YOKOSUKA, Japan — March 10, 2026 : The Japan Maritime Self-Defense Force (JMSDF) formally commissioned JS Chogei (SS-517), the fifth submarine of the Taigei-class diesel-electric attack submarine program. The vessel was delivered earlier the same day by Mitsubishi Heavy Industries at its shipbuilding facility in Kobe before entering active service.
Following commissioning, JS Chogei was assigned to Submarine Division 2 of Submarine Flotilla 2, which is based at the Yokosuka Naval Base in Kanagawa Prefecture. The induction marks the latest step in Japan’s ongoing modernization of its conventional submarine fleet.
Dimensions, Crew and Design Characteristics
JS Chogei has a crew of approximately 70 personnel and measures 84 meters in overall length, with a beam of 9.1 meters and a draft of 10.4 meters. The submarine has a standard displacement of about 3,000 tonnes, making it slightly larger than submarines of the earlier Soryu-class submarine.
Soryu-class boats share the same length and beam but have a slightly smaller draft of 10.3 meters and a standard displacement of around 2,950 tonnes.
The Taigei-class was designed with improvements in detection capabilities and reduced acoustic signature, aimed at increasing survivability and situational awareness in underwater operations.
A notable design change introduced with this class is the inclusion of female-only accommodation spaces, allowing up to six female crew members to be housed onboard. This represents the first time such facilities have been incorporated into JMSDF submarines.
Naming Convention and Construction Cost
The submarine’s name, Chogei, translates to “long whale” in Japanese. All submarines in the Taigei class incorporate the word “Gei”, meaning whale, in their names. This continues a JMSDF naming pattern that previously used “Shio” (tide) and “Ryu” (dragon) for earlier submarine classes. The class name Taigei itself means “big whale.”
The construction cost of JS Chogei was approximately 68.4 billion yen, equivalent to roughly $434 million.
Propulsion and Performance
JS Chogei uses a diesel-electric propulsion system generating approximately 6,000 horsepower, enabling the submarine to reach a maximum submerged speed of about 20 knots.
The Taigei-class incorporates lithium-ion battery technology supplied by GS Yuasa, replacing traditional lead-acid batteries. This technology was first introduced on the final two Soryu-class submarines—Oryu (SS-511) and Toryu (SS-512).
Japan currently remains the only country operating diesel-electric submarines equipped with lithium-ion batteries, although South Korea plans to introduce similar systems on later variants of its KSS-III-class submarine during the late 2020s.
Engines and Power Generation Improvements
Beginning with the fourth boat in the class, Raigei (SS-516), the submarines incorporate the Kawasaki 12V25/31 diesel engine, developed by Kawasaki Heavy Industries. The engine is paired with an updated snorkel system designed to improve power generation efficiency and battery charging performance.
Earlier submarines in the class—Taigei (SS-513), Hakugei (SS-514), and Jingei (SS-515)—use two Kawasaki 12V25/25SB V-12 diesel engines. Despite these changes in propulsion components, the overall submerged performance across the class remains broadly similar.
Sensors and Combat Systems
The Taigei-class submarines are equipped with the ZQQ-8 high-performance sonar system, developed by Oki Electric Industry. This sonar suite replaces the earlier ZQQ-7 system used on the Soryu-class and provides enhanced underwater detection capability.
The class also incorporates a new combat management system that integrates sensor data, command-and-control functions, and weapon engagement capabilities into a unified operational framework.
Additional sensor and stealth improvements include:
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A new-generation sonar system using fiber-optic array technology
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An enhanced snorkel system designed to reduce acoustic and other signatures during snorkeling operations
The submarines are fitted exclusively with non-penetrating optronic masts, eliminating the traditional penetrating periscope. The system installed is the Optronic Sensor A-type Kai-1, developed by Mitsubishi Electric in cooperation with Nikon.
Weapons and Defensive Systems
The Taigei-class submarines are armed with six 533-millimeter torpedo tubes capable of launching the Type 18 torpedo, the successor to the earlier Type 89 torpedo. The Type 18 incorporates improvements in propulsion, target detection, and onboard processing.
In addition to torpedoes, the submarines can deploy the UGM-84L Harpoon Block II anti-ship missile, which has an operational range of approximately 248 kilometers. The missile provides the submarine with the capability to engage surface targets from standoff distances.
The class is also equipped with a torpedo countermeasure system similar to that installed on the final four Soryu-class submarines.
Taigei-Class Construction Timeline
The commissioning of JS Chogei continues the planned rollout of the Taigei-class program:
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Taigei (SS-513) — commissioned March 2022
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Hakugei (SS-514) — commissioned March 2023
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Jingei (SS-515) — commissioned March 2024
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Raigei (SS-516) — commissioned March 2025
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Chogei (SS-517) — commissioned March 2026
The sixth submarine in the class, Sogei (SS-518), was launched in October 2025 and is currently undergoing outfitting, with commissioning planned for March 2027.
Additional submarines are under construction as part of Japan’s long-term naval modernization program.
Future Procurement and Regional Naval Context
In its FY2026 defense budget, Japan’s Ministry of Defense allocated 120.8 billion yen (approximately $766 million) for construction of the 10th submarine of the Taigei class, indicating continued expansion of the program.
The commissioning of JS Chogei takes place amid evolving regional naval developments. On November 5, 2025, the People's Liberation Army Navy commissioned its third aircraft carrier, Chinese aircraft carrier Fujian, joining the carriers Chinese aircraft carrier Liaoning and Chinese aircraft carrier Shandong.
These developments have expanded China’s carrier force to three operational carriers, enabling broader naval operations beyond the First Island Chain and into the Western Pacific.
Within this context, Japan’s submarine fleet plays a key role in maritime surveillance and deterrence, particularly around the country’s southwestern island chain, where monitoring of increased naval activity has become an operational priority for the JMSDF.
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