Japan commissions fifth Taigei-class submarine JS Chōgei for stealthier Indo-Pacific patrols.

Japan commissioned the fifth Taigei-class submarine JS Chōgei (SS 517) on March 10, 2026, during a handover ceremony at Mitsubishi Heavy Industries Kobe shipyard.

Japan commissioned the fifth Taigei-class submarine JS Chōgei (SS 517) on March 10, 2026, during a handover ceremony at Mitsubishi Heavy Industries Kobe shipyard. The diesel-electric submarine will enter operational service with the Japan Maritime Self-Defense Force (JMSDF)'s Submarine Division 2 based at Yokosuka Naval Base.
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The JS Chogei submarine has a standard displacement of about 3,000 tonnes and measures 84 meters in overall length with a beam of 9.1 meters and a draught of 10.4 meters, placing it slightly above the preceding Soryu-class submarines. (Picture source: MHI)

On March 10, 2026, the Japan Maritime Self-Defense Force (JMSDF) received the fifth diesel-electric submarine of the Taigei-class, the JS Chogei (SS 517), during a handover ceremony conducted at the Mitsubishi Heavy Industries shipyard in Kobe. The submarine forms part of a planned series of eight boats ordered to renew Japan’s conventional submarine fleet and is scheduled to enter operational service in March 2026. Construction of the vessel was financed under Japan’s fiscal year 2021 defense budget with a program cost of 64.8 billion yen, equivalent to $443 million. After delivery, the vessel joined Submarine Division 2 of Submarine Flotilla 2 and was assigned to Yokosuka Naval Base in Kanagawa Prefecture.

The submarine represents the third Japanese vessel to bear the name Chogei, following a former Imperial Japanese Navy transport ship and a submarine tender. In Japanese naval naming practice, submarines of the class carry the suffix “gei,” meaning whale, continuing a naming sequence after the earlier “shio” and “ryu” series used for previous generations. The name Taigei itself means “big whale,” while Chogei translates as “long whale.” The Taigei-class was developed as the successor to the Soryu-class vessels that have formed the backbone of the Japan Maritime Self-Defense Force’s submarine fleet since 2009. This next generation of diesel-electric submarines also introduces updated propulsion systems, new sensor technology, and improvements in acoustic stealth characteristics.

Eight submarines are planned within the class, although the number may increase depending on future defense requirements, particularly in waters surrounding the Japanese archipelago and the wider Indo-Pacific region. The JS Chōgei is the second Taigei-class submarine delivered from the Kobe shipyard after the JS Jingei in March 2024, as part of a production sequence shared between Mitsubishi Heavy Industries and Kawasaki Heavy Industries, both of which maintain submarine construction facilities in Kobe. This dual-shipyard approach supports continuous production capacity and preserves industrial expertise related to submarine design and assembly within Japan. The lead submarine JS Taigei (SS 513) entered service in March 2022, followed by JS Hakugei (SS 514) in March 2023 and JS Jingei (SS 515) in March 2024.

The fourth unit, JS Raigei (SS 516), was launched in October 2023 and is scheduled to enter service in March 2025 following acceptance trials. As of 2026, the sixth submarine of the class, the JS Sōgei (SS 518), is under construction at the Kawasaki yard, while the seventh, the SS 519, is being assembled by Mitsubishi Heavy Industries. The JS Chōgei incorporates a diesel-electric propulsion system supported by lithium-ion batteries as its primary source of stored electrical energy for underwater operations. Earlier units of the class used two Kawasaki 12V 25/25SB diesel engines as their main engines, while later submarines, including Raigei and Chogei, are fitted with the more powerful Kawasaki 12V 25/31 diesel engines, which are compatible with a newly developed snorkel system designed to improve power generation efficiency when operating near the surface.

The propulsion system now delivers a shaft output estimated at about 6,000 horsepower, allowing the submarine to reach submerged speeds exceeding 20 knots. Lithium-ion batteries supplied by the Kyoto-based manufacturer GS Yuasa provide a higher energy density than the traditional lead-acid batteries used in earlier conventional submarines, allowing longer underwater endurance and reducing the frequency of snorkeling required to recharge the battery banks. Japan became the first country to deploy such lithium-ion battery technology in operational diesel-electric submarines when the final two Soryu-class boats, Oryu (SS 511) and Toryu (SS 512), introduced the system.

Electricity stored in the battery system powers the submarine while submerged, while diesel engines operate to recharge the batteries when the vessel uses its snorkel mast near the surface. Compared with the preceding Soryu-class submarines, the Taigei-class introduces several improvements to increase the sonar detection capability and reduce the acoustic emissions produced by the hull and onboard machinery. A lower acoustic signature remains a central factor in submarine survivability and operational effectiveness during submerged operations. The JS Chōgei submarine has a standard displacement of about 3,000 tons and measures 84 meters in length, with a beam of 9.1 meters and a draught of 10.4 meters.

These dimensions place it slightly above the preceding Soryu-class submarines, which have a displacement of about 2,950 tonnes, but still correspond to the general size of modern diesel-electric attack submarines used for patrol missions. The hull adopts a cigar-shaped configuration introduced in Japanese submarines beginning with the Oyashio-class and retained through the Soryu and Taigei generations. Earlier Japanese submarines, up to the Harushio-class, used a teardrop hull design before the shift to the cylindrical shape optimized for submerged cruising performance. The stern control surfaces follow an X-shaped rudder configuration adopted from the Soryu-class onward instead of the earlier cross-shaped arrangement.

This configuration improves underwater maneuverability and reduces the risk of structural damage when the submarine operates in shallow waters, and the rudders contact the seabed. The structural design also incorporates high-strength steel and acoustic absorption materials to reduce detectability during underwater operations. The Taigei-class incorporates an integrated combat management system, which includes the OYX-1 processing system, the ZQX-12 submarine tactical display system, and the ZQQ-8 sonar suite using fiber-optic array technology. Additional onboard equipment includes the NZLR-2 electronic support measures system used to detect electronic emissions and the ZPS-6H radar system used when operating at periscope depth or on the surface.

The sonar configuration combines bow-mounted arrays, flank-mounted sensors, and towed array sensors deployed behind the submarine to detect targets across different thermal layers. This configuration allows improved detection of surface vessels and other submarines in complex acoustic environments. The design also integrates a domestically produced non-penetrating periscope developed by Mitsubishi Electric. The non-penetrating design eliminates the need for a physical periscope shaft penetrating the pressure hull and transmits visual data electronically to internal displays. Armament carried by the submarine includes the Type 18 heavyweight torpedo, which replaces the earlier Type 89 torpedo used by the Japan Maritime Self-Defense Force.

The Type 18 incorporates improvements in propulsion performance, target detection capability, and onboard processing. The submarine can also launch UGM-84L Harpoon Block II anti-ship missiles from its torpedo tubes for engagements against surface vessels. These missiles have a range of about 248 kilometers and provide a capability to strike ships beyond visual range. Defensive systems include torpedo countermeasures that deploy acoustic decoys designed to replicate the sound signature of the submarine’s propeller in order to divert incoming torpedoes. The same countermeasure system had previously been introduced on later Soryu-class boats such as Sekiryu. Japan is also developing a submarine-launched cruise missile derived from the Type 12 Surface-to-Ship Missile with an expected range of about 1,500 kilometers.

Construction of JS Chōgei began with the keel laying on April 19, 2022, at the Mitsubishi Heavy Industries Kobe Shipyard located in Hyogo Prefecture. The submarine was launched on October 4, 2024, marking the transition from the structural assembly stage to the outfitting and testing phase. During the post-launch period, shipyard teams installed onboard systems, integrated propulsion components, and conducted harbor tests prior to sea trials. Sea trials are used to verify propulsion performance, navigation systems, sensors, and combat systems under operational conditions. After these trials confirmed that the submarine met the required performance standards, the vessel proceeded toward final delivery to the Ministry of Defense.

The handover on March 10, 2026, concluded a construction timeline lasting nearly four years from keel laying to commissioning. This timeline reflects the complexity of modern submarine construction and the extensive integration work required before operational deployment. The submarine, which is operated by a crew of about 70 personnel, incorporates modifications to its internal layout to accommodate female personnel within the crew. Restrictions on the assignment of female members of the Japan Maritime Self-Defense Force to submarines were removed in recent years, leading to adjustments in the design of new vessels entering service.

To support mixed-gender crews, partitions were installed within the living quarters to create a dedicated sleeping area for female sailors. Curtains were also installed along the corridor leading to the shower room to ensure privacy within the accommodation spaces. These changes were integrated during the design stage rather than being retrofitted after construction. The modifications reflect personnel policy adjustments within Japan’s defense forces and are intended to allow female crew members to operate aboard submarines while maintaining appropriate onboard facilities.

Similar arrangements are expected to appear in additional submarines built under the same class. Mitsubishi Heavy Industries manages defense equipment activities covering land, sea, and air systems under an integrated organizational structure intended to share engineering expertise across different sectors. Within naval construction, the company is responsible for the design, engineering, and assembly of advanced submarines and surface vessels for the Japan Maritime Self-Defense Force. The company also supports maintenance and sustainment activities after vessels enter service to maintain operational readiness. Development efforts within its naval programs include work on reduced crew requirements, unmanned systems integration, cyber security resilience, and reductions in lifecycle costs. These research directions reflect broader trends in naval engineering and defense procurement priorities. As additional units are completed and commissioned, the Taigei-class will represent a significant portion of Japan’s operational submarine fleet during the coming decades.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.