Salisbury-class frigate
The Type 61 "Salisbury" class was a class of the Royal Navy aircraft direction (AD) frigate, built in the 1950s. The purpose of the aircraft direction ships was to provide radar picket duties at some distance from a carrier task force and offer interception guidance to aircraft operating in their area. The class was part of a multi-purpose frigate concept that also included the Type 41 "Leopard"-class anti-aircraft ships and the cancelled Type 11 anti-submarine variant. Together, they were the first ships in the Royal Navy to use diesel propulsion. Improvements in conventional steam  turbine power erased the range advantage of the diesel and led to future purchases of Type 61 and 41 being cancelled or converted to the new Type 12 frigate. Design. The "Salisbury" class frigates were conceived as part of the 1944 project for common-hull diesel-powered 1700-ton anti-submarine (A/S), air-warfare (AW) and aircraft direction (AD) vessels. The first two prototypes were ordered in the late 1945 construction programme and were built as HMS "Salisbury" and a second diesel, HMS "Leopard" (T41) as a suitable A/S frigate or sloop prototype could not proceed at the  time. By 1947 the legend (i.e: the complete plan of the RN warship, delivered to the shipyard to start building) of the Type 61 AD frigates and its sister Type 41 AW light destroyer were complete. The design of the new air defence frigates could proceed faster as the requirements were clearer and less complex and fuel-efficient diesel power was adequate for convoy escort picket ships. Destroyers could be converted for faster picket duties with carrier task forces. The design of new anti-submarine frigates was delayed due to the expectation that the Soviet Union would build submarines that  were much faster underwater, based on the German Type XXI and Type XXIII submarines, which operated at 12–16 knot underwater and HP (hydrogen peroxide) submarines running at 22–25 knots, submerged. This led the navy to revert to steam turbines and the slower evolution of the Type 12 variant. Steam turbines provided the quietness and speed desirable for anti-submarine applications. However, the design of efficient powerful steam turbines, for affordable common hulled anti-sub frigates with the range to escort Atlantic convoys and speed to screen carrier task forces, took years and was never entirely achieved. Neither the AD "Dido"-class  cruisers nor the "Daring"-class destroyers had the space required to combine the processing of radar and communications with dual-purpose AA guns. This integration was complicated, making the new diesel air warning and aircraft direction frigates an even higher priority. The legend of the lead Type 12 anti-submarine frigate was not drawn until 1950 as a steam version of the diesel Type 61. The original steam Type 11 frigate concept was abandoned in 1945 and was never designed. The Type 61 was the first of the new generation frigates laid down in 1951. Without the second Mk 6 4.5 in gun turret of the T41 variant, the T61 had a hundred tons extra capacity for fuel and the longest range of the frigate variants, 5000 nm at 15k compared with 4500 nm for the T41 or "officially" at 12k for T12(steam) actual range was only 2500-3000 nm, just Pearl Harbour to San Diego at 14k It was seen as much more important than the related Type 41 ("Leopard"-class) frigates, but with reduced armament (one twin 4.5-inch mount instead of two) to make way for more aircraft direction equipment. The Type 61 carried the Type 982 rake Air  Warning 200 km range radar and 277M search and height finder in addition to the radars on the Type 41 but the two additional systems added only 23 tons compared with the saving of 115 tons by excluding the second 4.5 turret. The Type 61 was later refitted with the high mounted four-ton antenna of the Type 965 (AKE-2) radar. The aircraft direction and air-warning frigates provided extra stations to the aircraft carriers to track incoming air attacks, and direct and communicate with defensive Royal Navy (RN) and land-based fighters. This role of AD cruisers was  seen in Operation Musketeer during the Suez Crisis, 1956, in which Hawker Sea Hawk ground attack and English Electric Canberra and Vickers Valiant bombers struck land air bases and other targets. Directing carrier-based air interception and strike operations was far more important than the "little cat" Type 41s or "big cat", Tiger-class cruiser's guns. In the mid-1950s, the Royal Navy was largely operating small light fleet carriers and first-generation jets which could takeoff from slow-moving carriers. In 1960, a second flotilla of four extra Type 61 AD frigates was planned. However, by 1961–62, the big  carrier HMS "Ark Royal"s problems were debugged, reconstructed small carriers HMS "Victorious and HMS "Hermes" came into effective service (with second-generation de Havilland Sea Vixens and Supermarine Scimitars) and the RN best carrier HMS "Eagle" was being reconstructed. Only the four Battle-class AD conversions were suitable as fast carrier pickets, as the Type 61's diesel power plant lacked the speed for operations with fast carrier groups. In 1962, orders for extra Type 61s were cancelled, long after the second flotilla of Type 41s was abandoned in 1955–1957, and a 2,000-ton 'East Coast convoy' Type 42 frigate  (a 25 knot derivative of the T41/61 diesel hull with 3/N5 4-inch automatic Vickers guns with 2/4 MR3 DCT and 40mm the T42 (1955) was a pocket RN diesel version of Chile's Vickers-built Almirante-class destroyers) was cancelled with the 1957 Defence White Paper. The role of the Type 61 was as a seaworthy air-ocean surveillance ship and air-control ship to escort slow task forces, such as amphibious warfare task forces. In the 1960s the T61 were still seen as important units and their modernisation was much more substantial than that of the  Type 41. The election of another Labour Government in 1974 threatened to bring the T61 service life to a premature end and the two T61 still deployed East of Suez in Jan 1976,"Chichester" was struck and "Llandaff" sold to Bangladesh by the end of 1976. Seacat missile-fitted T61 had a life extension in 1976, due to the Cod War confrontations. The T41 HMS "Jaguar" and T61 HMS "Lincoln" were refitted as specialised rammers with a reinforced bow to present a higher-level threat to Icelandic gunboats. The stronger UK/RN stand led to the parties in the 3rd Cod  war settling before "Lincoln" finished its trials, but with the expanded 200 mile offshore zones, partly stemming from the Cod Wars, she remained in commission to late 1977 on deep water patrol. The possibility of a new generation of diesel-electric anti-submarine Type 23 frigates resulted in the diesel-electric powered, Type 61 HMS "Lincoln" being refitted in 1978 to NATO operational frigate standard, to test certain hull characteristics and silencing of diesel electric engines, relative to passive sonar operation. The primary aircraft direction equipment fitted to the Type 61s was initially the Type 960 radar for aircraft warning  and Type 982M radar for a degree of 3D cover and better air control over land. The Type 960 radar was replaced by Type 965P at refit as follows: The Type 965 (AKE-2), had a large "double bedstead" antenna, while the Type 982M radar had a smaller "hayrake" antenna. The Seacat missile system was fitted to "Lincoln" in a long refit from 1966 to 1968 and in "Salisbury" from 1967 to 1970. It was the same GWS 20 optically guided system being refitted at the time to the "Rothesay"-class frigates. "Llandaff" continued to carry the twin MK 5  Bofors until sold to Bangladesh. In the late 1960s "Lincoln", "Salisbury", "Llandaff" and the aircraft carriers "Ark Royal" and "Bulwark" were all refitted with the new Type 986 radar using the 982 antenna, as a partial substitute for the 984 3D radar capability lost with the phaseout of the RN strike carriers. The Type 986 radar was intended to partially replace one of the roles of 984, giving more accurate, short-range definition of closing air targets to . It was only a partial replacement, as it lacked the 984 system's ability to rank and prioritize large numbers of targets  for interrogation and air interception. The 965 twin array radar was limited and obsolete by the 1970s. In 1973, HMS "Chichester" was downgraded to a Hong Kong guardship with a reduced gun armament of twin 4.5-inch; one 40 mm and two 20 mm and air surveillance radars removed. HMS "Lincoln" was seriously damaged in the second Cod War. In 1974, the new Labour Government made a policy decision that only anti-submarine frigates would be operational in the frigate fleet from then on. Therefore, for the rest of the decade, "Salisbury" and "Lincoln" alternated between the standby squadron and  lengthy re-activations under a number of pretexts. HMS "Salisbury", under the first Frigate command of Hugo White (later Admiral of the Fleet), was extensively involved in the third Cod War, holding the line against Icelandic gunboats within of multinational fishing fleets, colliding seven times with the Iceland gunboats "Tyr" and "Aegir" in March and April 1976. Following serious damage to RN frigates in the Cod War, HMS "Lincoln" was repaired and returned to service until the end of the decade. After refits, it returned to the status of an operational RN frigate declared to NATO. Construction programme. Three further  ships of the class were planned. Two of these, intended as HMS "Exeter" and "Gloucester", were cancelled under the 1957 Defence Review, while a third, HMS "Coventry", was suspended. It was hoped to order "Coventry" in 1961, but in the end it was decided to order the planned hull as a that became .