In the final months of WW2 the Imperial Japanese Navy was facing a situation they once thought impossible. Their mighty fleet had been destroyed, their aircraft carriers and battleships sunk by US torpedoes and bombers.
Worse, the inviolable Japanese homeland was herself under threat. On one thing the Japanese were clear: the US Navy could not be allowed to reach Japan, and in her defense everything should be considered.
This is the thinking which led to the Kamikaze attacks on US warships, Japanese pilots willingly crashing into the US fleet and committing suicide in order to bring death and destruction to the enemy. And this is the thinking which, in a similar vein, led to the Kaiten.
In many ways the Kaiten was an even more horrific strategy than that of Kamikaze. Japanese engineers modified a type 93 torpedo, equipping it with 1.5 tonnes of explosives and a human crew.
The Kaiten went down in history not due to technological innovation but because of the tragic fate of its operators. Unlike conventional torpedoes, the human torpedo was operated by Imperial Japanese Navy soldiers, who used these suicide crafts to target enemy vessels directly. Once in the Kaiten, there was no escape for the men.
The weapon’s name, “Kaiten,” held significant meaning, translating to “the heaven shaker” or “the turn toward heaven” in English, symbolizing its intended impact on the enemy’s resolve. The torpedo was used by the Japanese military despite its great human cost when they believed they were losing power and their prospects of winning the war.
A Monstrous Weapon
The first Kaiten was an innovative suicide weapon designed by Japanese naval engineers using an existing type 93 torpedo. This was a large torpedo with a warhead containing nearly a tonne of explosives, but all that explosive power would often go to waste as it could easily miss its target.
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To resolve this problem, and to make it even more deadly, it was scaled up and a human crew added to guide the torpedo to its target. This was The Type-1 human torpedo with a larger-warheaded oxygen-fueled torpedo, and 50% more explosives.
A tiny periscope, driving controls, and remote explosion mechanisms for the weapon were all placed in the pilot’s cabin. There were batteries to power these controls, and an air filter inside the craft to prolong the longevity of the pilot.
This Type-1 torpedo comprised impressive specifications, including a substantial 3,420-pound (1,550 kg) warhead, a cruising speed of 12 knots, and a maximum speed of 30 knots. It could operate at a maximum depth of 250 feet (76 m) and had a long range of up to 42 nautical miles (78 km).
Hiroshi Kuroki, a talented young lieutenant and submariner, invented the powerful torpedo, Kaiten. He got the idea of creating the torpedo after the devastating Japanese defeat at Midway in June 1942.
Hiroshi came to believe that Japan needed an important and powerful weapon for winning the war as a result of the Japanese imperial navy being almost completely destroyed at Midway. Therefore, shortly after, he came up with the idea for the torpedo submarine.
Despite the heavy defeat at Midway, Hiroshi’s innovative concept of the Kaiten torpedo faced reluctance from the Japanese high command. His suggestion was at first rejected with sorrow and respect for his devotion to his nation since it was thought to be too excessive within Japan’s warrior culture.
Hiroshi didn’t give up and kept trying for over two years to get the military ministry and his superiors to approve of him. To support his project, he once went so far as to write Admiral Yamamoto, the mastermind of the assault on Pearl Harbor, a letter in his own blood.
The war seemed to go out of hand in Japan, and the future appeared to be extremely hopeless. That’s when Hiroshi was allowed to develop a model for the torpedo. The actual construction of the torpedo started in 1944 at the Kure Naval shipyard, approximately 12 miles (20 km) from Hiroshima.
It was designed to be launched from a submarine, meaning that if the circumstances were right it could attack the US Navy without any warning at all. Torpedo testing started on Tsushima Island, which today serves as the Kaiten Memorial Museum.
And the converted Type 93 torpedo was just the start: over time, Japan developed six distinct human torpedo variations, each based on different torpedo types. However the principal Type 1 based on the Type 93 was the most common, and the most effective.
It is believed that some 420 torpedoes were manufactured in total, each crewed.
The Kaiten in Battle
For these were not some rare or theoretical weapon. The Kaiten saw action in the Pacific, becoming known as the “Long Lance” due to its exceptional range. With a capacity for twice as many warheads and the extraordinary ability to hit targets up to 42 miles (78 km) away while moving at 36 knots, this torpedo significantly outperformed its American contemporaries.
Additionally, it could be programmed to go at an amazing 49 knots for half that distance. No American or Allied torpedo could match the Long Lance’s exceptional performance, and defending against such a fearsome weapon was extremely difficult.
On November 20, 1944, a loud explosion echoed through the Ulithi Atoll in the West Caroline Islands at around 5:45 am in the morning. The USS Mississinewa, an oil tanker stationed near the entrance of the Ulithi lagoon, serving as a vital refueling base for over 200 US Navy vessels fighting against the Japanese in the South Pacific, was directly struck by a new top-secret weapon of the Imperial Japanese Navy.
One of the Kaiten torpedoes’ creators, Lt. Sekio Nishina, took charge of the weapon on a particular morning in November and fired it from a mothership submarine. It is believed that his human torpedo collided with the USS Mississinewa.
The massive 3,000-pound (1,360 kg) warhead of the human torpedo caused a spectacular explosion, creating a major hole in the port side of the Mississinewa. Then, an aviation gas tank’s vapors caught fire and caused another explosion.
Over 100 of the torpedoes would go on to be used in suicide missions against the US Navy. The Kaiten became among the most feared of Japanese weapons, due to its exceptional range, its potential to deal enormous damage, and its terrifying accuracy.
The Imperial Japanese Navy carried out eleven significant human torpedo actions during the course of the remaining fighting. One significant achievement was the July 1945 destruction of the destroyer escort USS Underhill, which resulted in the deaths of 113 crew members.
Additionally, the army’s landing craft LCI-600 was sunk by the torpedo attack, leading to the deaths of three men. Attacks by human torpedoes also caused damage to a number of other ships, causing the US Navy to adopt more defensive tactics for fear of losing their precious fleet to this new Japanese assault.
A 1945-developed Type-4 human torpedo is currently on display at the USS Bowfin Submarine Park & Museum in Pearl Harbor, Hawaii. There were between 45 and 50 Type-4 Kaitens manufactured, but they were never put into service.
But the losses to Japan were severe. In addition to 900 submariners or support staff, the Japanese torpedo program also lost eight motherships of torpedo submarines.
But even though the Japanese believed their strategy could change the outcome of World War II, it was all for naught. They had no answer for the two nuclear bombs dropped on Hiroshima and Nagasaki in August 1945. These even more terrifying weapons would demand their surrender.
Top Image: A Kaiten from the movie The Human Torpedo, 1968. Source: Pouazity3 / CC BY-SA 4.0.
By Bipin Dimri