The U.S. Navy continues to entertain an internal debate over the issue of just how effective non-nuclear submarines would be in wartime, and whether the U.S. should buy some of these non-nuclear boats itself. This radical proposal is based on two compelling factors. First, the U.S. Navy may not get enough money to maintain a force of 40-50 SSNs (attack subs.) Second, the quietness of modern diesel-electric boats puts nuclear subs at a serious disadvantage, especially in coastal waters.
In an attempt to settle the matter, from 2005 to 2007, the United States leased a Swedish sub (Sweden only has five subs in service), and its crew, to help train American anti-submarine forces. This Swedish boat was a "worst case" scenario, an approach that is preferred for training. The Gotland class Swedish subs involved are small (1,500 tons, 200 feet long) and have a small crew of 25. The Gotland was based in San Diego, along with three dozen civilian technicians to help with maintenance.
For many years before the Gotland arrived, the U.S. Navy had trained against Australian diesel-electric subs, and often came out second. The Gotland has one advantage over the Australian boats, because of its AIP system (which allows it to stay under water, silently, for several weeks at a time). Thus the Gotland is something of a worst case in terms of what American surface ships and submarines might have to face in a future naval war. None of America's most likely naval opponents (China, North Korea or Iran), have AIP boats yet, but they do have plenty of diesel-electric subs which, in the hands of skilled crews, can be pretty deadly.
One solution to the problem would be a new French design, the Andrasta class coastal boats. The Andrastas are 855 ton, 153 foot long subs, with a crew of 19 (plus 8 passengers, usually commandos). The boat can stay underwater for up to five days. Surfaced, it can travel up to 5,400 kilometers, at slow (170 kilometers a day) speed. There are sufficient supplies on board to keep the boat out up to 30 days. Thus the Andrastas can be deployed, slowly, anywhere in the world. Most missions would be more like two weeks, before the boat returned to port, or a sub tender (a support ship carrying fuel, supplies, maintenance capabilities, and even relief crews). The Andrasta has six forward firing torpedo tubes, which can also carry mines or anti-ship missiles. There are no reloads, all the weapons are stored in the torpedo tubes. There is a special chamber for letting divers exit the boat while underwater. These boats cost $200 million, a tenth of the cost of a Virginia class SSN (a 7,700 ton, 377 foot long boat with a crew of 134 and over 30 torpedoes and missiles on board).
Based on the experience with Australian and Swedish subs, the U.S. Navy has been developing new anti-submarine tactics and equipment. In secret, obviously. But apparently the modern, quiet diesel electric boats continue to be a major threat to U.S. surface warships and subs. Meanwhile, potential enemies build more of their cheaper, and higher quality, diesel-electric boats, and train their crews by having them stalk actual warships (including U.S. ones.) The subs are getting more numerous, while U.S. defenses are limping along because of the sheer technical problems of finding quiet diesel-electric boats in coastal waters.