The largest warship ever sunk by a submarine was a secret, unfinished Japanese carrier.
In short: The Shinano was a massive Yamato-class battleship converted into a support carrier after Japan's defeat at Midway. Despite its immense size and heavy armor, the ship was lost just ten days after commissioning, having never completed its sea trials or sealed its internal compartments against the sea.
Originally laid down in 1940 as the third Yamato-class battleship, the Shinano was repurposed as an aircraft carrier following the catastrophic loss of four Japanese fleet carriers at the Battle of Midway in 1942. Because the hull was already partially complete, the Imperial Japanese Navy opted to design it as a heavily armored support vessel intended to carry reserve aircraft, fuel, and ordnance for other ships. It was a project shrouded in extreme secrecy; workers were confined to the yard, and the ship was never officially photographed during its construction.
The carrier's final voyage in November 1944 was plagued by poor preparation. Captain Toshio Abe requested a delay to allow for essential safety testing, as the ship lacked functional bailing systems, fire mains, and properly sealed bulkheads. These requests were denied. When the U.S. submarine Archerfish intercepted the task group, four torpedoes struck the Shinano. While the damage was initially deemed manageable, the ship's lack of watertight integrity and the crew's overconfidence led to progressive flooding that could not be contained.
Post-conflict investigations revealed that the ship suffered from critical structural vulnerabilities, particularly at the joint where the armor belt met the anti-torpedo bulge. This design flaw allowed torpedoes to cause more damage than intended. Furthermore, the failure to conduct air-pressure tests on compartments meant that water flowed freely through unsealed gaps in bulkheads and doors. The ship eventually capsized and sank on 29 November 1944, taking 1,435 lives with it. It remains the largest vessel ever destroyed by a submarine, a record that highlights the dangers of rushing a complex warship into service before it is truly ready for the ocean.
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Sources
- Summary by Intellect · source: Japanese aircraft carrier Shinano
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