Lead orders for escort carriers (CVEs) were placed in early 1942, but urgent measures were required to close the mid-Atlantic gap during their building, One such was the CAM ship, the other the MAC (Merchant Aircraft Carrier), an ingenious solution later copied by the Japanese. The Ministry of War Transport was, understandably, reluctant to release good-class cargo tonnage^ for conversion to dedicated CVEs, but the MAC retained the greater part of its cargo capacity while having a flight deck topside. Breakbulkers required hatches and cargo-handling gear to function, but grain carriers required only small apertures to their holds, through which the hoses for loading and discharging grain could be inserted, This arrangement was fully compatible with fitting a flightdeck. Like the CAMs, the MACs sailed under the red ensign, only their flight personnel being Royal Navy. Similarly, they were integrated more with the convoy than its escort, though requiring more manoeuvring space. Appropriately carrying Empire 'Mac' names (particularly so as they came from Scottish yards), the first six were all converted from incomplete ships, with 129 by 19 m (423 by 62 ft) flightdecks and a diminutive hangar aft capable of accommodating four Fairey Swordfish aircraft.
From the point of view of cargo requirements and dimensions, tankers were also very suitable candidates for conversion, but the Admiralty had grave doubts of the fire risk. AngloSaxon Petroleum (now Shell) resolved the problem on their behalf, resulting in nine of their tankers being converted (though retaining their familiar Shell names) and four more liquidcargo Empire 'Macs' being launched for the job. The main difference between the wet and dry cargo carriers lay in the lack of a hangar on the tankers, the aircraft remaining topside in all weathers. Despite the urgency of the programme, it was April 1943 before the first MAC entered service. They were exceedingly fortunate ships, all 19 surviving the war to be reconverted.
The dry-cargo 'Empire Mac' class ships were the Empire Macalpine, Empire Macandrew, Empire Maccallum, Empire Macdermott, Empire Mackendrick and Empire Macrae. The equivalent tanker 'Empire Mac' class comprised the Empire Maccabe, Empire Maccoll, Empire Mackay and Empire Macmahon. Finally, the units of the 'Shell' class were the Acavus, Adula, Alexia, Amastra, Ancylus, Cadila, Macoma, Miralda and Rapana.
Specification 'Empire Mac' dry class Type: merchant aircraft-carrier Displacement: 7,930 to 8,250 gross tons Dimensions: length between 135.6 and 139.9 m (445 and 459 ft); beam between 17.1 and 17.7 m (56 and 58 ft); draught 7.5 m (24.66 ft) Propulsion: 1-shaft diesel delivering 3,300bhp(2461kW) Speed:12.5kts Armour: none Armament: one 102-mm (4-in), two 40mm AA and some smaller guns Aircraft: four Complement: 110
Specification 'Empire Mac' tanker class Type: merchant aircraft-carrier Displacement: 8,850 to 9,250 gross tons Dimensions: length 146.7 to 148 m (481.3 to 485.5 ft); beam 18.0 to 18.8 m (59 to 61.66 ft); draught 8.0 to 8.4 m (26.25 to 27.5 ft)
Propulsion: 1-shaft diesel delivering 3,300bhp(2461kW) Speed: 11.5 lets Armour: none Armament: one 102-mm (4-in) and some smaller guns Aircraft: four Complement: 110
Specification 'Shell' class Type: merchant aircraft-carrier Displacement: 8,000 gross tons Dimensions: length 146.5 to 147 m (480,66 to 482.25 ft); beam 18,0 m (59 ft); draught 8.4 m (27.66 ft) Propulsion: 1-shaft diesel delivering 3,750bhp(2796kW) Speed: 13 kts Armour: none Armament: one 102-mm (4-in) and some smaller guns Aircraft: four Complement: 105