in World War II. However, when the US designed the "treaty cruisers", beginning with launched in 1927, geared turbines were used to conserve weight, and remained in use for all fast steam-powered ships thereafter. Current usage Since the 1980s, steam turbines have been replaced by gas turbines on fast ships and by diesel engines on other ships; exceptions are nuclear-powered ships and submarines and LNG carriers. Some auxiliary ships continue to use steam propulsion. In the U.S. Navy, the conventionally powered steam turbine is still in use on all but one of the Wasp-class amphibious assault ships. The Royal Navy decommissioned its last conventional steam-powered surface warship class, the , in 2002, with the Italian Navy following in 2006 by decommissioning its last conventional steam-powered surface warships, the s. In 2013, the French Navy ended its steam era with the decommissioning of its last . Amongst the other blue-water navies, the Russian Navy currently operates steam-powered s and s. The Indian Navy currently operates INS Vikramaditya, a modified ; it also operates three s commissioned in the early 2000s. The Chinese Navy currently operates steam-powered s, s along with s and the lone Type 051B destroyer. Most other naval forces have either retired or re-engined their steam-powered warships. As of 2020, the Mexican Navy operates four steam-powered former U.S. s. The Egyptian Navy and the Republic of China Navy respectively operate two and six former U.S. s. The Ecuadorian Navy currently operates two steam-powered s (modified s). Today, propulsion steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications. Diesel power plants also have lower operating costs since fewer operators are required. Thus, conventional steam power is used in very few new ships. An exception is LNG carriers which often find it more economical to use boil-off gas with a steam turbine than to re-liquify it. Nuclear-powered ships and submarines use a nuclear reactor to create steam for turbines. Nuclear power is often chosen where diesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's s is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire service life. Nuclear propulsion has only been applied to a very few commercial vessels due to the