Soryu-class submarine. Note x-shaped tail. Photo via Military-Today.com.

Japan has announced it will increase the size of its submarine fleet, from 18 patrol submarines and two trainers to “more than 20″, according to this article.

Japan is to increase its submarine fleet for the first time in 36 years, the Sankei Shimbun reported Sunday. The plan apparently aims to counter China’s naval build-up by partially filling the void created by the U.S. reduction of submarines in the Pacific area.

The paper said the Japanese government plans to increase the number of submarines from the current 18 including two trainer submarines to more than 20 when it revises its Defense Program Guidelines by year’s end.

Tokyo has maintained 18 submarines since it first formulated the guidelines in 1976, although it has strengthened their capability by replacing superannuated vessels and with new ones. (Link)

The article has a picture of a Soryu-class diesel electric submarine. At 4,200 tons, Soryu-class is one of the largest classes of submarines Japan has ever operated–only the World War II-era  aircraft-carrying I-400 class submarines were larger.

I-400 submarine.

The Soryu-class is an improved version of the Oyashio-class submarines which were commissioned in the late 1990s through 2008. Soryu boats differ in having an X-shaped tail plane configuration and an Air Independent Propulsion (AIP) system. Soryu boats are larger, but only by 200 tons.

Asashio-class submarine Arashio, SS-565. Commissioned 1969, scrapped 1986, aged 17 years. Photo via Wikipedia.

Japan tends to keep submarines in service for twenty years or less:

(in chronological order:)
Asashio class: average 17  years in commission
Uzushio class: average 16 years in commission
Yushio class: average 16 years in commission
Harushio: 5 still in service (+2 converted to trainers) oldest is 20 years old
Oyashio class: 11 in service, oldest is 12 years old
Soryu class: 2 in service +4 under construction

These figures, drawn from Wikipedia, give Japan 18 active duty patrol submarines and 2 Harushio-class trainers. The Chosun Ilbo article above shorts Japan 2 patrol submarines. It’s possible that, given the fast-paced world of Japanese submarines, the trainers cited in Wikipedia have already been decommissioned and replacements drawn from the 5 Harushio boats listed in service.

Assuming Japan plans to increase its submarine fleet to “more than 20″ (let’s call it 21) Japan would have to build another four Soryu boats beyond current plans to replace the Harushios, all of which are getting quite old by Japanese standards. Given the pace of submarine shipbuilding, that should be completed by 2019 or so. That would give Japan 11 Oyashios and 10 Soryus. And in 2019, Japan will want to start retiring the lead Oyashio boats. That must keep Kawasaki and Mitsubishi pretty busy.

Question: if Japan thinks it can get by with such a slight increase in submarines while the Chinese sub building program keeps cranking them out like sausages, does it have some sort of ace in the hole to give Japan a technological lead? The Type 89 torpedo is almost 20 years old. The Harpoon missile is old, slow, and doesn’t pack much of a punch. Curious: can some version of the XASM-3 missile fit in a 533mm torpedo tube?

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A contributor and editor at the blog War Is Boring, Kyle Mizokami started Japan Security Watch in 2010 to further understand Japan's defenses and security policy.
Kyle Mizokami has 536 post(s) on Japan Security Watch