With war against Japan seeming more than probable, and with the added possibility even of a Japanese invasion, the Australian Ministry of Munitions first considered the idea of building tanks as early as July 1940. At this time, Britain’s tank strength was inadequate for home defence, and there seemed little possibility of Australia receiving tanks from this source for some time to come. The Army Design Section (part of the Directorate of Mechanisation) was therefore asked to examine design characteristics and production problems, and in November 1940, the Australian General Staff drew up precise requirements for the sort of tank they thought necessary. They called for a 16-20 (long) tons vehicle, with 2pdr main armament, crew of 4-5, a range of 150 miles, and armour maximum of 50mm. They estimated that 2,000 would be needed, with first deliveries in July 1941 and output of 70 a week from then on.
The Ministry of Munitions asked the British General Staff for the services of a tank design expert from Britain, and, accordingly, a Colonel Watson was sent to Australia in December 1940. Watson travelled via America, where he had the chance to see the designs being drawn up for the M3 medium tank (qv), and on arrival in Australia he was appointed Director of Design. For the proposed vehicle, AC I (AC: Australian Cruiser), Watson planned to use a copy of the M3 final drive and gearbox since he had been impressed by the mechanical features of this vehicle. For a power plant, Guiberson diesel motors were planned but since it seemed probable that there would be difficulty in obtaining these, three commercial automobile engines, Ford at first, then more powerful Cadillac engines, were adopted, arranged in “clover leaf’ formation. A leading Australian automobile engineer was co-opted to advise on development and installation.
In early 1941 a wooden mock-up of AC I was built. The vehicle was to have cast or rolled armour throughout, utilising only alloys available in Australia. By April 1941, drawings of the M3 final drive arrived from America, when it found that this installation was too sophisticated to be manufactured in Australia with existing facilities. Suitable machinery could not be delivered from Britain or America for at least another year. Meanwhile, the United States suggested that Australia produced a new design which could utilise components supplied from America. This proposal, envisaged the use of commercial truck engine and mechanical components. In July 1941, therefore, it was decided to go ahead with a new design which was designated AC II. The limitations which soon became evident using truck engines and drive, however, were many; principally the weight had to be kept below 16-18 tons with consequent reduction in armour thickness, and armament could be no heavier than a 2pdr gun. The truck mechanical components were not powerful enough to cope with a vehicle heavier than this. In September 1941, therefore, the AC II design was abandoned, and attention was given once more the the AC 1.
It was found that by redesigning the final drive to a much more simplified form it would be possible to build the necessary components in Australia. Meanwhile, redesign had also been carried out on the bogies; originally vertical volute bogies of the M3 type were planned, but these were changed to horizontal volute pattern and proved much superior. The first cast hull was successfully manufactured in October 1941, and the prototype AC I was completed in January 1942. The hull and turret castings were in them- selves a great achievement as nothing so complicated as this had previously been attempted by Australian industry.
Modifications were made to the prototype vehicle after trials, and in August 1942 the first production vehicle was completed at Chullora [NOT Chullona] Tank Assembly Shops, NSW, only a year after the first over-optimistic (and unrealistic) estimate. Chullora Shops had been built starting in January 1942 specially to produce tanks, and were erected and managed by New South Wales State Railways, based on the American tank arsenals. A total of 66 AC Is were built when production ceased and all orders were cancelled in July 1943. By this time the tank supply situation had changed and the USA was able to provide all vehicles necessary for equipping the 1st Australian Armoured Division which had meanwhile been formed. The AC Is already completed were therefore used only for training and never saw combat service.
The Australian AC tank, named Sentinel, was a most remarkable achievement for a nation with only limited heavy engineering facilities and no previous experience of tank production. The arrangement of the Cadillac “clover leaf’ power plant, and the cast one-piece hull were novel features which made a strong, tough, powerful vehicle capable of much future development. Plans for upgunned versions of the AC I (detailed below) never went beyond prototype stage, however, when AC production was prematurely terminated. Had AC manufacture continued, it was also planned to commence building ACs at the Geelong Tank Assembly Shops, Victoria, then being built, which were to be managed by Ford Motor Co (Australia).
AC III: This was an upgunned design of the AC I mounting a 25pdr in place of the 2pdr. This necessitated considerable modification, mainly the provision of a larger turret and turret ring, which was increased from 54in to 64in diameter. The engine installation was redesigned with a common crankcase, allowing room for extra fuel tanks, and the bow machine gun was eliminated to give increased ammunition stowage. The bow machine gunner was also, of course, dispensed with, reducing the crew to four. A prototype for the AC III underwent trials in February 1943 and AC III production was to replace the AC I at Chullora from May 1943. However, in view of cancellation of the AC programme it seems probable that very few, if any, AC III were actually completed.
AC IV: The AC III prototype was subsequently tested in March 1943 with two 25pdrs in a co-axial mount, so that the feasibility of mounting the new 17pdr high velocity gun in the AC series could be investigated. Fired together, the two 25pdrs gave a recoil 20 % greater than the recoil of a 17pdr gun with no adverse effect on the turret or vehicle. Plans thus went ahead to fit the 17pdr in the AC III design, and a prototype was completed and tested in late 1943. However, by then AC production had ceased, and no further production orders followed. With the 17pdr, the vehicle was designated AC IV. Undoubtedly it would have proved a most potent vehicle.
One AC hull was modified with torsion bar suspension in an attempt to provide superior riding qualities for the proposed upgunned models. Though completed and run, there was, of course, no opportunity of incorporating the new suspension in production vehicles.
Designation: Cruiser Tank AC I and AC III, Sentinel
Crew: 5 (commander, driver, hull gunner, gunner, loader)(No hull gunner in AC III).
Battle weight: 62,7201b
Track width 16tin
Dimensions: Length 20ft 9in Track centres/tread 7ft 6tin Height 8ft 5in . Width 9ft lin
Armament: Main: 1 x 2pdr OQF (AC I) I x 25pdr (AC III)
Secondary: 2 x Vickers· 303 cal MG (one in AC TTl)
Armour thickness: Maximum 65mm Minimum 25mm
Engine: 3 x Cadillac V-8 petrol, 117hp each unit (AC I) Perrier-Cadillac triple engine (common crankcase), 397hp(AC III)
Maximum speed: 30mph
Maximum cross-country speed: 20mph (approx)
Suspension type: HVSS, Hotchkiss type
Road radius: 200 miles (AC I), 229 miles (AC III)
Fording depth: 4ft
Vertical obstacle: 2ft (AC I), 4ft (AC III)
Trench crossing: 9ft 6in
Ammunition stowage: 130 rounds 2pdr (AC I) 4,250 rounds· 303 cal (AC I)
Special features/remarks: Cast one-piece hull with prominent armoured sleeve for bow machine gun mount. HVSS copied from French Hotchkiss design in place of M3 type vertical volute suspension at first planned. Very low, stable, fast vehicle, with good armour protection and development potential. Tracks were American rubber block type. Bren AA machine gun mount fitted on cupola of all marks.