are fitted with Voith Schneider Propellers, giving it the highest manoeuvrability in the navy. The RSN also operates the Protector unmanned surface vehicles. They were deployed together with the Endurance-class landing ships tank to the North Persian Gulf for peacekeeping operations in 2005, where they performed surveillance and reconnaissance, as well as force protection duties for more than eight hours at a go. Unmanned assets and the mine countermeasures ships form the Sixth Flotilla of the RSN. Submarines Challenger class In 1995, the RSN acquired a from the Swedish Navy and rechristened it the . Another three were transferred in 1997, making them Singapore's first underwater platforms. As the submarines were designed for operations in the Baltic Sea, various modifications were required to suit them to tropical waters. A comprehensive tropicalisation refitting programme was implemented for all four submarines, which involved the installation of air conditioning, marine growth protection systems and corrosion-resistant piping. It was believed that the Challenger class were purchased to develop the required submarine operations expertise before selecting a modern class of submarines to replace them, since all the boats were then over 40 years old. RSS Challenger and RSS Centurion were retired from service in 2015. Archer class The submarines were also former Swedish Navy combatants and known as the . In November 2005, an agreement was signed with Kockums for the same tropicalisation modifications for two submarines. Amidst the refitting process, an air independent propulsion system was installed, which necessitated the cutting apart of the submarine's hull to fit new components. RSS Archer was relaunched on 16 June 2009 and recommissioned on 2 December 2011, with her sister boat RSS Swordsman being commissioned on 30 April 2013. The AIP modification enables the submarines to have longer submerged endurance and lower noise signature, enhancing its stealth capability. The advanced sonar system is capable of detecting contacts at a further distance, while the torpedo system has a better target acquisition capability, which allows the submarines to engage contacts at a further range. An upgrade program was conducted between 2016 and early 2019, which involved the installation of CM010 optronic periscopes and new combat management, sonar and countermeasure systems. Invincible class The , also known as Type 218SG, is a submarine class ordered from Germany's ThyssenKrupp Marine Systems. Two Invincible-class submarines were procured in November 2013 and another two in May 2017. Several German industry experts commented then