Japan’s Silent Killers: The Japan Maritime Self-Defense Force’s Submarine Fleet

The Japan Maritime Self-Defense Force (JMSDF) fields the most capable conventional submarine force in East Asia.

Japanese submariners have gained admiration and respect for their professionalism and their boats’ excellent performance characteristics from allies all over the world, including the Royal Australian Navy and U.S. Navy.

During military exercises such as the U.S.-led Rim of the Pacific exercise, Japanese subs have repeatedly succeeded in penetrating multi-layered defense perimeters of carrier strike groups and successfully attacking surface warships.

Given the rapid buildup of China’s People’s Liberation Army Navy (PLAN) and continuous tensions on the Korean Peninsula and the East China Sea, Japan is increasing the size of its silent service from 17 to 22 boats by the early 2020s. It will be the first such increase since the 1970s.

The JMSDF submarine force is divided up into two flotillas, Submarine Flotilla 1 and Submarine Flotilla 2, based in Kure, Hiroshima Prefecture and Yokosuka, Kanagawa Prefecture (also home to the central headquarters of the JMSDF and U.S. Navy in Japan) respectively.

Whereas Flotilla 1 is divided into three submarine squadrons, Flotilla 2 consists of two squadrons and the JMSDF’s Submarine Training Command. The co-location of the training command with the U.S. Navy headquarters in Japan purportedly guarantees the effective integration of the silent services of Japan and the United States.

Japan’s submarine force currently is composed of 17 diesel-electric attack submarines (SSKs) divided up into two boat classes, the 4,000-ton (submerged) Oyashio-class and the technically more advanced and slightly bigger 4,200-ton Soryu-class of SSKs. Soryu-class and Oyashio-class boats both roughly fit a crew of 70. The JMSDF currently fields nine Oyashio-class and eight Soryu-class submarines, with five more boats of the latter class to be inducted by 2021.

Two additional Oyashio-class boats serve as training submarines for new crews and as a test platforms for new underwater technology. About a third of the JMSDF submarine force is deployed at any given time, with the other boats either being retrofitted or engaged in training exercises.

While the exact locations of operational deployment of the submarine fleet remains top secret, JMSDF sources have repeatedly confirmed that the operational focus of JMSDF submarines has shifted from the Soya, Tsugaru, and Tsushima Straits to the Ryukyu Islands chain (known in Japanese as the Nansei islands), which stretches southwest from Kyushu to Taiwan.

In particular, the Tsushima, Miyako, and Osumi Straits are seen as strategic chokepoints and passageways for PLAN warships to move from the East China Sea to the Pacific Ocean.  In addition, the 150-kilometer wide Bashi Channel between Taiwan and the Philippines is seen as an important gateway for PLAN ships into the Pacific and is consequently patrolled by Japanese subs.

The Soryu-class subs, like its predecessor the Oyashio-class, are built by two defense contractors: Kawasaki Heavy Industries (KHI) and Mitsubishi Heavy Industries (MHI). Since the end of World War II, the former has built 27 and the latter 26 submarines for the JMSDF. According to an analysis in Sentaku Magazine, KHI is known for its welding skills, whereas MHI has a particular expertise in reducing vibrations — a key to a submarine’s stealthiness.

Both submarine classes feature the same armament, including six HU-606 533 mm torpedo tubes that can accommodate Type 89 homing torpedoes and all weather, over-the-horizon UGM-84 Harpoon anti-ship missiles. Both submarine types also are equipped with a towed array sonar capable of detecting ships over 70 kilometers away.

However, in comparison to the Oyashio-class, the Soryu-class is the JMSDF’s first class of submarines fitted with an air-independent propulsion (AIP) system. Beginning in August 2018,  Japan will retrofit its submarine fleet with advanced lithium-ion batteries — the design of which is one of Japan’s top military secrets — in order to improve the fleet’s underwater endurance.

The Soryu-class is also fitted with a computer-controlled, Swedish-designed X rudder to increase the ship’s maneuverability when operating in shallow waters close to the seabed, particularly useful for operations in the shallow East China Sea. According to Sentaku Magazine, JMSDF submarines excel in small-turn performance over topographically complicated sea floors.

It remains to be seen how new Japanese security legislation, the so-called Permanent International Peace Support Law and the Legislation for Peace and Security, which came into force in March 2016, will affect JMSDF submarine operations. The new legislation for the first time allows JMSDF subs to use force while protecting allied assets if certain criteria are met in peacetime.

In the event of a military conflict with the PLAN, the JMSDF submarines’ primary objective would be to skillfully deploy along the Ryukyu Islands chain and Bashi channel and prohibit the PLAN from entering the open waters of the Pacific Ocean until the U.S. Navy and other allied navies can deploy in full force.

According to some estimates, the JMSDF will need around eight boats, six for the Ryukyu Islands chain and two for the Bashi channel, to prohibit the PLAN from breaking through in the event of war. Given that for every submarine deployed, two backup boats are required for training and maintenance, it would not be surprising if the Japanese government were to decide to build two additional SSKs to bring up to total strengths to 24 submarines during the next budget cycle.