The Kaiten were manned torpedoes used by the Imperial Japanese Navy during World War II, designed for suicide attacks against Allied ships.

Inspired by the success of kamikaze tactics, the Kaiten program sought to replicate that success underwater, with pilots guiding these modified torpedoes directly to their targets.

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Historical Context

After a series of initial successes following the attack on Pearl Harbor in 1941, Japan’s fortunes in the Pacific War began to wane. By 1944, the strategic initiative had decisively shifted to the Allies, particularly the United States, which had rebounded from early losses and was systematically island-hopping towards the Japanese mainland.

This change in fortunes was the result of several factors. The United States’ industrial capabilities had been fully harnessed for war production, churning out ships, planes, and armaments at a rate that Japan could not hope to match. Additionally, American cryptographers had broken key Japanese naval codes, allowing the U.S. Navy to preempt and counter Japanese movements effectively. These factors, combined with the loss of experienced pilots and attrition of naval forces, left Japan in an increasingly perilous position.

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In response to this unfolding disaster, the Japanese military establishment began to seek out new strategies that could help redress the imbalance. It was within this context of desperation that they turned to more drastic and unorthodox methods of warfare. The kamikaze tactics, in which pilots would deliberately crash their aircraft into enemy vessels, were an embodiment of this shift towards what could be described as a ‘total warfare’ mindset, where no sacrifice was deemed too great in the defense of the homeland.

USS Bunker Hill after being hit by kamikazes.

The Kaiten project was born out of this strategic desperation. It was conceptualized as an extension of the kamikaze approach, utilizing the sea rather than the air to deliver catastrophic blows against the American naval forces. The rationale behind the Kaiten was twofold: firstly, it aimed to exploit the proven effectiveness of the conventional torpedo, and secondly, it sought to address the increasing difficulty of reaching well-defended enemy ships by utilizing human pilots to guide the torpedoes to their targets.

This strategy was also deeply rooted in the cultural and military ethos of Japan at the time. The samurai tradition, with its emphasis on honor and sacrifice, permeated the military culture. The idea of dying for one’s country was not only valorized but was also seen as the ultimate expression of patriotism and duty. The young men who would pilot the Kaiten were seen, and often saw themselves, as modern-day samurai, making the ultimate sacrifice for the Emperor and their country.

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Despite these cultural underpinnings, there was also a grim recognition among Japanese military leaders that the war was not going favorably. The Kaiten and other similar efforts were seen as potential game-changers, a way to even the odds, or at the very least, to inflict significant damage on the enemy and boost the morale of the Japanese people.

Design Of The Kaiten

The original design of the Kaiten drew heavily from the existing Type 93 torpedo, known to the Allies as the “Long Lance.” The Type 93 was already a formidable weapon, famous for its range and destructive power, far surpassing that of the torpedoes used by Allied forces at the beginning of the war. However, the Kaiten’s modifications would transform the Long Lance from an unguided projectile into a piloted vessel of destruction.

A Type 1 Kaiten. Image by Nick-D CC BY-SA 3.0

The first step in this transformation was the addition of a small, cramped cockpit where the pilot would sit. This cockpit contained the necessary controls for steering the torpedo and a periscope for navigation. Though rudimentary by any standard, these controls gave the pilot the ability to make adjustments in the torpedo’s course, thus greatly increasing the chances of a successful strike compared to conventional torpedoes which followed a preset course after being launched.

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Beyond controls for navigation and propulsion, the Kaiten had to be adapted to sustain human life, even if just for the duration of the mission. Oxygen tanks were installed, providing the pilot with a breathable atmosphere while submerged. This modification underscored the tragic reality of the Kaiten’s purpose; it was not designed for the pilot to return. The inclusion of life-support systems was purely to ensure the pilot could complete their mission before the inevitable impact.

Furthermore, the Kaiten’s designers had to consider the balance and buoyancy of the modified torpedo. Adjustments to the ballast tanks were made so that the Kaiten could maintain neutral buoyancy at periscope depth, allowing the pilot to approach the target while minimizing the risk of detection.

A Type 1 Kaiten is launched from a Japanese cruiser during trials.

The explosive charge in the Kaiten was another area where the design reflected its suicidal mission. The warhead was enormous, carrying nearly three tons of high explosives. This was far more than standard torpedoes and ensured that if a Kaiten reached its target, it would likely result in catastrophic damage.

The piloted nature of the Kaiten posed unique design challenges as well. Since the pilot would have to enter the torpedo while it was submerged, a reliable hatch that could be sealed to prevent water from flooding the interior was necessary. The interior space had to be designed to maximize efficiency, fitting controls, life support systems, and navigation instruments into an area barely larger than a coffin.

Lastly, the launch mechanism for the Kaiten was another aspect that required innovation. While some were designed to be launched from submarines, necessitating a compatible launch system, others were deployed from surface ships or shore installations, where rails and ramps were constructed to send the Kaiten into the sea.

The Kaiten In Action

After its development and initial tests, the first Kaiten was operationally deployed in late 1944. The early tests had demonstrated the risks and technical challenges inherent in the weapon, but the desperate situation faced by Japan meant that these were often overlooked or accepted in the push for an operational advantage.

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Kaiten operations began with a series of deployments aimed at disrupting and damaging Allied shipping, primarily targeting American warships and transports. The first mission, which took place on November 20, 1944, involved the submarine I-36 launching Kaiten against Allied ships at Ulithi Atoll, a major staging area for the U.S. Navy. While the mission resulted in the loss of the Kaiten pilots, no significant damage was inflicted on the target.

USS Mississinewa was destroyed by a Kaiten, resulting in the loss of 63 men, November 20, 1944.

The most notable success of the Kaiten program was the sinking of the USS Underhill on July 24, 1945. The destroyer escort was hit by a Kaiten launched from the I-53 and broke in two, resulting in the loss of 112 crew members.

However, successes such as the sinking of the USS Underhill were exceptions rather than the rule. More often than not, Kaiten missions failed to achieve their intended military goals. The difficulty of navigating the Kaiten and the increasing effectiveness of Allied anti-submarine tactics rendered many of the missions ineffective.

The US Navy destroyer USS Underhill, 1943.

By the war’s end, it is estimated that around 100 Kaiten missions were carried out, involving over 400 Kaiten torpedoes. Despite this considerable effort, only a handful of these missions resulted in damage to Allied vessels, and the strategic situation remained unchanged.

With Japan’s surrender in August 1945, the Kaiten program came to an abrupt end. The remaining Kaiten, along with much of Japan’s remaining naval assets, were scuttled or dismantled during the post-war occupation.