Merchant aircraft carrier
Merchant aircraft carrier (MAC) ships were bulk grain ships and oil tankers fitted with flight decks enabling them to operate anti-submarine aircraft in support of convoys during the Battle of the Atlantic. Despite their quasi-military function, MACs retained their mercantile status, continued to carry cargo and operated under civilian command. MACs began entering service in May 1943 and although originally intended as an interim measure pending the introduction of escort carriers,they remained operational until the end of the war in Europe.
Various schemes for converting merchant ships into aircraft carriers had been suggested to the Admiralty as far back as the First World War but none seem to have been taken up. In 1940, Captain M S Slattery RN, Director of Air Material at the Admiralty, resurrected the idea as a follow-up to the CAM Ship project. Slattery proposed fitting a flight deck equipped with two arrester wires and a safety barrier onto an existing merchant ship hull. The resulting 'auxiliary fighter carrier' would be capable of operating six Hurricane fighters while retaining its cargo-carrying ability. The stumbling block for Slattery's proposal turned out to be objections from the Ministry of Supply that combining the merchant and aircraft carrier roles would be too complicated. While this would turn out to be over-stated, it seems to have had the effect of diverting attention away from the idea of hybrid merchant-warships towards the alternative of converting merchant ships into fully-fledged warships designated 'auxiliary aircraft carriers', the first of which, converted from the captured German cargo ship Hannover, entered service as Empire Audacity (later HMS Audacity) in June 1941.
The hybrid concept re-emerged early in 1942 when, in the face of mounting losses from U-Boat attacks, it became apparent that escort carriers building in the US could not be delivered quickly enough in the numbers required. Various people have been credited with re-inventing the idea, including Captain B B Schofield RN, Director Trade Division, Sir Douglas Thomson of Ben Line and the Ministry of War Transport, and Mr John Lamb, Marine Technical Manager of the Anglo-Saxon Petroleum Company. Sir James Lithgow, Controller of Merchant Shipbuilding and Repair and joint-owner of Lithgows Ltd, the Clyde-based shipbuilders, also helped overcome Admiralty reservations about MACs. Lithgow is said to have sketched a rough design for one on the back of an envelope and offered to convert two ships about to be built at his family's shipyard on condition that "I am not interfered with by the Admiralty". While the timing of Lithgow's intervention is uncertain, his deputy, Sir Amos Ayre, the Director of Merchant Shipbuilding, was certainly discussing the requirements for MACs by May 1942 and the first two were ordered in June 1942 from the Burntisland Shipbuilding Company on the Firth of Forth and William Denny & Brothers of Dumbarton. These ships, which were not strictly conversions but brand-new grain ships that had not yet been laid down, would eventually enter service as Empire MacAlpine and Empire MacAndrew respectively.
The merchant aircraft carriers were modified bulk grain carriers or tankers built with flight decks and small island structures. Minimal aircraft handling and accommodation facilities were available. The bulk nature of the cargoes did not need deck mounted cargo-handling gear used for general cargoes. The MAC-ships were manned by a Merchant Navy crew, under merchant colours, the Royal Navy provided the air department and the guns were manned by DEMS soldiers. They carried their regular cargo in addition to operating aircraft. Tanker MAC-ships were able to carry about 80 percent of their original cargo, the remaining space being taken for Avgas stowage to Admiralty safety standards. Grain carrier MAC-ships suffered a reduction of about 3,000 tons (or 30 per cent) of the original cargo of grain. Their Fairey Swordfish aircraft carried out anti-submarine patrols around the convoy.
The scale of the conversion was small, hence it could be completed in a short time; five months has been quoted as being typical.
The fuel piping arrangements in a tanker made it impossible to build a hangar under the flight deck, so tanker conversions were limited to embarking three Fairey Swordfish which had to be kept on deck. Collapsible deck-edge wind breaks were fitted on the flight deck aft to provide some protection for parked aircraft. The grain carriers had a small hangar aft with a single lift which allowed the carriage of four Swordfish. Flight decks were around 460 feet (140 m) long on the tankers and between 413 ft (126 m) and 424 ft (129 m) for the grain ships, width was 62 feet (19 m) in all cases. All the MAC-ships were fitted with four arrester wires. The permanent deck park of the tanker MAC-ships required a barrier for aircraft recovery. Aircrew accommodation was minimal and ammunition and fuel stores were neither armoured nor partitioned.
Many of the ships were given names with the prefix "Mac-", in a reference to their designation as MACs; the "Empire" prefix was used on ships built by the Ministry of War Transport to distinguish them from pre-war privately owned ships. Since these ships were owned by the Government and only on loan to the shipping companies it was easier to take them out of service for conversion.
The 'air party' was the Fleet Air Arm personnel who flew and supported the aircraft. It consisted of an air staff officer (lieutenant commander), the aircrew, a doctor, a batsman (landing control officer), ten ship's gunners, four signalmen, and three or four able seamen. Under Board of Trade regulations and to comply with the Geneva Convention they were signed onto the ship's articles as merchant ship officers and ratings. Officially they were entitled to the nominal sum of 1 shilling per month on articles, but in practice this was rarely if ever paid. The Merchant Navy part of the crew might have an extra mate and/or wireless operator but was otherwise unchanged.
The aircrew on each ship formed one flight of the MAC-ship wing which consisted of 836 and 860 Naval Air Squadrons, based at HMS Shrike, RNAS Maydown, near Derry in Northern Ireland. 836 NAS was manned by the Royal Navy and 860 NAS was manned by Dutch personnel and assigned to the two Dutch MAC-ships.
An informal, illegal benefit for aircrews was their ability to smuggle contraband without paying import duties. Aircraft were flown off to RNAS Maydown laden with goods which were usually unloaded and hidden before HM Customs could intervene.
Two aircraft types were considered for use on MAC ships: the Swordfish and the Vought SB2U Vindicator (known in British service as the "Chesapeake"). Fifty Chesapeakes were delivered, but after a three-month trial, they were deemed unsuitable by the Admiralty and relegated to a training role.
However, there is evidence that Sea Hurricanes were deployed aboard MAC ships. According to the Fleet Air Arm historical website:
"The Sea Hurricane Mk I were followed by about 300 Mk Is converted to Sea Hurricane Mk IB configuration, these having catapult spools plus a V-frame arrester hook: in addition 25 Mk IIA were modified as Sea Hurricane IB or Hooked Hurricane II fighters. Their initial role was a considerable improvement on CAM-ship deployment, for from October 1941 they began to go to sea aboard MAC-ships, these being large Merchant ships fitted with a small flight deck." 
In practice, the MAC ships were successful. The apparently antiquated, but robust, Swordfish aircraft were suited to the conditions and their patrols were very effective. They flew more than 4,000 sorties in all. Wartime records show that no convoy with an accompanying MAC ship ever lost a vessel to U-boats, nor was any MAC ship lost.
Empire-class grain carriers
Approximately 8,000 tons deep load, 12 knots, 4 aircraft, crew 107, launched December 1942-January 1944. Equipped with hangar and lift. Armament: 1 x single 4 in (102 mm) QF MK IV, 2 x single 40 mm Bofors, 4 x single 20 mm Oerlikon cannons.
- MV Empire MacAlpine
- MV Empire MacAndrew
- MV Empire MacCallum
- MV Empire MacDermott
- MV Empire MacKendrick
- MV Empire MacRae
Empire-class oil tankers
Approximately 9,000 tons deep load, 11 knots, 3 aircraft, crew 122, launched May–July 1943. BP tankers. No hangar and lift; aircraft maintained and stored on deck. Armament: 1 x single 4 in (102 mm) QF MK IV, 8 x single 20 mm Oerlikon cannons.
Rapana-class oil tankers
8,000 tons standard, 16,000 tons deep load, 12 knots, 3 aircraft, crew 118 (64 RN plus 54 MN), converted 1942-44. Anglo-Saxon Petroleum Company tankers. No hangar and lift; aircraft maintained and stored on deck. Armament: 1 x single 4 in (102 mm) QF MK IV, 2 x single 40 mm Bofors, 6 x single 20 mm Oerlikon cannons.
- MV Acavus
- MV Adula
- MV Alexia
- MV Amastra
- MV Ancylus: operated Swordfish II of ‘O’ Flight, 860 NAS
- MV Gadila: operated under the Dutch ensign with Swordfish of 'S' Flight, 860 NAS
- MV Macoma: operated under the Dutch ensign with Swordfish of 'O' (later 'F') Flight, 860 NAS
- MV Miralda
- MV Rapana: operated Swordfish II of ‘L’ Flight, 836 NAS
Amastra and Ancylus ceased operating Swordfish in late 1944, the empty decks often being used to ferry aircraft from the USA to the UK.
- Wragg, David (2005). The Escort Carrier in World War II: Combustible, Vulnerable, Expendable!. Barnsley, UK: Pen & Sword Maritime. pp. 12–24. ISBN 1-84415-220-0.
- The National Archives: Public Record Office (TNA:PRO), ADM 1/17628, Voyage Report - mv Alexia
- TNA:PRO, ADM 1/14950, First Lord of the Admiralty to Mr G Klass dated 30 July1943
- TNA:PRO, ADM 234/383, The Development of British Naval Aviation, Volume I, p. 152.
- Friedman, Norman (1988). British Carrier Aviation: The Evolution of the Ships and their Aircraft. London, UK: Conway Maritime Press. pp. 179–181. ISBN 0-85177-488-1.
- Roskill, Captain S.W. (1954). The War at Sea 1939-1945, Volume I. London, UK: HMSO. p. 477.
- TNA:PRO, ADM 234/384, The Development of British Naval Aviation, Volume II, p. 110.
- Schofield, B. B., 'The Defeat of the U-Boats During World War II' in Journal of Contemporary History Vol. 16, No. 1 (January 1981), p. 125.
- Lenaghan, J; R. Baker, W. J. Holt, A. J. Sims, A. W. Watson (1983). Selected Papers on British Warship Design in World War II. London, UK: Conway Maritime Press. pp. 43–62. ISBN 0-85177-284-6.
- Poolman, Kenneth (1972). Escort Carrier 1941-1945: An Account of British Escort carriers in Trade Protection. London, UK: Ian Allan.
- National Maritime Museum (NMM), Ship's Cover 667Y, Escort Carriers Converted from Grain Ships etc.
- NMM, Ship's Cover 667Y, Minute, DMB to DNC dated 9 June 1942.
- Mitchell, W H, and Sawyer, L A (1990). The Empire Ships. London, New York, Hamburg, Hong Kong: Lloyd's of London Press Ltd. ISBN 1-85044-275-4.
- Hobbs, David (1996). Aircraft Carriers of the Royal and Commonwealth Navies: The Complete Illustrated Encyclopedia from World War I to the Present. London: Greenhill Books. pp. 231–248. ISBN 1-85367-252-1.
- October 1941 is inconsistent with the first MAC ship becoming operational in early 1943, and the same article also says "by 1943 the Sea Hurricane had all but disappeared from service". This may be confusion between MAC ships and escort carriers.