The latter two boats underwent additional retrofitting from the year 2000: this includes above all a new section with the AIP system of Stirling engines . A new air conditioning system was installed as well as a lock for the deployment of special units . They were then put back into service as the Södermanland class .
During the modernization, the boats were cut apart immediately aft of the tower and lengthened to 60 m by inserting a new 12 m long section. The new section houses the new Stirling AIP system, liquid oxygen tanks and electric generators. After the modernization, the boats can now also be used in tropical waters. In the past, the boats were only designed for use in cold Nordic waters. Higher temperatures of the surrounding water could have led to problems with increased internal temperature and increased condensation, because previously seawater was pumped on board for cooling. Now the submarines are equipped with heat exchangers instead . In addition to improving the stealth properties of the boats, the on-board and weapon systems were also brought up to the state of the art. With the Södermanland class, the Swedish Navy now only has submarines with independent air propulsion.
Archer- class
The other two were put on reserve service until November 2005 before being sold to the Singapore Navy, the Republic of Singapore Navy (RSN). They will also be upgraded to the Södermanland- class standard and will receive additional air conditioning for service in tropical waters. The Defense Science and Technology Agency (DSTA) of Singapore and ST also equipped the boats with new command and control systems and fire control systems. The boats are now 60.5 m long and have a crew of 27 men. Training of RSN personnel began in 2007, and the Archer (ex Hälsingland ) arrived at Changi Naval Base on August 17, 2011, where it entered service on December 2, 2010. The Swordsman (ex Västergötland ) was taken over in Changi on December 31, 2012 and is scheduled to enter service in early 2013. They form the "171. Squadron "(squadron).
