HMS Gotland became the first submarine to use an operational Stirling-engine air-independent propulsion system.
The Swedish Gotland class entered service in the late 1990s. Its Stirling engines generate electricity using liquid oxygen and diesel fuel, allowing the boat to remain submerged far longer than a conventional diesel-electric submarine running only on batteries.
Air-independent propulsion does not mean unlimited underwater endurance: the submarine still needs fuel and oxygen, and it eventually requires resupply. However, it greatly reduces the need to surface or raise a snorkel. The Gotland class attracted international attention when HMS Gotland trained with the United States Navy in 2005–2007 and successfully simulated attacks against much larger surface forces.
The class includes Gotland, Uppland, and Halland. Its technology influenced later Swedish submarine designs and several foreign AIP programs.