By Jia Feimao | 2025-11-07

Japan's navy is upgrading its submarines as China continues to cast a shadow over the Indo-Pacific.

The Japan Maritime Self-Defense Force (JMSDF) christened its sixth Taigei-class diesel-electric attack submarine, JS Sōgei, at Kawasaki Heavy Industries' Kobe yard on October 14. It is slated to enter service in March 2027.

Powered by lithium-ion batteries for endurance, sprint performance and quiet running underwater, the boat reflects the ongoing modernization of Japan's submarine arm.

According to the JMSDF, the Sōgei (hull number SS-518) cost about 73.6 billion JPY ($480.6 million). The submarine is roughly 84 meters long, displaces about 3,000 tons standard and carries a crew of around 70.

The Sōgei has six 533mm tubes for heavyweight torpedoes and tube-launched antiship missiles.

Since 2020, Japan has kept up a steady drumbeat of submarine production, launching almost one Taigei boat a year.

Open sources peg the class at about 20 knots submerged and roughly 6,000 hp installed, numbers consistent with a fast and quiet coastal hunter optimized for the strategic first island chain that includes Japan, Taiwan and the Philippines.

The latest christening comes amid rising regional tensions. In June, Chinese aircraft carriers Liaoning and Shandong drilled together in the Pacific and operated near Japan's southern islands. Liaoning also became the first Chinese carrier to cross the second island chain.

The second chain includes Guam, the Mariana Islands and New Guinea.

Meanwhile, China on November 5 commissioned its third carrier, the Fujian.

As China, North Korea and Russia flex military strength around Japan, pressure on JMSDF submarines is increasing, The Diplomat reported in October.

A Taiwan contingency could draw in Japanese submarines, a risk that has driven Tokyo's maritime buildup.

Challenging Chinese blue-water naval ambitions

Japan remains the only known operator of lithium-ion batteries on diesel-electric submarines.

Compared to lead-acid and air-independent propulsion systems, lithium-ion systems afford faster recharging, higher discharge capacity and greater energy density. They enable quieter running, higher transit and sprint speeds and extended underwater endurance.

Lithium-ion submarines stay underwater longer and are harder to detect than other subs, Shu Hsiao-huang, an associate research fellow at Taiwan's Institute for National Defense and Security Research, told Focus.

"Taigei-class submarines can quietly and quickly reach the core of a Chinese carrier strike group, posing a major threat to Beijing's blue-water ambitions," he said.

Japan is also developing a torpedo-tube-launched cruise missile to add long-range strike options against land and sea targets.

The Defense Ministry has signed two contracts with Mitsubishi Heavy Industries for an upgraded ship-launched Type 12 antiship missile and an unnamed submarine-launched cruise missile. The latter is expected to exceed 998km in range and to enter service in the latter half of the 2020s.

The Taigei class currently fields a tube-launched antiship missile with a range of 248km.

However, it shares torpedo tubes with other weapons, slowing reloads. To fire long-range weapons more efficiently, Japan has considered adding vertical launch systems (VLS) to its future submarines.

"A VLS would enable Japan's next-gen subs to fire both more, larger (and longer-range) weapons than they can now, turning them into more lethal strike platforms," Jeffrey Hornung, a Japan defense specialist at RAND, told the Japan Times in October.