A merchant aircraft carrier (also known as a MAC ship, the Admiralty's official 'short name') was a limited-purpose aircraft carrier operated under British and Dutch civilian registry during World War II. MAC ships were adapted by adding a flight deck to a bulk grain ship or oil tanker enabling it to operate anti-submarine aircraft in support of Allied convoys during the Battle of the Atlantic.
Despite their quasi-military function, MAC ships retained their mercantile status, continued to carry cargo and operated under civilian command. MAC ships entered service from May 1943 when they began to supplement and supplant escort carriers, and remained operational until the end of the war in Europe.
Development In 1940, Captain M. S. Slattery RN, Director of Air Matériel at the Admiralty, proposed a scheme for converting merchant ships into aircraft carriers as a follow-up to the CAM ship project.The National Archives: Public Record Office (TNA:PRO), ADM 234/383, The Development of British Naval Aviation, Volume I. Slattery proposed fitting a flight deck equipped with two arrestor 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 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 ) 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 being built in the US could not be delivered quickly enough in the numbers required.TNA:PRO, ADM 234/384, The Development of British Naval Aviation, Volume II. Various people have been credited with re-inventing the idea, including Captain B. B. Schofield RN, Director Trade Division, and John Lamb, Marine Technical Manager of Anglo-Saxon Petroleum.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. 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 MAC ships. 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 possibly apocryphal intervention is uncertain, his deputy, Sir Amos Ayre, the Director of Merchant Shipbuilding, was certainly discussing the requirements for MAC ships by May 1942.National Maritime Museum (NMM), Ship's Cover 667Y, Escort Carriers Converted from Grain Ships etc. Ayre himself credits Sir Douglas Thomson of Ben Line and the Ministry of War Transport with having first suggested the idea.
There was some initial resistance to the MAC ship concept, in particular through concerns about operating aircraft from short, relatively slow ships. The Admiralty considered that a flight deck length of was needed for safe take-offs and landings and a speed of . to provide a sufficient margin over convoy speeds and they were especially doubtful that tankers, with their low freeboards and volatile cargoes, could be utilised.TNA:PRO, MT 59/1966, Minute dated 29 June 1942 As the U-boat situation worsened, however, such concerns were out-weighed by the urgent need to provide convoy air support, and once it was accepted that the grain ship variant at least could be made to work, the Admiralty became more enthusiastic about the project.
By September 1942, the Admiralty was asking for "about 50" MAC ships to be made available, to allow one to be included in every North Atlantic convoy and in October the requirement was set at 52.TNA:PRO, MT 59/1966, Admiralty to Ministry of War Transport dated 1 September 1942.NMM, Ship's Cover 667Y, Controller to ACNS(T) dated 1 October 1942. This ambitious target had been scaled back to 40 by the time the MAC ship project was formally endorsed by the War Cabinet in October 1942, but it was apparent that even this reduced number could not be produced sufficiently quickly without American help.TNA:PRO, FO 371/32584, Construction of Merchant Aircraft Carriers: Joint Memorandum by First Lord of the Admiralty and Minister of War Transport dated 27 October 1942. The US was therefore asked to begin construction of 30 MAC ships to be delivered during the first half of 1943, but a Navy Department committee specially formed to consider the request turned it down because of concerns about the experimental nature of the ships.TNA:PRO, CAB 65/28.17, War Cabinet Conclusions 147(42) of 29 October 1942.TNA:PRO, MT 59/1966, BADW to Admiralty dated 27 November 1942. In the event, all the MAC ships were built and/or converted in British shipyards.
Conversion programme
The first two MAC ships 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 and respectively.NMM, Ship's Cover 667Y, DMB to DNC dated 9 June 1942. Empire MacAlpine was launched on 23 December 1942 and completed more-or-less on schedule on 21 April 1943. Five more new-build grain ships, , , , and followed at approximately two-month intervals, with Empire MacDermott entering service in March 1944.
By late September 1942 it was finally agreed that tanker-MAC ships could be operated safely subject to various limitations about the cargoes they would be allowed to carry. Four new-build tankers were scheduled for conversion but work on these did not start until May 1943. entered service in October 1943, followed at intervals by , and in November 1943. Additional hulls were still needed, however, and it was decided that existing tankers would have to be withdrawn from trade. The most efficient way to approach the task of converting these was to select ships of similar design and, at the beginning of 1943, Anglo-Saxon Petroleum, which had actively promoted the MAC ship concept, offered up its entire fleet of British-registered 'Triple Twelve' tankers (sometimes referred to as the class) for government charter.TNA:PRO, MT 59/1966, Minute dated 12 November 1942.TNA:PRO, MT 59/1966, Anglo-Saxon to Ministry of War Transport dated 2 February 1943.
The desirability of converting foreign-owned tankers was also considered and in January 1943 the Ministry of War Transport asked the Norwegian government-in-exile if the modern Norwegian tanker could be converted to a MAC ship under British command. The initial response was not helpful with Norway insisting on financial arrangements that War Transport officials described as "wholly unreasonable as between allies", but although these were later ameliorated, the proposal eventually foundered because the design effort was judged too much for what would have been a one-off conversion.TNA:PRO, MT 59/1494, Minute dated 19 January 1943.TNA:PRO, MT 59/1494, Edelsten to Hurcomb dated 11 February 1943. At about the same time however, a further three tankers of the same class as the Anglo-Saxon 'Triple Twelves' were identified operating under Dutch registry (Anglo-Saxon Petroleum was a subsidiary of Shell). The Netherlands authorities approved the British request but on condition that the ships, as MAC ships, would fly the Dutch flag and be under Dutch civilian command to which the Admiralty, citing potential language difficulties, only reluctantly agreed.TNA:PRO, MT 59/1494, Minute dated 17 February 1943.TNA:PRO, MT 59/1494, Hurcomb to Moore dated 11 May 1943. In the event, only two of the Dutch ships were taken up as MAC ships but these, and , crewed entirely by Dutch merchant seamen and with aircraft flights drawn from the Royal Netherlands Navy-manned 860 Naval Air Squadron would have the distinction of becoming the Netherlands' first aircraft carriers.
Civilian status
Despite their military appearance and combatant function, MAC ships were civilian ships that did not appear in the Navy List nor were they commanded by commissioned officers: this unusual status is one of their defining characteristics. The hybrid nature of the MAC ships raised from the outset the question of whether they would be commissioned warships like the new escort carriers, or if they would operate as merchant ships under the Red Ensign like the earlier CAM ships. The Admiralty's preference was to operate them as regular warships but it soon became clear that there were not enough personnel available to man them to naval standards without causing serious shortfalls in other areas, in particular the large numbers of escort carriers that would soon arrive.TNA:PRO, FO 371/32584, Waldock to Steel dated 27 December 1942. The Admiralty and Ministry of War Transport therefore agreed that the MAC ships would be civilian-manned. Serious reservations about the proposed civilian status were raised, however, by Foreign Office officials, who were concerned that the ships would be de facto warships under international law but, without the protection afforded by the 1907 Hague Convention which applied only to members of the armed forces, their civilian crews would be liable to be treated as unlawful combatants, or francs-tireurs.TNA:PRO, FO 371/32584, Construction of Merchant Ship Aircraft Carriers, Steel to Dean dated 29 October 1942. The potential for such action had been clearly demonstrated by the execution in 1916 of Captain Charles Fryatt, for attempting to ram a U-boat with his cross-Channel ferry. The risk to MAC ship crews was considered so great that it was explicitly put before the War Cabinet:
The War Cabinet ordered a review of the situation but the manning difficulties could not be resolved. The Foreign Office reluctantly agreed that the MAC ships would have to operate under the Red Ensign, provided that the merchant seamen and their unions were made aware of the risks involved, and that every possible precaution would be taken to ensure that the men did not fall into enemy hands.TNA:PRO, FO 371/32584, Le Rougetel to Waldock dated 12 January 1943. In the event, there is little to suggest that the MAC ships' crews were ever formally notified that they might be at special risk. There is evidence, however, that the authorities remained sensitive about the status of MAC ships which were specifically excluded from the publicity that was arranged for other 'special service' merchant ships, e.g. rescue ships.TNA:PRO, ADM 1/16111, Minute by DTD dated 18 September 1944. As late as September 1944, masters were reminded to point out to their crews the need to maintain secrecy about the MAC ships' operations and functions.TNA:PRO, ADM 1/16111, Admiralty to the Masters mv Acavus, Amastra, Ancylus, Rapana dated 25 September 1944.
Ship details