In theory, all submarines are stealthy. They travel and attack from underwater, where they can’t be seen, and where radar is mostly useless. And subs should be quiet enough to make them difficult to detect by sonar. But the Russians used to — and the Chinese still do — build some nuclear-powered subs so noisy that their stealth was questionable at best.

One way to build a really scarily freaking quiet sub was to make it a diesel-electric boat, or SS. When running on battery power, they’re damn-near silent. The weakness of those boats is when they would have to run at or near the surface on their noisy diesel engines to recharge the battery. Also, compared to nuclear-powered boats (SSN), diesel-electrics have shorter range, shorter endurance, and much lower top speeds.

But damn if an SS can’t sometimes get right up close and put the fear of God into you when you sail into their waters. They’re also cheaper to build and operate — perfect for a not-as-rich nation wanting to defend their home waters from, say, a US aircraft carrier battle group, and/or an American SSN or three.

And now there are diesel-electrics with the new Air Independent Propulsion system, known as SSP. They don’t have to run near the surface and put up a snorkel to run the diesel motor to charge their batteries — eliminating the biggest weakness of the SS boats.

All of that is why this is news:

Russia has launched its new state-of-the-art Novorossiysk submarine, which set sail from a St Petersburg shipyard to become the first of six diesel-electric stealth subs delivered to the Russian Black Sea fleet in the next two years.

The Novorossiysk belongs to the Varshavyanka-class (Project 636), which is characterized by advanced stealth technology, making it virtually undetectable when submerged.

“Our potential opponents call it the ‘Black Hole’ due to the very low noise emission and visibility of the submarine,” Konstantin Tabachny, captain of the Novorossiysk, told Channel One TV. “To be undetectable is the main quality for a submarine. And this whole project really fits its purpose.”

At least two more of these SSP boats are under construction. I can’t find any information on how many more might be on order.

If I’m reading Global Security’s writeup correctly, Project 636 is a very modern update to Russia’s tried-and-true Kilo-class SS program — and will presumably be made available to export to nations like China, Venezuela, and really anybody with ready cash to spend.

The real wrinkle is that there’s never been a shooting match between SS/SSP boats and SSNs. Or between SS/SSPs and a modern surface fleet like the US Navy. But the Russians and Chinese have managed to take us by surprise at sea a few times, so I suspect a hot war at sea would be like a knife fight — everybody bleeds.