Japan can quickly and inexpensively increase its force of diesel-electric combat submarines from 22 to at least 28, if it stops prematurely retiring them.

That would provide more of the one category of warship that the armed forces of democratic countries could safely operate close to China in wartime. Moreover, additional Japanese submarines would ease pressure on the US Navy, which is straining to maintain submarine numbers.

Last month Japan decommissioned the first of its 11 Oyashio class boats. They are contemporaries of Australia’s Collins class, which Canberra is not remotely close to withdrawing from service.

JS Oyashio was 25 years old and was only 17 when pulled from the combat force to be converted into a training submarine. Most navies would regard that as waste, just as most air forces would not follow Japan’s policy of discarding F-15 fighters that are nowhere near worn out.

Japan’s early decommissioning of submarines seems especially improvident given the importance of its boats in helping to deter a Chinese attack on Taiwan. They are based near China and are operated by people highly experienced in the waters that would be the main naval theatre in such a conflict.

Consider, for example, the results of table-top simulations of a Taiwanese war published by the Center for Strategic and International Studies in Washington in January. After Japan’s forces in the games suffered initial blows, the researchers reported that: ‘most valuable were the Japanese submarines, which could strike Chinese amphibious ships and the Chinese picket line around Taiwan.’

For decades, Japan has almost always built a submarine a year. The current and planned fleet is 22 plus two: 22 for operations, including one routinely assigned to development work but presumably fully armed, and two converted for training. On average, they’ll be retiring after about 24 years in service.

Elsewhere, submarine service lives of 30 years are unremarkable.

If Japan suspended submarine retirements for six years and raised the average decommissioning age to 30, it would increase its fleet by six boats without having to spend even one more yen on construction. It could enlarge the fleet at a rate of one a year.

Using each submarine for 32 years would enlarge the fleet by eight. The US Navy has found that its Los Angeles class nuclear submarines are good for 36 years.

The idea of keeping Japanese submarines going for longer is not new. US Congressional Research Service analyst Ronald O’Rourke said in 2020 that the number-one opportunity to expand the naval power of the US and its allies was by enlarging Japan’s submarine force. And in 2021 I proposed that, since Japan planned to throw away Oyashios before they were worn out, they’d make fine temporary additions to the Royal Australian Navy while it awaited delivery of nuclear submarines.

Keeping the remaining Oyashios in Japanese service would be much more valuable.

Japan has expanded its fleet once before by lengthening service lives. The fleet target of 22 plus two, achieved last year, was set in 2010 when Japan had 18 boats (16 plus two) and retired them generally before they got to even 20 years.

Another immediate opportunity is for Japan to end its practice of dedicating two submarines to training. A Japanese naval source says that converting a boat for that purpose every few years involves removing combat equipment, much reducing its war-fighting capability.

Other navies teach sailors the ropes in front-line submarines. If Japan did that, it could add two submarines to its combat force without lifting the building rate.

So, a quick addition of 10 submarines to the Japan Maritime Self-Defense Force is quite conceivable.

Japan would have to train enough people for one extra crew each year, though that should not be difficult. It was doing that several years ago as it lifted the force to 22 plus two.

It would also need expanded maintenance capacity, additional weapons such as torpedoes, and a larger budget to cover running costs.

But the biggest additional financial burden may be in shipyard work.

The 2010 decision to operate the submarines for longer required life-extending overhauls for the Oyashios, which began in 2013 and were followed by modernisation to almost the standard of the succeeding Soryu class, according to Japan’s government. If Japan lifts the submarine retirement age to 30 or 32, its submarines will presumably need more or deeper overhauls and modernisation.

That would cost much less than new submarines.

But, however well modernised, will diesel-electric submarines continue to be useful?

Peter Dutton, who was defence minister when Australia decided in 2021 it needed nuclear-powered submarines, wrote last year that the advice from our experts was clear. Diesel-electric submarines would not be able to compete in hostilities in the South China Sea beyond 2035. The diesel-electric submarine needs to come up near to the surface to ‘snort’—raising a snorkel to run its diesel engines and recharge its batteries—and would be detected by emerging radar technologies, Dutton said.

That assessment should not discourage Japan from extending the service lives of its diesel-electric submarines. That would yield a larger Japanese fleet well before 2035, which cannot be regarded as a sharp turning point at which the conventionally powered submarine concept will suddenly become obsolete. Such changes come gradually.

Also, Japanese submarines would operate in wartime not so much in the South China Sea, where the presence of Chinese airborne radar surveillance might be uncontested, but mainly in the East China Sea and nearby waters, close to Japanese and Taiwanese air bases.

And all Japanese submarines delivered since 2009 (after the Oyashio class) have either air-independent propulsion systems or large-capacity lithium-ion batteries enabling them to ‘loiter’. Both technologies allow a commander to reduce or entirely avoid snorting in dangerous locations. Australia’s previously planned diesel-electric submarines lacked such features.

Although Japan plans to double its defence share of GDP to 2% by 2027, a force structure plan issued in December confirmed that the 22-plus-two submarine fleet size would stay.

Why the government and navy rejected the possibility of further expansion is unclear. Possible explanations include competition within the navy for funds or a simply a disinclination to accept the disruption of expansion beyond the 2010 plan.

Whatever the reason, perhaps the one influence that can change minds in Tokyo is Japan’s US ally. If Washington, in seeking to deter China, values undersea strength as highly as it seems to, it needs to tell Japan to stop throwing away good submarines.