Sōryū-class submarine

The Sōryū-class submarines (16SS) are diesel-electric attack submarines. The first boat in the class entered service with the Japan Maritime Self-Defense Force in 2009. The design is an evolution of the Oyashio-class submarine, from which it can most easily be distinguished by its X-shaped stern combination diving planes and rudders. The Sōryūs have the largest displacement of any submarine used by post-war Japan.

The Sōryū-class is Japan's first air-independent propulsion submarine class. From Sōryū to Shōryū are fitted with Kockums Naval Solutions Stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for longer periods of time. The 11th submarine of the class, Ōryū, is the world's first lithium-ion battery submarine. The cost of the sixth submarine (Kokuryū) was estimated at US$540 million.

In 2023, the first of the replacements for the Sōryūs, the Taigei-class submarine, entered service.

Naming convention
Japanese submarines since World War II were named after ocean currents. The JMSDF changed its naming convention with the Sōryū, and submarines will now be named after mythological creatures. Sōryū (そうりゅう) means blue dragon in Japanese and shares its name with the World War II aircraft carrier JAPANESE AIRCRAFT CARRIER Sōryū, sunk during the Battle of Midway.

Variants
The eleventh Sōryū-class submarine (Ōryū) is the first Japanese submarine in the fleet to mount lithium-ion batteries. The JS Ōryū was given a budget of under the 2015 Japanese Defense Budget.

Lithium-ion batteries have almost double the electric storage capacity of traditional lead-acid batteries. Updated Sōryū-class boats also added more batteries by placing them within hull spaces previously occupied by AIP system machinery. These upgraded boats benefited by increasing both the size and energy density of their battery storage. The change to lithium-ion improved the underwater endurance significantly and will be an advantage over the slow recharge capability of the AIP system.

In any event, JMSDF believes that lithium-ion is the way forward and intends to 'trial' this new system and compare it to the previous AIP system for operational effectiveness.

Exports
Japan offered Sōryū-class submarines to Australia as replacements for the Royal Australian Navy's Collins-class submarines, as part of the Collins-class submarine replacement project. On 9 April 2014, then-Australian Minister for Defence, David Johnston, described the Sōryū class as "extremely impressive" while discussing Australia's future submarine options. On 26 April 2016, Australian Prime Minister Malcolm Turnbull announced that the Australian contract had been awarded to the French-designed Shortfin Barracuda, though this deal was eventually rescinded.

India, Morocco, Norway, Netherlands, and Taiwan have also approached Japan, and expressed an interest in buying Sōryū-class submarines. During a visit to Japan, India's then-Union Minister of Defence, Manohar Parrikar, invited the Japanese government to participate in their US$8.1 billion Project 75I-class submarine procurement program.