The 46 Knox-class frigates were the largest, last, and most numerous of the US Navy's second-generation anti-submarine warfare (ASW) escorts. Originally laid down as ocean escorts (formerly called destroyer escorts), they were all redesignated as frigates on 30 June 1975, in the 1975 ship reclassification plan and their hull designation changed from 'DE' to 'FF'. The Knox class was the Navy's last destroyer-type design with a steam turbine powerplant.
Due to their unequal comparison to destroyers then in service (larger size with lower speed and only a single propeller and 5-inch gun), they became known to a generation of destroyermen as "McNamara's Folly."
These ships were retired from the US Navy at the end of the Cold War due to their relatively high running costs, a declining defense budget, and the need for ships with a more advanced anti-submarine capability. None of the ships served more than 23 years in the US Navy, and by 1994, all of the class had been retired, although some remain in service with foreign nations such as Egypt, Taiwan, Thailand, and Mexico.
Learn more on Wikipedia