HSwMS Sjöormen (Sor)  HSwMS "Sjöormen" (Sor), 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 . The hulls was covered with rubber tiles to reduce the acoustic signature (Anechoic tiles), at this time a pioneer technology. The "Sjöormen"-class also pioneered the use of an x-shaped (as opposed to cross-shaped) rudder as a standard (as opposed to experimental) feature. 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.