In November 1940 the Australian General Staff issued a specification for a 16-20 ton tank, with a top speed of 30mph, armed with a 2-pdr gun and with armour 50mm thick. Due to the shortage of armour plate and the capability to produce rolled plate it was decided to use a number of castings bolted together in a similar manner to that used on the British Matilda. The design team then took this a stage further an proposed casting the entire hull and turret as a single castings, quite an achievement. The armour protection producred was 45mm for the front hull and 65mm for the turret. The Sentinel was powered by three Cadillac V-8 engines arranged in a ‘clover-leaf’ configuration. The drive shafts from all three engines passed to a transfer box mounted beneath the turret from which emerged a single drive shaft which passed to the front mounted transmission. The gearbox was a five speed crash and incorporated differential steering. The combined engines delivered 370 bhp which gave a maximum speed of some 40mph although this was limited to 30mph on production vehicles. The suspension system employed although appearing similar to that of the M3 is actually based on the French Hotchkiss type. The track was of a rubber block double pin pattern. The AC1 was first armed with a 2-pdr main gun with two Vickers .303 machine guns, one mounted co-axially with the main gun. The tank had a crew of 5. The first three prototypes were completed in January 1942 with the first full production vehicle being completed in August 1942. The tank has thus been taken from specification to production in just 22 months. Production continues through to July 1942 by which time 66 tanks had been produced.

The Sentinel was not without its design problems, the electric turret traverse ceased to function if the vehicle was on an incline and the cooling system was found to be inadequate. It was however the increased availability of American tanks that sealed the fate of the Australian Cruiser programme.

Unlike the British tanks of the time the basic design of Sentinel proved to be capable of being successfully up-gunned and in 1943 the AC3 prototype was produced which mounted the British 25 pdr. This vehicle had a modified hull with no hull machine gun and a larger turret ring. There were a number of variations on the AC3 design which are worthy of note. To cope with the extra weight of the AC3 a modified engine configuration consisting of four Gypsy Major engines was tried. A feasibility study was carried out to mount the 17 pdr and to test the effects of recoil special turret was made which mounted dual 25 pdrs which when fired simultaneously which produced some 20% more recoil than the 17pdr. One vehicle was subequently mounted with the 17pdr

There are only 4 surviving examples of the Sentinel, one in Bovington, which was shipped there for trials during WW2, another recently rebuilt to running condition at the School of Armour in Puckapunyal, Victoria.This vehicle is the mascot of the Royal Australian Armoured Corp.(RAAC) There is a 25pdr armed vehicle in the Australian War Memorial and the last vehicle is a shell in poor condition at a privately owned museum in Victoria again.

With thanks to Shane Abdoo for his assistance with this feature.