Germany and Japan employed advanced naval tactics, including the use of submarines. As one of the Axis powers, Japan had a formidable submarine fleet. The Yi-400 class submarine, built by the Japanese Navy, was a remarkable achievement during that time.

The Yi-400 class submarine was the largest submarine of its kind during World War II and remained the largest until the construction of nuclear-powered submarines in the 1960s. It measured 122 meters in length, 12 meters in width, and 7 meters in height. With a surface displacement of 3,550 tons and an underwater displacement of 6,560 tons, it had a remarkable range of 60,000 kilometers.

The I-400 submarine had a starting crew of 145, which was raised to 213 and carried 220 during battle, with a top surface speed of 20 knots, a submerged speed of 7, and a range of 33,000 nautical miles at 16 knots.

Its fuel capacity allowed it to circumnavigate the globe one and a half times. This made the Yi-400 class submarine a remarkable vessel before the advent of nuclear-powered submarines.

Admiral Yamamoto Isoroku, commander-in-chief of the Japanese Combined Fleet, conceived the idea of using submarine-launched aircraft to strike coastal cities in the United States shortly after the attack on Pearl Harbor. The Yi-400 class submarine, known as the “submersible aircraft carrier,” was born from this ambitious plan.

The hull design of the Yi-400 class submarine was unique. The designers employed a horizontal binocular pressure shell design resembling a pair of glasses. This design ensured the safety of the ammunition depot and the large fuel tank inside the submarine while enhancing overall stability.

Narrow corridors were crammed with food boxes and other supplies to keep the crew alive during long Pacific Ocean patrols.

Originally, each I-400 submarine could carry two aircraft, but due to reduced production, the carrying capacity was later changed to three per ship. The designers allocated a spacious 35-meter-long and 4-meter-wide hangar to accommodate the three small attack aircraft. The specially designed “Qinglan” water attack aircraft for the I-400 class utilized foldable main wings to save space.

The hangar featured three refueling systems that utilized the submarine’s operational heat to warm the fuel, enabling the fighters to take off immediately without the need for engine preheating.

Additionally, an 85-foot-long catapult zipline extended from the hangar mouth. The aircraft would be folded and stored in the hangar during normal operations, and when in combat, it would be dragged to the catapult, where its wings would unfold, refuel, and reload before launching. After completing their missions, the planes would land on the water near the mothership and be hoisted back onto the ship using a collapsible crane.

The I-400 submarines were equipped with powerful weapons in order to execute Admiral Yamamoto’s plan to attack the Panama Canal and strike Washington. The iconic “Qinglan” attack aircraft, the submarine’s primary carrier-based aircraft, was equipped with one “93-type” machine gun and capable of carrying up to 800kg of torpedoes or aerial bombs.

Although it had fewer armaments than conventional fighters, it could still inflict heavy damage on U.S. surface ships. The fighter’s combat preparation took only a few minutes, allowing for quick attacks and swift retreats from the battlefield.

As the U.S. military prepared to attack the Japanese mainland after occupying Okinawa, the Chief of Staff of the Japanese Navy realized that the original plan to bomb the Panama Canal would have little impact on the war.

Japan changed its strategy to counter the impending U.S. attack and targeted the U.S. military base at Ulithi Atoll instead. This shift in plans was prompted by the presence of 15 U.S. Navy aircraft carriers gathering at the Ulithi base, preparing to launch an assault on the Japanese mainland.

The Japanese Navy believed that a direct confrontation with the U.S. military was the only viable option to prevent their impending attack. By redirecting their efforts towards the Ulithi base, they aimed to disrupt the U.S. military’s plans and gain a strategic advantage.

The decision to focus on the Ulithi base highlighted the flexibility and adaptability of the Yi-400 class submarines. With their ability to carry and launch aircraft, these submarines provided Japan with a unique and unexpected means of engagement. The innovative design of the submarines, along with their powerful weapons and extensive range, made them formidable assets in the Pacific Ocean.

The Yi-400 class submarines played a crucial role in Japan’s naval operations during World War II. They demonstrated the Japanese Navy’s commitment to innovation and their determination to explore unconventional strategies.

The Yi-400: Japan's Revolutionary Submarine Ahead of its Time During World War II.

Although the overall outcome of the war was not in their favor, the Yi-400 class submarines represented a significant technological achievement and a testament to the ingenuity of their designers and crew.

Today, the Yi-400 class submarines serve as a reminder of the advancements made in naval warfare during World War II. Their legacy endures as a symbol of the strategic and tactical capabilities that emerged during one of the most challenging periods in modern history.

Despite their impressive capabilities, the Yi-400 class submarines did not see significant combat action during the war. Only three submarines of this class were completed, namely the I-400, I-401, and I-402. These submarines were commissioned in 1945 but were unable to fulfill their intended mission of launching airstrikes on the Ulithi base due to the changing tides of the war.

By the time the submarines were ready for deployment, Japan was already on the defensive, and the tide had turned against them. The U.S. military had gained the upper hand in the Pacific, and Japan’s resources and manpower were dwindling. As a result, the ambitious plan to attack the Ulithi base was deemed unfeasible.

The submarines were subsequently utilized for reconnaissance missions and targeted attacks on enemy ships. The I-401, in particular, played a role in the surrender of Japan. It was used to transport Japanese envoys to deliver surrender documents to Allied forces.

After the war, the remaining Yi-400 class submarines were surrendered to the U.S. Navy. American naval experts thoroughly studied and evaluated the submarines and recognized their advanced engineering and technological features. Eventually, the submarines were scuttled in 1946 as part of an agreement to disarm Japan.

While the Yi-400 class submarines did not achieve their original objectives, their legacy lives on. They represented a significant milestone in submarine design and technology, showcasing Japan’s ability to push the boundaries of naval warfare. The concept of a submarine capable of launching aircraft was a testament to the innovative thinking of the Japanese Navy.

Today, the Yi-400 class submarines are remembered as pioneering vessels that paved the way for future advancements in submarine technology. Their unique design and capabilities continue to captivate historians and naval enthusiasts alike, serving as a testament to the ingenuity and ambition of their creators.

In conclusion, the Yi-400 class submarines were extraordinary vessels that pushed the boundaries of naval warfare during World War II. Despite their limited combat action, their innovative design and capabilities mark them as significant contributors to the development of submarine technology. Their story serves as a reminder of the relentless pursuit of innovation and the ever-evolving nature of warfare.