The Implacable-class aircraft carrier consisted of two aircraft carriers built for the Royal Navy during World War II. Derived from the design of the , they were faster and carried more aircraft than the older ships. They were initially assigned to the Home Fleet when completed in 1944 and attacked targets in Norway as well as the . Subsequently, they were assigned to the British Pacific Fleet (BPF).
was the first ship to go to the Pacific and attacked Japanese-controlled oil refineries in Sumatra en route. She participated in Operation Iceberg, the invasion of Okinawa in March–April 1945. s arrival in the Pacific was delayed by a refit and she did not begin operations against the Japanese until June. The sister ships participated in the attacks on the Japanese Home Islands in July and August. Indefatigable was the only carrier chosen to continue operations after most of the BPF withdrew to prepare for further operations in early August. After the Japanese formal surrender in September, Implacable ferried Allied troops and prisoners of war back to Australia and Canada for the rest of the year.
The sisters returned home in 1946; Indefatigable was used for the rest of the year to transport troops before being placed in reserve in 1947 and Implacable became the training carrier for Home Fleet. Indefatigable was converted into a training ship and reactivated in 1950 for service with the Home Fleet. Implacable was relegated to the reserve that same year and modified into a training ship in 1952. The sisters were scheduled for modernisation during the mid-1950s, but it was cancelled as the modernisation of Victorious before them proved expensive and lengthy. The sisters were decommissioned in 1954 and sold for scrap in 1955–1956.
Background and description The Implacable class had its origin as an improved version of the Illustrious-class aircraft carriers for the 1938 Naval Programme while still remaining within the available from the tonnage allowed by the Second London Naval Treaty. The initial change was to increase the carriers' speed to no less than which required the addition of a fourth steam turbine and associated propeller shaft. Offsetting the additional weight of the machinery meant reductions in armour thicknesses in the hangar deck and the bulkheads at the ends of the hangar. At the same time the Director of Naval Construction (DNC) was developing a different modified Illustrious design (Design D) to carry an additional dozen aircraft (a total of 48) in a lower hangar that also incorporated the additional machinery of the initial design with the sacrifice of even more armour. Hangar height was initially planned as in the upper hangar to accommodate the new Fairey Albacore torpedo bomber and in the lower hangar to accommodate taller amphibious aircraft, but a later change in policy raised the upper hangar height to . Design D was submitted to the Board of Admiralty on 2 August 1938 and approved on 17 November. In April 1939 the lower hangar's height was reduced to 14 feet to compensate for the thickening of the hangar side armour to and the idea of carrying amphibians in the hangar was abandoned.Friedman, pp. 141–144
The Implacable-class ships were long overall and at the waterline. Their beam was Friedman, p. 366 at the waterline and they had a draught of at deep load. The ships were significantly overweight and displaced at deep load.Brown 1977, p. 49 Their complement was approximately 2,300 officers and enlisted men in 1945.Hobbs, p. 109 They had metacentric heights of at light load and at deep load as completed.Friedman, p. 151
The ships were fitted with four Parsons geared steam turbines, each driving one shaft using steam supplied by eight Admiralty 3-drum boilers.Chesneau 1980, p. 20 The turbines were designed to produce a total of , enough to give them a maximum speed of .Friedman, p. 367 On their sea trials, the ships reached speeds of with . The Implacable class carried a maximum of of fuel oil which gave them a range of at .
The armoured flight deck had a maximum width of .Friedman, p. 154 The arrestor cables, crash barricades, aircraft catapult and lifts were designed to handle aircraft up to in weight. The carriers were fitted with nine arrestor cables aft that were designed to stop landing, at speeds of up to . They were backed up by three crash barricades to prevent landing aircraft from crashing into aircraft parked on the ship's bow. In case of damage to the rear flight deck, the Implacable-class ships also mounted three additional forward arrestor cables to permit aircraft to land over the bow. A single BH3 hydraulic catapult was fitted on the forward part of the flight deck to launch 20,000-pound aircraft at ; lighter aircraft could be launched at a maximum speed of . The ships were equipped with two lifts on the centreline, the forward of which measured and served only the upper hangar, and the aft lift () which served both hangars. The upper hangar was long and the lower hangar was long; both had a uniform width of .Roberts, pp. 62–64 Both hangars had a height of only 14 feet which precluded storage of Lend-Lease Vought F4U Corsair fighters as well as many post-war aircraft and helicopters. In case of fire the upper hangar could be divided by two fire curtains and the lower hangar had one fire curtain. Designed to stow 48 aircraft in their hangars, the use of a permanent deck park allowed the Implacable class to accommodate up to 81 aircraft. The crewmen, maintenance personnel and facilities needed to support these additional aircraft were housed in the lower hangar. The ships were provided with of petrol,Brown 1977, pp. 48–49, Friedman, p. 144 only enough for approximately five sorties per aircraft.Brown 1977, pp. 48–49, Hobbs, p. 108
Armament, electronics, and armour The ships' main armament consisted of sixteen QF Mark II dual-purpose guns in eight powered RP 10 Mk II** twin-gun turrets, four in sponsons on each side of the hull. Unlike the Illustrious-class ships, the roofs of the gun turrets were flat and flush with the flight deck. The gun had a maximum range of at an elevation of +45° and a ceiling of .Campbell, pp. 52, 55 Their light anti-aircraft defences included five octuple mounts for QF 2-pounder ("pom-pom") anti-aircraft guns, two on the flight deck forward of the island, one on the aft part of the island and two in sponsons on the port side of the hull. A single quadruple 2-pounder mount was also fitted on the port side of the hull.Friedman, pp. 144, 367 The 2-pounder gun had a maximum range of .Campbell, p. 71 The two ships were also fitted with approximately sixty Oerlikon 20 mm autocannon in varying numbers of single and twin-gun mounts. These guns had a maximum range of , but many were replaced by 40 mm Bofors AA guns when the ships were transferred to the Pacific War as the 20 mm shell was unlikely to destroy a kamikaze before it hit the ship. The Bofors gun had a maximum range of .Campbell, pp. 67, 75 Two additional quadruple "pom-pom" mounts were added to Implacable before she joined the British Pacific Fleet in 1945. After the war, more Oerlikons were exchanged for Bofors guns. By April 1946, the sisters had 11–12 Bofors guns and 19–30 Oerlikons each.Friedman, pp. 149, 154
The 4.5-inch guns were controlled by four Mk V* (M) fire-control directors, each mounting a Type 285 gunnery radar. Two of the directors were positioned on the flight deck, one each fore and aft of the island, a third was on the island, aft of the funnel, and the fourth director was on the port side of the hull, below the flight deck. Each director sent its data to a Fuze Keeping Clock AA fire-control system for gunnery calculations. Each "pom-pom" was provided with its own Mk IV director that carried a range-only Type 282 gunnery radar.Campbell, pp. 15–16, 18–19, Friedman, p. 144, Hobbs, p. 111
The specifics of the Implacable-class ships' radar suite is not readily available in published sources. They were fitted with the Type 277 surface-search/height-finding radar on top of the bridge and a Type 293 target indicator radar on the foremast. The ships probably carried Type 279 and Type 281B early-warning radars, based on the radars fitted aboard the Illustrious-class carrier late in the war.Friedman, pp. 145, 149, 151, Hobbs, pp. 111, 113
The Implacable-class ships had a flight deck protected by of armour. The sides of the hangars were designed to be thick to protect the hangar from low-level attacks with semi-armour-piercing bombs, but were supposedly thickened to late in the design process at the cost of reducing the height of the lower hangar. Naval historian Norman Friedman wrote: "Ironically, it appears that the ships were actually built with 1.5-inch ... armour."Friedman, pp. 142–143 The ends of the hangars were protected by 2-inch bulkheads and the armour of the hangar deck ranged from in thickness.Friedman, p. 142
The waterline armour belt was thick, but only covered the central portion of the ship to form the armoured citadel. The belt was closed by 1.5 to 2-inch transverse bulkheads fore and aft. The underwater defence system was a layered system of liquid- and air-filled compartments as used in the Illustrious class and was estimated to be able to resist a explosive charge. The magazines for the 4.5-inch guns lay outside the armoured citadel and were protected by 2 to 3-inch thick roofs, 4.5-inch sides and 1.5 to 2-inch ends.Friedman, pp. 134, 139, 142
Planned modernisation The two Implacables were tentatively scheduled to be modernised in 1953–55 with Implacable following Victorious. The draft Staff Requirements were drawn up in July 1951. This included combining the two hangars into a single hangar, strengthening the flight deck and aircraft handling equipment to deal with aircraft, enlarging the lifts to , adding a gallery deck between the hangar and the flight deck to accommodate the additional personnel required, the addition of steam catapults, and the increase of her aviation fuel stowage to . Other desired improvements were new boilers to increase her endurance, more space for the latest radars, and the replacement of her anti-aircraft armament with the British version of the 3"/70 Mark 26 gun and sextuple mounts for the Bofors guns.Friedman, pp. 305–310
By October 1951, the estimated completion date for Victoriouss modernisation was already a year past the initial estimate of April 1954. Implacable was scheduled to begin her modernisation in April 1953 for completion in 1956, but the Director of Dockyards pointed out that existing schedules prevented her from beginning any earlier than April 1955 unless the modernisations of two cruisers and the guided missile test ship RFA Girdle Ness were delayed. The Controller of the Navy asked if the time and cost of the reconstruction could be reduced, but the minimum modifications were the most expensive as they involved structural alterations. The Controller ordered the Director of Dockyards to plan for rebuilding Implacable between June 1953 and December 1956 even after the latter protested that even a limited modernisation would require about three-quarters of the structural work of the original plan and that the shortage of skilled workers (already insufficient for Victorious by herself) would delay work on both ships. In order to reduce the amount of structural work, the requirement to replace the boilers was cancelled and the ship would receive existing radars instead of systems then still under development. In January 1952, the ship's new armament was finalized at six twin-gun 3"/70 mounts and three sextuple Bofors mounts. Five months later the Admiralty decided that Victorious would be the last fleet carrier modernised as experience showed that the process would take longer and cost more than was practicable.Friedman, pp. 310–311, Hobbs, p. 109
Ships