China's submarine fleet is largely limited to a coastal defensive role, but still could not prevent infiltration by US undersea boats, according to a recent analysis by Owen Cote Jr. of the Massachusetts Institute of Technology's Security Studies Programme.

Only big shifts in doctrine and technology could alter the current equation, Cote explained.

Cote's assessment is a fresh reminder of the obstacles China faces in securing its own waters – to say nothing of extending its influence farther into the Pacific. The report is also a reminder that, despite the high visibility of aircraft carriers, jet fighters and ballistic missiles, submarines are still the most decisive weapon in the evolving rivalry between Washington and Beijing.

With their stealthiness and tremendous firepower, submarines pose a serious threat to surface vessels. For that reason, they are ideally suited to so-called sea denial – that is, keeping enemy fleets out of a given patch of ocean.

That's precisely what Beijing intends for its force of roughly 50 small diesel-electric submarines and 10 or so larger nuclear boats. ‘China plans on using its diesel attack submarines…for coastal defence,’ Cote wrote.

Likewise, the nuke boats could be used in an attempt ‘to deny or limit the access of Western navies to the larger sea space between what (Chinese officials) call the 'first and second island chains' – or, roughly speaking, the Philippine Sea.’

But the US Navy’s own submarines plus its patrol planes, helicopters, surface ships and underwater ‘listening’ arrays, concentrated in geographic choke points, could probably detect most Chinese subs attempting to reach the US fleet beyond the first island chain, Cote asserted.

At the same time, ‘China has very limited (Anti-Submarine Warfare) capabilities and appears not to be making major investments to improve them.’

So while the submarines of the People's Liberation Army Navy might be able to deny US and allied surface vessels access to the South China Sea, they can’t reliably extend that denial into the Philippine Sea. Nor could they even detect American undersea vessels anywhere. ‘US submarines can currently operate freely in Chinese coastal waters.’

This imbalance is a result of the United States’ huge investment in submarine technology during the decades of the Cold War. The US has demonstrated ‘the ability to establish and consistently maintain significant acoustic advantages for its nuclear submarines in a competition with a peer competitor.’

The United States’ 55 nuclear attack subs are so quiet ‘that their passage at close range can actually cause a dip in noise levels because background noise is being blocked by the submarine's hull.’

The superiority of US subs could severely limit the potential of China's own undersea fleet in a prolonged conflict, even degrading its coastal defence capability. Cote described US boats taking up positions outside Chinese ports, tracking and destroying PLAN subs as they left or entered.

There’s a catch. The US Navy maintains only 10 submarines on routine forward deployment all over the world during peacetime. Many more boats could ‘surge’ in the event of hostilities, but would require days or weeks to reach Chinese waters. For that reason, ‘the US can do little to stop an initial sortie by a large portion of the Chinese submarine fleet.’

The American sub fleet's advantage would increase over time as more vessels arrived.

Whether Chinese subs could exploit their moment of relative freedom is debatable. Even US submarines with their highly sophisticated sensors and combat systems rely on ‘cueing’ by land-based over-the-horizon radars, satellites or high-altitude surveillance aircraft that can steer them towards their targets.

China possesses a rudimentary cueing capability in the form of several OTH radars and a small constellation of surveillance satellites. Beijing is also developing drone spy planes. The United States could find a new role for its submarines in destroying some of these cueing assets, further handicapping an already inferior Chinese undersea fleet.

US subs are already armed with land-attack cruise missiles. For their own cueing against OTH radars on land, US boats could deploy small aerial drones – a capability already in development.

Thus equipped, US submarines would also be capable of destroying a wider range of land targets, including China's hundreds of mobile missile launchers.

‘Just as I argue that the US could seek new areas of military competition with China by giving its submarines new missions, the Chinese might seek to do the same,’ Cote explained. ‘But the situation for the Chinese is different because they would not be building on a situation of current or inherent advantage like the US would be.’