Sweden has finally settled on a replacement for its Cold War era Gotland class submarines. Two A26 subs will enter service in 2018 and 2019 and eventually replace the three Gotlands that entered service in 1996. The U.S. Navy had a high opinion of the Gotlands as they leased one of them (along with Swedish crew) for two years (2006-7) to be a vital part of an anti-submarine warfare training program.
The diesel-electric Gotland was the first submarine designed from the start to use air-independent propulsion (AIP) and could remain submerged for 19 days. Gotlands were among the quietest non-nuclear submarines in the world. The three Gotland class boats are highly automated, with a crew of 30. They displace 1,494 tons, are 60.4 meters (198 feet) long and have four 533mm torpedo tubes (with 12 torpedoes) and two 400mm tubes (with six torpedoes). They can also carry 48 mines externally.
The A26 is a 1,900 ton boat that is 63 meters (207) feet long and armed in a similar fashion to the Gotlands but with about 20 percent more mines and torpedoes. Each one will cost about $500 million. Underwater endurance (with an improved AIP) is the same as the Gotlands 18 days with overall endurance 45 days. The A26 crew is smaller (about). The A26 has better electronics and can dive a bit deeper (at least 200 meters/650 feet). Both designs were mainly intended for coastal waters and the relatively shallow Baltic Sea (average depth 55 meters and max depth 459 meters). The A26 is also equipped to carry naval commandos and has a special chamber for the commandos to leave and enter the submerged sub.
Meanwhile the three Gotland boats are undergoing refurbishment and upgrades, which include some of the new gear the A26s will receive. The refurbed Gotlands can serve into the late 2020s if need be. There are many nations who seek to buy second-hand Swedish subs and that’s what may happen to the Gotlands.
The Gotland leased to the United States was based in San Diego, along with three dozen civilian technicians to help with maintenance. The U.S. Navy has also trained against Australian diesel-electric subs, and often come out second. The Gotland was more modern than the Australian boats, particularly because of its AIP system. 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, but they do have plenty of diesel-electric subs which, in the hands of skilled crews, can be pretty deadly. China is building now building subs with AIP.
Training against the Gotland enabled the U.S. Navy to improve its anti-submarine tactics and techniques, as well as getting much valuable data from inside the Gotland. All the results of this training is highly classified, but it was apparently successful enough to get the one year program extended for another year.