injured its trainee pilot.
Kaiten were intended to be launched either by carrier submarine, from cruisers and destroyers via a rear ramp, or from a ramp or slip on land. Only the former system was ever used operationally.
Kaiten Type 1 Specifications:
Length: 14.5m (45ft)
Diameter: 1m (3.05ft)
Payload: 1550kg (3410lbs) HE
Total weight: 8 tonnes (8.05 long tons)
Speed/Range:
78,000m (85,332yds) at 12 knots
43,000m (47,042yds) at 20 knots
23,000m (25,162yds) at 30 knots
The Kaiten Type 1 was the only type used in actual combat. It was produced by the simple expedient of bolting the Type 93 Model 3 torpedo, less warhead but with larger fins and rudders, into the rear of a new one-metre (39in) diameter forward section, comprising the cockpit and warhead. A conical section faired the connection. The warhead held 3410lbs (1550kg) of explosive and was capable of inflicting devastating damage. The pilot held in his hand the pistol handle for the electric detonator. At the moment of impact with the target his weight was thrown forward, closing the switch.
The torpedo was fitted with an electric gyroscope which was preset by the navigator of the carrier submarine, to despatch the pilot in the direction of a previously detected enemy vessel. Control was extremely difficult. At over 5 knots the pilot’s view was obscured by spray, so a special fuel reduction valve had to be installed to produce a minimum speed of 5 knots. But then as the oxygen fuel was used up, the weight of the torpedo decreased, control became more difficult and it was impossible to submerge. So the pilot had to be provided with manual means of introducing and purging water ballast to maintain his chosen depth and trim. Normally the Kaiten ran at a nose-down angle of between 1 and 3 degrees.
The Kaiten would be carried on wooden blocks on the deck of the mother submarine, secured by a clamp ring. A tube connected the Kaiten’s cockpit with the interior of the submarine, through which the pilot would pass, after which the hatch would be bolted shut.
The mother submarine at periscope depth of 9m (30ft), gave final target instructions to the pilot by telephone. At between 6000 and 7000m (6500–7700yds) from the target, the Kaiten was then released. If carried forward of the conning tower, the pilot started his engine before release to avoid being run down by the submarine. The Kaiten’s gyro automatically followed the set course, at 6m (20ft) depth for a predetermined period of time before surfacing, bringing the target to about 1000m (1100yds) range. The pilot took over control from the gyro, adjusted his course to strike the target amidships, and set the depth according to his estimate of the ship’s draught. After running the 1000m, if he missed he could turn and come in again for a second attempt.
Many attacks had to be called off when the mother submarine was within launch range, because the Kaiten was found to be unserviceable. When in enemy-dominated waters, the mother submarine remained submerged during the day at depths of 30–40m (100–130ft), and since the Type 93 had not been designed to operate for long periods at such pressures, its engine compartment became waterlogged. Leaks also occurred at the body join, and around the entry hatch.
A total of 130 Kaiten pilots died in action, and a further fifteen in training accidents. In addition, over six hundred crewmen died in eight mother submarines sunk during Kaiten operations. In exchange for these heavy casualties, Kaiten scored only two confirmed sinkings, plus several ships damaged. On 20 November 1944, fleet tanker USS Mississinewa was struck by a Kaiten, probably launched from submarine I 47 in Ulithi anchorage; sixty-three men were lost in the sinking. (A colour photo of her on fire is at NHCC, photo ref K-5510. For nearly sixty years the wreck lay on the bottom, posing a considerable environmental hazard, until in February 2003 a US Navy salvage team recovered nearly two million US gallons of oil.) On 24 July 1945 the destroyer escort USS Underhill was escorting a convoy to Leyte when several sonar contacts were reported. PC 804 claimed a successful depth-charge attack on a submarine, but her depth charges had blown to the surface two Kaiten, close aboard on either side of Underhill. Her captain shaped to ram the Kaiten to port, when Underhill was blown in two by massive explosions, killing 112 officers and men.
The Type 2 Kaiten was an attempt to gain more range and speed with a larger payload, using hydrogen peroxide as fuel. Benefiting from German technical advice, the IJN experimented for some time with this dangerous fuel, achieving an engine output of 1500bhp, from a double-bank engine layout of two sets of four cylinders side by side, with two crankshafts geared together at the rear. Accepting that there would be insufficient time left to complete this ambitious project, it was abandoned in March 1944. The work done was not wasted, however, since the basic hull layout and engine were adopted for the Kaiten Type 4.