How stealthy are the world’s subs?
There used to be a rough ranking system for people who followed sub warfare. The best diesel submarines are the most quiet when they aren't running their diesel, followed by the best nuclear submarines, followed by crappy and older subs.
But over the past couple of decades, submarine technology has gotten so advanced that the engine might not even be the limiting factor. Now, sub hunters look for a lot more than a bit of engine or pump noise under the water.
They search for heat trails created by friction between the water and the hull, listen for bubbles that form in the low pressure zones on the backs of propellers, and search for magnetic signatures given off by some sub components. Though modern submarine hulls are often made out of non-magnetic or low-magnetic materials to reduce this signature, some components are naturally magnetic and electrical currents passing through circuits and motors creates small magnetic fields.
Taking a look at these minute details, it's clear why submarine technology is so heavily guarded. If the enemy finds out that your new motor is quieter but makes a magnetic field that is larger than old designs, they'll buy better magnetic anomaly detectors (yes, that's a real name). And if they find out your engine is stealthier than their engine, they'll try to steal it (looking at you, China).
So, what's the hierarchy of subs look like right now?
At the top are a few kinds of air-independent propulsion systems, meaning that the subs never or very rarely have to surface to let in oxygen during a cruise. There are two major kinds of AIP submarines, those that use diesel or similar fuel and those that use nuclear power.
In general, the stealthiest subs are generally acknowledged to be non-nuclear boats when they're running on battery power. Sweden has a sub that fits this bill that is well-regarded across the world and has managed to evade a U.S. carrier group's anti-submarine screen so well that it "killed" a U.S. aircraft carrier during an exercise. China and Russia also have subs in this category and use them for shoreline defense.
Just beneath that group is nuclear submarines that generate electricity and then use an electric motor to drive the propeller or the pump jets (pump jets are preferred because they are less likely to create cavitation, more on that in the next paragraph). America's newest submarines fit into this category. They have a small disadvantage against advanced AIP diesel-electric because the nuclear reactors must be continuously cooled using pumps which generate some noise.
Many of America's subs were created before pump jets matured and have more traditional propellers. At the right depths and propeller speeds, propellers cause cavitation where the water boils in the low-pressure zone near the propeller despite the low temperatures. The telltale bubbles collapse almost immediately, letting good sonar operators follow the noise directly to the enemy sub.
Regardless of whether the sub is using pump jets or conventional propellers, it's less stealthy when the reactors provide power directly to the propeller. U.S. subs are transitioning to only generating power and then using the electrical power to control the engines. China recently claimed to have developed the components necessary for the same upgrade.
Another step down for diesel subs is when they have low-capacity batteries. Having a low capacity forces the sub to surface and run its engines more often, making them much more likely to be found via radar or satellite.
The oldest diesel subs are also less likely to be designed with sufficiently quiet engines or sound dampening. These older diesel subs are also more likely to be made with steel that can be detected by magnetic anomaly detectors, but at this point, we're only talking about navies like North Korea's.
The fact is, however, even at the level of antiquated diesel submarines with direct power going to the engines, small batteries, and little sound dampening, it takes a relatively advanced navy to detect enemy subs.
Sub hunters need solid sonar systems and well-trained operators that can distinguish an enemy sub running quiet from the surrounding ocean noise, especially if the sub moves into a noisy patch of ocean like littoral or tidal areas, where the water rushing over rocks and coral hides the acoustic signatures of all but the noisiest submarines.
While all truly modern navies can do this, not all ships are capable of hunting even older submarines, so older models still give an asymmetric advantage to a nation. But for modern navies like the U.S. and China, the competing sailors have to use every trick in their toolbox to retain an edge.
This is a relatively new development since Chinese subs were known as being laughably loud to U.S. forces just a few years ago. While it's unclear in the unclassified world just how much China has closed the gap, they've made claims that they're actually slightly ahead of the U.S. This seems unlikely, but China has shown off advanced technologies, like pump jets, that could put its tech within striking distance of the U.S.'
And its subs have twice threatened U.S. carriers, once surfacing well within torpedo range and once shadowing a U.S. carrier near Japan. The U.S. Navy might have spotted the subs and decided to not risk starting a war by engaging it, but it's also possible that the Chinese subs actually got the jump on them.
Meanwhile, the U.S. has completed its own submarine surprise against China. In 2010, the U.S. surfaced three submarines simultaneously, one each near South Korea, The Philippines, and Diego Garcia, all within range of Chinese forces or the Chinese mainland. Between the three boats, they could carry 462 Tomahawk Land Attack Missiles.
So, it seems that in submarine warfare, the advantage still lies with the subs. But modern submariners are still counting on every advantage that their training, scientists, and engineers can give them, because in a small metal tube hundreds of feet underwater is a horrible time to find out you're not as stealthy as you'd hoped.