In 1995, Singapore purchased 4 A-11 Challenger Class (ex-Sjöbjörnen Class), the first of which was launched in 1968. The Challenger Class were not only refitted for tropical conditions, but also received weapons system and sensor upgrades. These submarines performed very well in exercises against US Navy and Royal Australian Navy units, proving that these old boats, when handled well in littoral conditions, are quiet and maneuverable subs that are able to give a pretty good account of themselves.
Of the 4 submarines in the Vastergotland class that Sweden built between 1987 and 1990, the first two boats, the HMS HMS Vastergotland and HMS Halsingland were taken out of Royal Swedish Navy active service in 2004, while the other two, the HMS Sodermanland and HMS Ostergotland, received such significant upgrades in 2003-2004 that they are considered a new class.
In particular, apart from weapons system and stealth system improvements, these boats received a 12 metre (36 foot) hull extension in order to use the Stirling Air Independent Propulsion System, making them, along with the 3 submarines of the follow on A-19 Gotland Class, probably the quietest and most advanced conventional submarines currently in operational service the world. The Stirling AIP system is a propulsion system that uses diesel fuel and Liquid Oxygen in a closed system, and greatly increases a conventional submarine's submerged endurance. With AIP systems, conventional submarines can go for weeks without having to surface or snorkel in order to recharge their batteries.
Singapore has purchased the deactivated HMS Vastergotland and HMS Halsingland, and it is more than likely that part of the upgrades that these boats will receive will include the Stirling AIP hull extension, making them virtually identical to the Sodermanland Class. This means that by 2010, the RSN will be operating two of the quietest and most lethal conventional submarines in the world, and probably at a pretty good price - definitely below the cost of a new Gotland sub, which has been suggested at $100 million each, while the German U-212 Class has been estimated at $250 million a copy.
Just how lethal will these subs be? Well, take into consideration that the state-of-the-art Gotland Class, which is essentially an improved Vastergotland Class with the Stirling AIP system incorporated from the beginning, is considered so quiet and so deadly that the USN has leased the HMS Gotland for a year in order to practice and develop its ASW tactics against a first class opponent.
Singapore will buy 2 Swedish A-17 Vastergotland submarines, which are expected to enter service starting in 2010, replacing some of current Challenger-class submarines. This fits into Singapore's traditional method of acquiring new capabilities, where old refurbished platforms are first purchased to gain operational experience before newer weapons systems are acquired.