The Royal Australian Navy (RAN) fleet is made up of 38 commissioned warships and 6 non-commissioned . The main strength is the eight frigates and three destroyers of the surface combatant force: eight Anzac class frigates and three Hobart class destroyers. Six Collins-class boats make up the submarine service, although due to the maintenance cycle not all submarines are active at any time. The issues have now been fixed and five submarines are available for service. Amphibious warfare assets include two Canberra-class landing helicopter dock ships and the landing ship . Eight Armidale-class patrol boats perform coastal and economic exclusion zone patrols, and four Huon-class vessels are used for minehunting and clearance (another two are commissioned but in reserve since October 2011, for sale as 2018). Replenishment at sea is provided by two Supply-class replenishment oilers, while the two Leeuwin-class and two Paluma-class vessels perform survey and charting duties. In addition to the commissioned warships, the RAN operates the sail training ship Young Endeavour and five Cape-class patrol boats. Other auxiliaries and small craft are not operated by the RAN, but by DMS Maritime, who are contracted to provide support services.<ref name=SaundersPhilpott35>Saunders & Philpott (eds.), IHS Jane's Fighting Ships 2015–2016, p. 35</ref> The lion's share of the RAN fleet is divided between Fleet Base East (, in Sydney) and Fleet Base West (, near Perth). Mine warfare assets are located at (also in Sydney), while in Cairns and in Darwin host the navy's patrol and survey vessels. Submarines Collins class Australia operates a single class of diesel-electric submarines, the six Collins-class boats which began entering service in 1993. The Collins was designed by the Swedish submarine builder Kockums as the Type 471 specifically to meet Australian requirements, many of which were derived from Australia's need for great range without utilizing a nuclear propulsion system. The boats themselves were built in Australia by the Australian Submarine Corporation in Adelaide. The submarines are classified by the RAN as guided missile submarines (SSG), but are often referred to as hunter-killer submarines (SSK) in the international press. While these vessels represented a major increase in capability for the RAN, they have found themselves mired in numerous technical and operational problems. Meanwhile, the RAN has struggled to sufficiently crew their submarine fleet, with at times no more than two qualified crews available. The Collins was to be replaced by twelve Shortfin Barracudas a conventionally-powered design