When originally designed in 1980 to replace the Leander and Broadsword Class ships the Type 23 Frigate was intended to be a simple and relatively inexpensive ship, with minimal armament, which would be supported by new generation Royal Fleet Auxiliaries that would provide hanger, repair and maintenance facilities for the frigate’s helicopter as well as area defence with a Seawolf missile system. As a result of changing operational requirements the ship was redesigned to operate the Sea King and Merlin helicopters while its armament was enhanced by installation of Seawolf and Harpoon missile systems as well as two additional torpedo tubes and the replacement of the OTO Melera 76mm gun with the more powerful Vickers 4.5 inch gun. These design changes resulted in a more sophisticated, larger and therefore more expensive ship. The Type 23 Frigates are powered by Combined Diesel, Electric and Gas Turbine Engines (CODLAG) with the gas turbines being used for cruising while the electric motors allow for virtually silent running when performing anti-submarine duties. To further reduce the noise signature of the vessels the engines are fitted above the waterline and the propellers have small holes around the edges. The ships are designed to be "stealthy" with the hull having rounded edges and all vertical surfaces having a slope of 7º to reduce the radar signature of the vessel while the funnels are fitted with cooling devices to reduce the heat signature.