Key Points and Summary – Japan’s Sōryū-class submarines are the backbone of the JMSDF’s undersea force and some of the most capable non-nuclear subs afloat.
-Evolving from the Oyashio-class, they added air-independent propulsion, powerful sonar suites, and heavyweight Type 89 torpedoes plus Harpoon missiles, giving Japan a stealthy, long-endurance hunter in the first island chain.
Taigei-Class Submarine Japan Navy. Image Credit: Creative Commons.
-Later Sōryū variants traded AIP for cutting-edge lithium-ion batteries, boosting underwater performance and recharge flexibility.
-Built by Mitsubishi and Kawasaki, these large, quiet boats were designed with China’s naval rise in mind and will remain a key deterrent even as Japan transitions to the next-generation Taigei class.
Inside Japan’s Sōryū-Class: The Diesel Sub That Acts Like Nuclear
Japan’s Sōryū-class submarines are among the most advanced conventional submarine designs in the world.
They’re also a key pillar of Japan’s Maritime Self-Defense Force (JMSDF).
Built between 2005 and 2021, these diesel-electric attack submarines were Japan’s first to use air-independent propulsion (AIP), which allowed them to remain submerged for extended periods without surfacing to recharge batteries.
The first of twelve vessels, JS Sōryū (SS-501), entered service in 2009 and represented a significant leap for Japan’s undersea warfare capability.
The submarine’s entry into operational service coincided with the country’s growing recognition of the threat posed by Chinese naval forces in the East and South China Seas.
By 2021, Japan had completed ten AIP-powered Sōryū vessels and two later variants equipped with lithium-ion batteries—the first operational submarines of their kind in the world.
SOUDA BAY, Greece (March 27, 2022) The Ohio-class ballistic-missile submarine USS Georgia (SSGN 729) near Souda Bay, Greece, during training with U.S. Marines from Task Force 61/2 (TF-61/2), conducting launch and recovery training with their combat rubber raiding craft, March 27, 2022. TF-61/2 will temporarily provide command and control support to the commander of U.S. 6th Fleet, to synchronize Navy and Marine Corps units and capabilities already in theater, in support of regional Allies and Partners and U.S. national security interests. (U.S. Marine Corps photo by Sgt. Dylan Chagnon)
The Sōryū-class – also known as 16SS – is a development of the earlier Oyashio-class design. It was initially conceived to improve the range, stealth, and underwater endurance of Japan’s regional defense posture, making Japan a stronger naval force at a time when China had begun investing heavily in its naval and general military assets and operations.
Built jointly by Mitsubishi Heavy Industries (MHI) and Kawasaki Heavy Industries (KHI), the class features a distinctive X-shaped stern that provides superior maneuverability in shallow waters.
It’s a feature that many modern submarine programs have since copied.
The submarines are advanced for good reason; their primary mission is to defend Japan’s sea lanes, conduct surveillance in the first island chain, and perform anti-ship and anti-submarine warfare in coordination with the United States and allied forces.
How AIP Technology Works
The first ten Sōryū boats used a Stirling-engine air-independent propulsion system, designed to extend underwater endurance without running noisy diesel engines or surfacing to the air.
Each submarine is powered by two Kawasaki 12V 25/25 SB diesel engines for surface and battery charging, as well as four Kawasaki-licensed Kockums V4-275R Stirling engines that burn diesel fuel using liquid oxygen to generate electricity while underwater.
Those systems allow the submarine to remain submerged silently for as long as two weeks while traveling at slow patrol speeds of between five and seven knots, significantly reducing its acoustic signature and therefore its detectability.
There is, however, a trade-off—and it comes in the form of reduced power output and increased mechanical complexity.
That aside, the AIP system was a significant technological milestone for Japanese shipbuilders and remains an impressive feat.
Technical Specifications
The Sōryū-class is among the largest conventional submarines to have ever been built, measuring around 84 meters long and 9.1 meters wide, while also displacing roughly 4,200 tonnes submerged.
Despite being non-nuclear, its size and range give it near-blue-water capability, enabling it to operate far from home waters for extended periods.
The submarines are crewed by 65 sailors, including specialists in sonar operations, engineering, weapons, and navigation.
Underwater, the boats can reach speeds of up to 20 knots (or roughly 37 km/h), which is comparable to other advanced diesel-electric designs like Germany’s Type 212CD and South Korea’s KSS-III.
Their endurance is significantly extended by either air-independent propulsion (AIP) or, on the later units, lithium-ion batteries.
AIP Gotland-Class Submarines. Image Credit: Creative Commons.
Each submarine is fitted with the ZQQ-7 sonar suite, a Japanese-built integrated detection system that includes bow-mounted, flank-mounted, and towed-array sensors.
And these features matter. Flank arrays are long panels along the hull sides that are designed to detect sound from targets located off the submarine’s bow, increasing situational awareness and helping cthe rew identify surface ships or other submarines at medium range.
The towed array, meanwhile, is an extended cable deployed behind the submarines, equipped with hydrophones (underwater microphones) that can pick up extremely faint sounds over long distances.
And because the microphone trails far behind and is separated from the submarine noise, it is one of the most sensitive tools for tracking adversaries who may also be operating in stealth.
Together, the two systems feed into onboard combat computers that display acoustic data in real time, allowing crews to triangulate and classify other vessels with precision.
As for armament and combat systems, the Sōryū-class carries six 533mm torpedo tubes in its bow, capable of launching Type 89 heavyweight wire-guided torpedoes – Japan’s primary anti-submarine and anti-ship weapon.
The Type 89 is equivalent in performance to the U.S. Mk 48 ADCAP, with active/passive homing and a range exceeding 27 nautical miles.
Harpoon Missile Onboard USS Iowa. Image Credit: National Security Journal.
The tubes can also launch UGM-84 Harpoon anti-ship missiles, giving the boats the ability to strike surface targets at extended ranges, as well as deploy naval mines.
A modern combat management system also integrates sonar, radar, and weapon data, enabling targets to be tracked and engaged simultaneously under electronic countermeasure conditions.
The Sōryū-class is an essential asset for Japan and will remain in service for years.
Still, it is already being gradually replaced by the new Taigei-class submarines, which rely entirely on lithium-ion batteries.
The new class will mark a significant evolution in conventional undersea warfare.
About the Author:
Jack Buckby is a British author, counter-extremism researcher, and journalist based in New York. Reporting on the U.K., Europe, and the U.S., he works to analyze and understand left-wing and right-wing radicalization, and reports on Western governments’ approaches to the pressing issues of today. His books and research papers explore these themes and propose pragmatic solutions to our increasingly polarized society. His latest book is The Truth Teller: RFK Jr. and the Case for a Post-Partisan Presidency.
