Leopard-class frigate
The Type 41 or "Leopard" class were a class of anti-aircraft defence frigates built for the Royal Navy (4 ships) and Indian Navy (3 ships) in the 1950s. The Type 41, together with the Type 61 variant introduced diesel propulsion into the Royal Navy, the perceived benefits being long range, low fuel use, reduced crew (especially skilled artificers), and reduced complexity. Although successful, improvements in traditional steam turbine technology erased the fuel economy advantage of the diesel powerplants and led to production being curtailed in favour of the Type 12 frigate, which was similar in overall  design. Design. These ships were designed to provide anti-aircraft escorts to convoys and amphibious groups and act as light destroyers on detached duties. They were not intended to operate with fleet carrier task forces which had speeds of over 28 knots and were escorted by destroyers and similar vessels, and therefore made only . They were envisioned in late World War II and immediately after as part of a 1945 project for anti-submarine, anti-aircraft, and Aircraft Direction frigates which would all share a common hull and propulsion, and the design of the Type 41 was completed by December  1947. Like the 1950 RAN Battle-class variant (actually the Royal Navy variant, for war emergency production) and the unbuilt 1942 two-turret RN G destroyer, which the 1944 common hull escort closely resembles (shipyards building the Type 41, like Dennys Glasgow yard, had been provided with the full 1944 "Gallant"-class plans), the Type 41 "Leopard" class used the latest twin semi-auto 4.5" Mk6 turrets. This meant that, unlike other post-war frigates, the Type 41 had a full destroyer armament of two twin 4.5" Mk6 gun turrets, giving them a more powerful armament than the Battle- or Weapon -class destroyers. The first production orders were in the 1951/2 and 1952/3 programmes. In 1953 eleven additional Type 41s, also with cat names like "Cougar" and "Cheetah", were planned, together with ten Type 61 or "Salisbury"-class frigates, with which they shared a common hull and machinery. Distinct from the Type 61, the Type 41 radar fit also supported surface fighting, whereas the radar fit of the Type 61 A/D ("Aircraft Direction") frigates was, when introduced, largely identical to the reconstructed "Dido"-class A/D cruiser "Royalist". To that end, HMS "Leopard" carried navigation radar, the new type 992 for long -range surface target indication, and the type 960M for LRAW as compared to the Type 61's four dedicated LRAW systems: types 293, 977M, 960M and 982M. An intended A/S version, the Type 11 class (see Type system of the Royal Navy), was cancelled due to the low 24-knot top speed being insufficient for accompanying fast carrier task forces, particularly with HMS "Eagle", the flagship, commissioned in 1951. However, in practice, frigates and destroyers moving at more than 25 knots create turbulence which blinds their own sonars and can only engage fast-moving subs by  using a helicopter with its own sonar. Thus the Type 41s were still fitted with the best late-1950s RN sonars, types 170 and 174 (which remained a good passive sonar into the 1970s), but were equipped with only a minimal A/S mortar battery. Through their diesel propulsion, the Type 41s achieved long range through their low fuel use. The ships had a total of twelve Admiralty Standard Range Mk.1 (ASR1) diesel engines disposed four-each in three engine rooms. In the fore and aft engine rooms, two engines were connected to the drive shafts by fluid clutches and reduction  gearboxes, while the other two were not connected to the shafts, but instead drove 360 kW alternators to provide electrical power. In the centre engine room, all four engines were connected to the shafts. "Jaguar" was fitted with controllable-pitch propellers. Initially the diesel engines proved somewhat unreliable, but these teething troubles were gradually overcome and reliability eventually became very satisfactory. The "Leopard" class was also fitted with an early type of hydraulic stabiliser system consisting of two fins that could be extended outside the main hull, to port and starboard, from a compartment between the two engine rooms. Gyro  controlled with a relatively simple control system, they proved very effective in use. During testing every three months at sea, the ship could be easily driven into a 20°+ roll from the manual control on the bridge. Prior warning had to be given over the ship's tannoy system before testing was carried out, to allow stowage of loose items. A slight reduction in top speed was also noticed when in use. However, by 1955 success had been achieved, with difficulty and limitations, in developing new steam turbines giving 30-knot speed and the range to take convoys  across the Atlantic, embodied in the "Whitby"-class Type 12 frigates. As a result, the orders for the new diesel-electric frigates were cancelled, changed to orders for Type 12, or sold to India. Within a few years of the Type 41's introduction in the late 1950s they were regarded as obsolete for their intended function as anti-aircraft convoy escorts. This was emphasized when the planned replacement of the 4.5" guns with 3"/70 AA guns was abandoned (in January 1955) due to cost and the view that AA guns were obsolete against jets and missiles. The addition of power -ramming for the twin 4.5" guns, intended to boost the rate of fire from 14rpm to 24rpm, failed. Replacement of the unreliable STAAG 40mm gun mount by Seacat surface-to-air missiles was cancelled on economic grounds, and the guns eventually replaced by a single, manually operated Bofors gun. Replacement of the experimental version of the fast rotating 992 target indicators with the slower standard 993 was also abandoned. Only a short range 262 radar MRS1 provided secondary AA fire control for the main armament. Service. In service, the "Leopard" class were used mainly as patrol frigates, notably on  the South American station, where their long range and impressive destroyer-like appearance were particularly advantageous. Operating out of Simonstown in South Africa, they in part replaced the "Dido"-class cruisers HMS "Euralyus" and "Cleopatra" usually deployed on these duties during 1946–1954. It was hoped a pair of Type 41 gunships with four twin-4.5" guns between them would be adequate to deter a single Russian "Sverdlov" cruiser, which British Naval Intelligence saw as having been in part conceived of to threaten the traditional trade from Buenos Aires to England. Later they were extensively used in the Far East during the 1963–68  confrontation with Indonesia over Borneo and Malaysia, for which all-gun-armed Type 41s were again well suited. In the 1970s they saw service on Cod War duties. In 1972 it was decided not to refit HMS "Puma" again, as purchasing the half-sister of the class, the former "Black Star" ordered by Ghana, and commissioning it as HMS "Mermaid", would cost less than a Type 41 refit. HMS "Leopard" finished its service in the 1975–1976 Cod War, having given an Icelandic gunboat a 30-second warning that it would open fire with its 4.5" guns. HMS "Lynx" was the  last of the class operational, in 1977 attending the Spithead fleet review. HMS "Jaguar" was reactivated from the standby squadron for the 3rd Cod War, but sprang too many leaks on the voyage to Iceland and instead returned to Chatham. HMS "Jaguar" and HMS "Lynx" were sold to the Bangladesh Navy in 1978 and March 1982 respectively. Had they been retained a few more years they could have been ideal during the Falklands War for specialized bombardment and the air defence of ships unloading in San Carlos Water. The destroyers and frigates remaining in RN service in 1982 had only  one gun turret, the new 4.5" Mk.8 often jammed, and those with the Mk.6 twin-4.5" (which required 40-45 men required for each turret) rarely even test fired the guns. As it was, the Bangladesh Navy found the "Leopard"-class satisfactory and useful for long life, the ships being active until they were retired in 2013. Construction programme. A fifth Royal Navy vessel, HMS "Panther" was ordered twice. The first was transferred to India in 1953 before being laid down, a replacement was cancelled in 1957, before being laid down.