Recently, the Taiwanese Navy detected an unidentified submarine outside one of its major naval bases. Ships and helicopters pursued the contact, but the suspected submarine left the area. A Chinese boat was suspected, mainly because for the last decade, Chinese subs have increasingly been showing up close to Japan and South Korea as well.
Two years ago, Japan increased anti-submarine patrols in international waters, just outside Japanese territorial waters. Chinese submarines were apparently exercising there more frequently, looking for Japanese, South Korean and American warships to play tag with. The U.S. has also redirected more of its space based naval search capabilities to assist the Japanese.
Chinese Song class diesel electric and Han class nuclear powered boats have been detected and tracked with increasing frequency over the last few years. In that time, one of each of these was spotted stalking the American carrier USS George Washington, as it headed to South Korea for a visit.
China is rapidly acquiring advanced submarine building capabilities, and providing money (for fuel and spare parts) to send its subs to sea more often. Moreover, new classes of boats are constantly appearing. The new Type 39A, or Yuan class, looks just like the Russian Kilo class. In the late 1990s, the Chinese began ordering Russian Kilo class subs, then one of the latest diesel-electric design available. Russia was selling new Kilos for about $200 million each, which is about half the price other Western nations sell similar boats for. The Kilos weigh 2,300 tons (surface displacement), have six torpedo tubes and a crew of 57. They are quiet, and can travel about 700 kilometers under water at a quiet speed of about five kilometers an hour. Kilos carry 18 torpedoes or SS-N-27 anti-ship missiles (with a range of 300 kilometers and launched underwater from the torpedo tubes.) The combination of quietness and cruise missiles makes Kilo very dangerous to American carriers. North Korea and Iran have also bought Kilos.
The Chinese have already built two Yuans, the second one an improvement on the first. These two boats have been at sea to try out the technology that was pilfered from the Russians. A third Yuan is under construction, and it also appears to be a bit different from the first two. The first Yuan appeared to be a copy of the early model Kilo (the model 877), while the second Yuan (referred to as a Type 39B) appeared to copy the late Kilos (model 636). The third Yuan may end up being a further evolution, or Type 39C.
Preceding the Yuans was the Type 39, or Song class. This was the first Chinese sub to have the teardrop shaped hull, and was based on the predecessor of the Kilo, the Romeo class. The Type 39A was thought to be just an improved Song, but on closer examination, especially by the Russians, it looked like a clone of the Kilos. The Yuan class also have AIP (Air Independent Propulsion), which allows non-nuclear boats to stay underwater for days at a time. China currently has 13 Song class, 12 Kilo class, two Yuan class and 25 Romeo class boats. There are only three Han class SSNs, as the Chinese are still having a lot of problems with nuclear power in subs. Despite that, the Hans are going to sea, even though they are noisy and easily detected by Western sensors.