SS-501 Soryu / 16SS / SS 2,900 ton Class

The Soryu class are the largest submaries Japan has operated since the end of World War II [the I-400 boats were larger]. At 84 m long and 4,200-tons submerged, Soryu is significantly larger than the original 11 Oyashio-class boats (81 m and 3,500 tons), in order to incorporate a Kockums Stirling air independent propulsion (AIP) system. With the Kockums Stirling AIP system the submarine doesn't have to surface to charge the batteries and thus increases the submerged endurance from days to weeks. Components for the AIP, which will enhance the vessel's stealth and special operations capability, were supplied by the Swedish company for assembly in Japan.

A follow-on to the Oyashio class, the design features higher automation (particularly in combat systems) and computer-aided X control planes. The revolutionary X rudder configuration was first developed by Kockums for the Gotland. The X rudder provides the submarine with extreme manoeuvrability and also enables it to operate very close to the seabed. The Soryu's hull is clad in anechoic coating and the interior features sound isolation of loud components. An anechoic coating has a major effect on the avoidance of detection. The hull is HY-80 alloy (the same as the USN's Los Angeles class SSN) steel. They are being built at the Kobe shipyard.

The first of three improved Oyashio-class attack submarines on order for the Japanese Maritime Self-Defence Force (JMSDF) was launched at Mitsubishi Heavy Industries' Kobe shipyard on 05 December 2007. It was formerly known as the "SS 2,900 ton" and the "16SS" project as it originated in the 16th year of the current Emperor's reign (2003).

The Stirling engine is a heat engine. Heat is produced in a combustion chamber separated from the actual engine. The heat is tranferred to the engine's working gas, operating in a completely closed system. The working gas forces the pistons in the engine to move, thus producing mechanical energy. In other words, all the Stirling engine needs is heat and it doesn't really matter how the heat is produced. It can come from anything that burns: oil, diesel, petrol or gas. Or the heat may come from the sun. Letting a concave mirror concentrate the sunbeams on a Stirling engine, driving a generator makes for the present the most efficient conversion of solar energy to electrical power. In addition, the Stirling engine is flexible, silent and practically vibration-free. All these characteristics open a wide field of applications.

A submarine disappears from sight when it dives, but it can still leave a trail of sound, heat, sonar reflections and magnetic anomalies. For the submarine to remain concealed, all of these signatures must be minimized. All emitted sound that can be detected by enemy hydrophones must be suppressed.

In the 1960s Kockums became involved in the development of a Stirling engine and in 1968 Kockums assumed responsibility for continued project planning. In the mid-1980s Kockums installed Stirling engines and a LOX (Liquid Oxygen) system from AGA Cryo aboard the French 500 ton civilian research submarine Saga, which has an operating depth of 600 meters and is fitted with a diving system for work at depths as deep as 450 meters. Intense R&D and the experiences gained from the Saga project paved way for the installation of a Stirling engine in the Royal Swedish Navy submarine Näcken in 1988. The submarine was placed in dry-dock and cut in two. A fully outfitted eight-meter AIP section was then inserted.

The AIP system of the SS-501 Soryu comprises the virtually vibration-free and silent Stirling engine. The fact that the submarine can operate at great depths for long periods of time is also an important stealth factor. The hydrodynamic design of the hull, rudder and propeller is of vital importance. The flow sound when the submarine travels through the water can disturb the submarine's own hydrophones and can also be heard by the enemy.

The JMSDF plans to operate 10 of these submarines. Japan decided in 2014 to power its new batch of Soryu-class submarines with Lithium-ion batteries instead of air-independent propulsion (AIP). the change would apply to the last four Soryu-class submarines in the 10-boat class. The new Soryus would retain the main propulsion diesels, but use Lithium-ion batteries that are more powerful and require far lower maintenance than lead-acid batteries.

The JMSDF is looking for a fleet of about 20 SSK's, and by 2015 a total of eight of this class had been built or were on order. The Japanese have an incredibly rapid turnover rate compared to international standards when it comes to her submarine fleet, with most boats being retired after around 20 years in service.

A report in the United Evening News on 14 April 2013 alleges that Taiwanese officials had discussed the possibility of a technological transfer from Japan during recent “private meetings.”

Michael Cole wrote "Although the defense ministry immediately denied that Taiwan had explored the possibility of acquiring submarine technology from Japan, the claims were confirmed by a Japanese defense official who spoke to The Diplomat on the condition of anonymity, citing the sensitive nature of the exchanges. Such talks, unofficial in nature, have indeed taken place, though the Taiwanese officials involved were well aware of the little likelihood that such exchanges could occur under the current circumstances."

The United States has been pushing for greater defense cooperation among Japan, India and Australia in order to counterbalance China's growing military power in the Asia-Pacific region and especially the South China Sea, which has long been the subject of fierce territorial disputes between Beijing and its neighbors.

Australian leadership were considering an arms deal with Japan to buy 12 submarines based on Soryu-class vessels. "Australia is considering buying top-secret technology from Japan to build a fleet of new generation submarines, a move that would risk reigniting diplomatic tensions with China only recently smoothed over," Bloomberg reported 18 December 2014.

The media outlet underscored that Australian Defense Minister David Johnston has acknowledged that the authorities are considering "unsolicited proposals" made by Japan, Germany, Sweden and France to provide the Australian fleet with new submarines. The minister confirmed that the country was planning to replace its six Collins-class diesel electric submarines by 2026.

New Delhi is considering a project to build six Japanese Soryu-class diesel-electric submarines in an Indian shipyard, according to Indian media reports. "In keeping with their expanding strategic partnership, the Modi government has asked the Shinzo Abe administration whether it would be interested in the over Rs 50,000 crore [$8.1 billion] project to build six stealth submarines in India," the Times of India reported 30 January 2015.

At 4,200 tons submerged, the Soryu-class is considerably larger than either the [German] Type 214, [French] Scorpene, or improved [Russian] Kilo, and can carry a much heavier weapons load. This size also makes them quieter and longer-ranged than the other boats on the market. At current price expectations of around $500 million, the Soryus are not wildly more expensive than the other boats.

Japan's shipbuilder Kawasaki Heavy Industries (KHI) launched the 12th and final Soryu-class diesel-electric attack submarine (SSK) for the Japan Maritime Self-Defense Force (JMSDF). JS Toryu (Fighting Dragon) SS-512 is the second submarine of the class to feature Li-Ion batteries. The launching ceremony was held today at the KHI shipyard in Chuo-ku, Kobe. About 380 people from Japan’s Ministry of Defense and KHI participated in the ceremony. As per tradition, Admiral Hiroshi Yamamura, Chief of Staff of the JMSDF, cut the rope which started the launch process and send the new submarine in the water for the first time. SS-512’s keel was laid in January 2017, and the submarine is set to be delivered to the JMSDF around March 2021.

All submarines of the class are named after dragons: Soryu means Blue Dragon, Hakuryu (2nd in the class) White Dragon, Sekiryu (8th in the class) Red Dragon, Shoryu (10th in the class) means Soaring Dragon, etc