The Leander-class, or Type 12I (Improved) frigates,^[1][2][3] comprising twenty-six vessels, was among the most numerous and long-lived classes of frigate in the Royal Navy's modern history. The class was built in three batches between 1959 and 1973. It had an unusually high public profile, due to the popular BBC television drama series Warship. The Leander silhouette became synonymous with the Royal Navy through the 1960s until the 1980s.

The Leander design or derivatives of it were built for other navies:

• Royal New Zealand Navy as the Leander class
• Chilean Navy: Condell class
• Royal Australian Navy: River class
• Indian Navy: Nilgiri class
• Royal Netherlands Navy: Van Speijk class

The policy adopted by the Royal Navy during the 1950s of acquiring separate types of frigates designed for specialised roles (i.e. anti-submarine, anti-aircraft and aircraft direction) had proved unsatisfactory. Although the designs themselves had proved successful, the lack of standardisation between the different classes led to increased costs during construction and also in maintenance once the ships became operational. Furthermore, it was not always possible to have the ships with the required capabilities available for a specific task. The first move towards creating a truly general-purpose frigate came with the Type 81 Tribal class which was initially ordered in 1956. The 24-knot speed of the Tribals was considered the maximum possible for tracking submarines with the new medium-range sonars,^[4] entering service. The type 81 gas turbine saw the frigates underway quickly, without taking hours flashing up steam turbines, and the provision of a helicopter for long-range attack were considered essential in the nuclear age. These ships were mainly intended to operate in the tropics but lacked the speed and armament required for the priority fleet carrier escort role East of Suez, where fast radar picket capability was important, as much as anti-submarine capability. So the new frigates would combine the roles of the T12 and T61. The fully air-conditioned Royal New Zealand Navy Rothesay class variant, HMNZS Otago, which gave all the crew a bunk and cafeteria messing^[5] and a RNZN commissioned design study for a more fully capable Type 12 frigate, which also assessed whether the Type 12 could carry 2 of the larger Wessex AS helicopters, was used as the basis of the RN Leander Improved Type 12 design.^[6]

On 7 March 1960, the Civil Lord of the Admiralty C. Ian Orr-Ewing stated that the "Type 12 Whitby-class anti-submarine frigates are proving particularly successful ... and we have decided to exploit their good qualities in an improved and more versatile ship. This improved Type 12 will be known as the Leander class. The hull and steam turbine machinery will be substantially the same as for the Whitbys. The main new features planned are a long-range air warning radar, the Seacat anti-aircraft guided missile, improved anti-submarine detection equipment and a light-weight helicopter armed with homing torpedoes. We shall also introduce air conditioning and better living conditions."^[7] The 1963 edition of Jane's Fighting Ships described it as a "mainly anti-submarine but flexible and all-purpose type".^[8]

The Leander class have the same hull and substantially the same steam turbine machinery as the Whitby class, but are a revised and advanced design and will fulfil a composite anti-submarine, anti-aircraft and air direction role. The 40mm guns will eventually be replaced by Seacat ship-to-air launchers. The ships are equipped with VDS (Variable Depth Sonar), formerly known as dipping asdic.

— Jane's Fighting Ships, ^[8]

The difference between the Leanders (Type 12I) and the Whitbys (Type 12) was most obviously that the stepped quarterdeck of the Type 12 had been done away with, resulting in a flush deck, with the exception of the raised forecastle. The superstructure had been combined into a single block amidships and a new bridge design gave improved visibility. A hangar and flight deck were provided aft for the Westland Wasp light anti-submarine helicopter, which was still at the prototype stage when the first ships were ordered. The ship was air conditioned throughout and there were no portholes in order to improve nuclear, biological and chemical defence. The ships were all given names which had previously been given to Royal Navy cruisers, mostly of characters from classical mythology, the exceptions being Cleopatra and Sirius.^[9]

The Y160 boiler variant used on the Batch 3 Leanders (such as Jupiter) also incorporated steam atomisation equipment on the fuel supply so the diesel fuel entering the boilers via the three main burners was atomised into a fine spray for better flame efficiency. Some ships with Y100 Boilers were also converted to steam atomisation, HMS Cleopatra being one of them. The superheat temperature of the Y160 was controlled manually by the boiler room petty officer of the watch between 750–850 °F (399–454 °C) and the steam supplied to the main turbines was at a pressure of 550 psi (3,800 kPa). The Leander-class frigates did have Babcock & Wilcox boilers but of a more conventional two-drum design, one water drum and one steam drum, much like a Yarrow boiler without the second water drum. The water drum was offset to one side and below the furnace and steam drum. The two boilers fitted were 'handed' with the water drum inboard on both. Many Leanders had six-burner furnaces (known as Five and a Half Boilers) and the output was varied by altering the number of burners in use.

The entire class were designed for a standard weapons fit when built, with a twin 4.5-inch Mark 6 gun mount, GWS-22 Seacat missile system and Limbo anti-submarine mortar, though the first seven entered service fitted with two single 40 mm Bofors guns on the hangar roof instead of Seacat, with the SAM system fitted later. All but one of the ships had Seacat GWS-22; the exception was Naiad, which had Seacat GWS-20.^[24] However, advances in weapons systems led to a number of different conversions being undertaken on various members of the class. This saw the class grouped into four broad batches:

• Ikara – installation of the Ikara ASW missile system in place of the 4.5-inch gun mount, plus an additional Seacat missile system.
• Exocet/Seacat – installation of Exocet anti-ship missile system in place of 4.5-inch gun mount, plus two additional Seacat missile systems.
• Exocet/Seawolf – installation of Exocet anti-ship missile system in place of 4.5-inch gun mount; replacement of Seacat with single GWS-25 Seawolf surface-to-air missile system.
• Gun – retained 4.5-inch gun mount and Seacat missile system.

This section needs additional citations for verification. (August 2018)

During their lengthy service with the Royal Navy Leanders were employed during the Indonesia–Malaysia confrontation in 1963–1966. The 1973 and 1975–1976 Cod Wars in the latter of which HMS Diomede suffered severe damage with a 30 feet gash in its hull after being rammed by an Icelandic Coast Guard gunboat. The frigates with their thin hulls being much less suited to this duty than the later offshore patrol vessels.

Four members of the class saw action during the 1982 Falklands War, three Batch Two conversions, HMS Argonaut, HMS Minerva and HMS Penelope ^[44] Argonaut experienced 15 air attacks in San Carlos Sound and was hit by a number of bombs and cannon fire; it was stranded for six days, with two bombs lodged in the forward Seacat magazine and boiler room.^[44] The first Seawolf conversion HMS Andromeda, was one of only three Seawolf fitted frigates available with the Royal Navy's newest missile in the war and served during the war as the critical "goalkeeper"- the last line of defence for the carrier HMS Invincible during the war.^[45] The five unconverted, gun-armed, broad-beam Leanders, arrived in the war zone in the last week of the conflict and immediately after it to serve with the post-war task force led by the brand new carrier HMS Illustrious. An Argentine naval dive team planned to place limpet mines on HMS Ariadne at Gibraltar during the conflict (Operation Algeciras). The last Leander commissioned in 1973, like the two built for Chile, carried special electronic warfare systems, for countering^{[clarification needed]} Exocet missiles, and the Argentine services may have anticipated the Ariadne was scheduled for service in the Total Exclusion Zone, which in fact did not happen until after the end of the war.^{[citation needed]}

The ships performed excellently in Royal Navy service, with relatively low noise levels giving the 2031(I) towed sonar a range of more than 100 miles, better than that of the more advanced 2031(Z) sonar when fitted in the Type 22 frigates. However, all Leanders in Royal Navy service were decommissioned by the early 1990s due to the ships' ageing design and the high number of crew.^{[citation needed]} Scylla was sunk on 27 March 2004 as an artificial reef off Cornwall, eleven years after her decommissioning in 1993.^[46]

Leander-class frigates were also successfully exported to serve in the Royal New Zealand Navy and Chilean Navy; in the latter they were designated as the Condell class. Further frigates were modelled on the Leander-class frigates and were built under licence in Australia as the River class for the Royal Australian Navy, India as the Nilgiri class and the Netherlands as the Van Speijk class. Royal Navy ships were sold to the navies of Chile, Ecuador, New Zealand (Bacchante becoming HMNZS Wellington and Dido becoming HMNZS Southland), India and Pakistan.

Starting in 1986, the six Van Speijk-class ships were sold to the Indonesian Navy and renamed the Ahmad Yani class, five of which are still in service.^{[Note 2]} Pakistan decommissioned the last of its Leander-class frigates, Zulfiqar, in January 2007,^[52] India decommissioned her last Leander class on 24 May 2012.^[53]

HMNZS Canterbury, the last steam-turbine driven Leander-class frigate in the Royal New Zealand Navy, was decommissioned in Auckland on 31 March 2005 after 33 years operational service. In 2006 it was announced that the ship was to be sunk as a dive attraction in the Bay of Islands, and this was carried out on 3 November 2007 at Deep Water Cove. She lies near her sister ship HMNZS Waikato.^[54]

Royal Navy

• List of naval ship classes in service
• Whitby-class frigate, the original Type 12 frigate.
• Rothesay-class frigate, the follow-on Type 12M frigate.
• Warship, a BBC television drama series.