Instead of the gas turbines or diesel-electric systems used for propulsion on many modern warships, the carriers use two A4W pressurized water reactors which drive four propeller shafts and can produce a maximum speed of over and maximum power of around . As a result of the use of nuclear power, the ships are capable of operating for over 20 years without refueling and are predicted to have a service life of over 50 years. They are categorized as nuclear-powered aircraft carriers and are numbered with consecutive hull numbers between CVN-68 and CVN-77.