At the time of their deployment they were regarded as one of the most advanced non-nuclear submarine-classes in the world, incorporating many new features including x-rudder and anechoic tiles.

In the late 1990s, all five submarines were acquired by the Republic of Singapore Navy (RSN) and four were relaunched as the Challenger class following modernisation and tropicalisation.

Designed for the confined waters of the Baltic sea, the vessels had an endurance of 21 days and a test depth of 150 metres (490 ft).

[2][3] In 1984–85, the Sjöormen class received upgraded Ericsson IBS-A17 fire control system and CSU-83 sonar suite.

In 1992, refits began on Sjölejonet and Sjöhunden that improved their electronics and their towed sonar array.