By early 1944 the British Pacific Fleet would badly need new aircraft maintenance ships equivalent to HMS Unicorn, the two candidates were an American supplied escort carrier and one of the newly Colossus class carrier under construction. The escort carrier would be equivalent to one third of a Unicorn, the light fleet carrier to one half. The British Pacific Fleet would need up to three Unicorn equivalents by the end of 1945, that is, nine escort carrier hulls or six light fleet carrier hulls. 

By this time the first four ships of the Colossus class (Colossus, Glory, Vengeance, and Venerable) were too far along to convert, but the next four (Edgar, Mars, Warrior, and Theseus) were viable candidates. Given an early enough decision, two ships (one unit) could be available at the end of 1944, followed by another in April 1945 and the third in the latter half of 1945. 

Finally, the Royal Navy adopts the decision to convert two light fleet carriers of the Colossus class into aircraft repair ships. 

The selected ships, were HMS Edgar and HMS Mars under construction at Vickers Armstrong, Newcastle, and before their completion's were altered internally and externally to their new task. 

Initially, they have a deckhouse aft to housed spare aircraft engines, an oxygen producing plant and an oxygen cylinder filling and stowage room. 

The boats, including a 48 ft aircraft lighter, could be stowed on deck just abaft amidships, served by the large crane just abaft the island. The built-up structure inboard of the island housed, among other things, a large crew recreation space and cinema, and an awing could be rigged over the remaining width of the former flight deck. A second 48 ft aircraft lighter could be stowed on deck just forward of the forward lift served by a 15 ton crane mounted to port. 

The first ship, HMS Edgar, was laid down 1 June 1942, launched 26 March 1944 and commissioned 19 October 1945 as HMS Perseus and served as a maintenance support ship and went to the Far East but arrived at the end of the war and was not needed. In June 1946 she went into reserve. In 1949, Perseus was reactivated and converted in 1949 to steam catapult trials ship (operating as such from July 1950 to June 1952). Later, Perseus became a ferry trooping carrier between November 1952 and May 1953. She, her sister Pioneer and Unicorn were all re-rated as ferry carriers in June 1953. Their official role was to transport aircraft to and from bases overseas and act as limited replenishment carriers, but in 1951 it was decided that no effort would be made to adapt them as full replenishment carriers until after the angled deck program was complete. Perseus functioned as reviewing stand during the 1953 Coronation Naval Review, and in 1954 worked up the first Royal Navy ASW helicopter squadron. 

Later, she carried stores and supplies to the Far East. Upon her return she paid off into reserve. She was to have been converted to a submarine tender, but with the reduction in naval forces in 1957 two rather than three such ships were required. The projected cost of conversion was discarded. Perseus lasted a few years longer in service, undergoing a refit in 1955 before entering reserve in 1957 and being sold for scrap in 1958 at Port Glasgow. 

The second ship, HMS Mars, laid down on 2 December 1942, launched on 20 May 1944 and commissioned on 8 February 1945 as HMS Pioneer to serve with the British Pacific Fleet. She was at Manaus when the Japanese in that area surrendered to the British forces. After the surrender, Pioneer returned to the United Kingdom, and was then laid up in reserve for seven years. In 1953, Pioneer was reactivated for service as a ferry carrier. However, that duty lasted only until September 1954, when the ship was decommissioned and sold for scrap. 

The Steam catapult and HMS Perseus 

Up to and during World War II most catapults were hydraulic. At the end of the Second World War the Royal Navy convinced that it was apparent that virtually all future high performance carrier aircraft would have to be launched by more powerful catapults. 

The British Admiralty decided to adopt slotted cylinder steam catapults in 1946, and by 1948 plans for a test unit, the BXS-1 catapult developed by Cdr. C. C. Mitchell, O. B. E., of Messrs. Brown Brothers & Co., Ltd., of Edinburgh, had been completed. Their pistons would be driven directly by boiler steam.  Because it acted directly (the piston in the slotted cylinder drives the aircraft) without sheaves or pulleys, the new catapults could be much more powerful than any existing unit. It was also much simpler. 

The BXS-1 prototype was installed aboard HMS Perseus in 1951, and extensive trials (including 1560 launches) followed in 1951-52, showed an aircraft weighing 30,000 lbs could be launched with a speed of over 90 knots, which was a considerable increase over the air hydraulic catapults then in service. The steam catapult also proved to be considerably more reliable. 

The new steam catapult then entered service aboard British carriers as BS4. It was installed in several modernized light fleet carriers. 

As the new catapult design was appeared, the US Navy were interested in this invention and as Norman Friedman says in his book British Carrier Aviation, in the first three months of 1952, the US Navy carried out tests of the new British developed steam catapult, launching US naval aircraft at sea from HMS Perseus, and on land at the Naval Shipyard, Philadelphia, and the Naval Operating Base, Norfolk. In April 1952, the US Navy announced that this catapult would be adopted for use on US aircraft carriers, with the first installation on the USS Hancock. 

Later, because this effectiveness the navies introduced these steam catapults in the mid 1950s, which were the only ones capable of launching the heavier jet fighters.
 

This model is one of the first resin “test copies” of my Colossus class HMS Vengeance aircraft carrier, and after this test I had the intention to use one of these hulls to make some variants of the class for my ship models collection. 

The silicone mold of the hull and the island was made by my friend Doni Alba. 

Then with the plans and some photographs kindly lent by A. D. Baker III, I started to build an HMS Perseus model and some of the parts were made by me with styrene sheets, and I use a WEM cruiser crane (similar to those used in this carriers) and the Gold Medals 1/700 British Aircraft Carrier photoetch set the rails. Also, in deck are a WEM Hawker Sea Fury, Fairey Firefly and a Sea Hornet with their respective photoetch fittings.
 

Friedman, Norman. British Carrier Aviation: the Evolution of the Ships and their Aircraft. Annapolis; US Naval Institute Press. 1989.

Friedman, Norman. Carrier Air Power. London; Conway Maritime Press. 1981.

Internet: 

• HMS_Perseus page on Wikipedia
• Fleet Airarmarchive.net Perseus page