HMS Sjöormen, Sw. meaning sea serpent, was the lead ship in the Swedish submarine class Sjöormen, project name A11. The planning of the ships included a number of different AIP-solutions including nuclear propulsion, however the ships where finally completed with for the time extremely large batteries. The ship was a single hull submarine, with hull shape influenced by the American experimental submarine USS Albacore (AGSS-569). The hulls was covered with rubber tiles to reduce the acoustic signature (Anechoic tiles), at this time a pioneer technology. The Sjöormen-class was also the first in the world to use an cross-rudder. The submarine served in the Swedish navy for almost 30 years and was then after upgrades sold to Singapore in 1997 together with its four sister ships.