The Sjöormen class (Sea serpent) was a class of submarines built for the Swedish Navy in the late 1960s. They had a teardrop hull shape and were capable of diving to 150 metres (490 ft). 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.^[1] Both speed and underwater endurance was at this time very high for a conventional submarine. The submarines were retired by Sweden in the early 1990s. In the late 1990s, four submarines were acquired by the Republic of Singapore Navy (RSN) and relaunched as the Challenger class following modernisation and tropicalisation.

As built, the Sjöormen class were designed with a teardrop hull shape, based on the United States' Barbel class. They had bow planes on the sail and their stern diving planes were configured in a x-shape.^[2] They had a standard displacement of 1,075 tonnes (1,058 long tons; 1,185 short tons) and 1,400 tonnes (1,400 long tons; 1,500 short tons) when dived.^{[3][lower-alpha 1]} The submarines had a waterline length of 50 metres (164.0 ft) and a length overall of 51 metres (167.3 ft). They had a beam of 6.1 metres (20.0 ft) and a draught of 5.8 metres (19.0 ft). The Sjöormen class was powered by a diesel-electric propulsion system composed of two Pielstick diesel engines providing power to an ASEA electric motor driving one shaft with a five-bladed propeller. The entire system was rated at 2,200 brake horsepower (1,600 kW). This gave the submarines a surfaced speed of 15 knots (28 km/h; 17 mph) and 20 knots (37 km/h; 23 mph) submerged. 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]

As built the Sjöormen class were equipped with surface search radar and sonar.^[3] The submarines were armed with four 533 mm (21 in) torpedo tubes located in the bow for surface attack and two 408 mm (16 in) torpedo tubes in the stern for either anti-submarine warfare or for naval mines. The Sjöormen class had a complement of 23 officers and enlisted.^[2][3]

The Sjöormen class were ordered by the Swedish Navy in 1961. The first boat in the class, Sjöormen, entered service in 1968. In 1992, two vessels in the class, Sjölejonet and Sjöhunden, underwent modernisation. The remaining three were supposed to remain in service until the Gotland class became operational, but due to lack of funding, all were laid up in 1993.^[2]

Four submarines were sold to the Republic of Singapore Navy (RSN) in the 1990s and entered service as the Challenger class following modernisation and tropicalisation.^[4][5]