HMS Unicorn (I72) - Design, Description and Construction

Design, Description and Construction

The Abyssinia Crisis of 1934–35 demonstrated to the Admiralty that it needed a depot ship to support the aircraft carriers in active service, just like submarine and destroyer tenders supported those types. Such a ship would be able to perform a wider range of aircraft repair and maintenance tasks than aircraft carriers and on the full range of aircraft operated by the Royal Navy, including amphibians. Admiral Reginald Henderson, Controller of the Navy was instrumental for gaining approval for the ship and ensuring that she had a complete flight deck that would allow her to land, service and launch aircraft on active operations. She was the first ship built in any navy that could "carry out the full range of aircraft maintenance and repair work in addition to the ability to operate aircraft from the flight deck". In practice, Unicorn proved the value of the concept and two similar support ships, Perseus and Pioneer were converted into aircraft maintenance ships by modifying light aircraft carriers still under construction.

Unicorn had an overall length of 640 feet (195.1 m), a beam of 90 feet 3 inches (27.5 m), and a draught of 23 feet (7.0 m) at deep load. She was somewhat overweight as completed and displaced 16,510 long tons (16,770 t) at standard load rather than her designed 14,750 long tons (14,990 t). Each of the ship's two sets of Parsons geared steam turbines drove one 15-foot (4.6 m) propeller. Steam was supplied by six Admiralty three-drum water-tube boilers operating at a pressure of 400 psi (2,758 kPa; 28 kgf/cm2). The turbines were designed for a total of 40,000 shaft horsepower (30,000 kW) and gave Unicorn a speed of 24 knots (44 km/h; 28 mph). The ship carried 3,000 long tons (3,000 t) of fuel oil which gave her a range of 7,000 nautical miles (13,000 km; 8,100 mi) at 13.5 knots (25.0 km/h; 15.5 mph).

In order to land aircraft aboard, Unicorn was therefore designed with a full-length, 600-foot (180 m)-long flight deck with arresting gear. She was also fitted with a catapult capable of launching a 14,000-pound (6,400 kg) aircraft to a speed of 66 knots (122 km/h; 76 mph). The ship had two hangars, each 16 feet 6 inches (5.03 m) high. Aircraft were transported between the hangars and the flight deck by two aircraft lifts (elevators); the forward lift measured 33 by 45 feet (10.1 m × 13.7 m) and the rear one 24 by 46 feet (7.3 m × 14.0 m). Unicorn could carry about 33 operational aircraft. Bulk petrol storage consisted of 36,500 imperial gallons (166,000 l; 43,800 US gal). During wartime, the ship's crew totaled 1200. The ship carried a self-propelled lighter under the rear of the flight deck to allow unflyable aircraft to be transferred between ships or to shore facilities. This lighter was lowered flush with the upper hangar deck so that an aircraft could be rolled onto to it or an aircraft could be lifted onto it once the lighter was in the water.

Unicorn was armed with four twin mounts for the 45-calibre QF 4-inch Mk XVI dual purpose gun. This mounting could elevate from −10 to +80°. The Mk XVI gun fired about 12 35-pound (16 kg) high-explosive shells per minute at a muzzle velocity of 2,660 ft/s (810 m/s). Against surface targets it had a range of 19,850 yards (18,150 m) and a maximum ceiling of 39,000 ft (12,000 m), but an effective anti-aircraft range of much less. She was also equipped with four quadruple mounts for the 40-millimetre (1.6 in) QF 2-pounder Mk VIII gun ("pom-pom"). These gun mounts could depress to −10° and elevate to a maximum of +80°. The Mk VIII 2-pounder gun fired a 40-millimetre (1.6 in) 0.91-pound (0.41 kg) shell at a muzzle velocity of 1,920 ft/s (590 m/s) to a distance of 3,800 yards (3,500 m). The gun's rate of fire was approximately 96–98 rounds per minute. Close-range air defence was provided by twelve 20 mm Oerlikon autocannon, in two twin mounts and eight single ones. The Oerlikon fired a 0.272-pound (0.123 kg) shell at a muzzle velocity of 2,750 ft/s (840 m/s). It had a rate of fire of 465–480 rounds per minute and a maximum range of 4,800 yards (4,400 m).

The ship was equipped with two High Angle Control System (HACS) directors on her island and each "pom-pom" mount had its own director as well. Unicorn was the first ship to mount a Type 281B early warning radar, and each HACS director was fitted with a Type 285 gunnery radar.

Unicorn was ordered on 14 April 1939, and laid down at Harland and Wolff in Belfast, Northern Ireland on 29 June. Her construction was delayed several times in favour of higher priority projects and she was not launched until 20 November 1941. The ship was completed on 12 March 1943. In order to accelerate the ship's completion, the Admiralty decided in 1942 that she would not be equipped with her full suite of maintenance and repair equipment. Excluding her armament, Unicorn cost £2,531,000. The name Unicorn already in use by the 1824-built wooden frigate Unicorn, which was being used as a drill ship in Dundee, Scotland. The frigate's name was changed to Unicorn II in February 1939, then to HMS Cressy on 20 November 1941. The frigate regained her original name on 14 July 1959 after the carrier was scrapped.