Cruiser Tank, Sentinel AC I

The Sentinel AC I cruiser tank was an Australian tank designed to help defend the country against a potential Japanese threat. Despite being a impressive technical achievement, the AC I entered production too late for its primary purpose, and instead was using as a training vehicle.

In the summer of 1940 Australia was faced with the potential of a war with Japan, while lacking any modern tanks. Britain, the obvious source of new equipment, was rearmed after losing almost all of its own modern tanks at Dunkirk. The most modern tank available in American was the Medium Tank M2, which was already obsolete, and was in the process of being replaced with the Medium Tank M3, but that wouldn't be ready for combat until 1942.

Sentinel AC I from the front-right
Sentinel AC I from the front-right

The Australian Ministry of Munitions began to consider building tanks in Australia in July 1940. In November 1940 the Australian General Staff issued the requirements for the new tank, based on recommendations from the Army Design Section. This called for a tank armed with a 2-pounder (40mm) main gun, up to 50mm armour, a range of 150 miles, a crew of 4-5, and weighting 16-20 tons. The eventual AC I at least equalled all of these requirements.  

The production schedule was rather too ambitious. The General Staff thought they'd need 2,000 tanks, with deliveries to begin in July 1941, and to run at 70 per week.

The Australians asked Britain to send out a tank export. Colonel W.D. Watson, an artillery officer with tank design experience, was sent via the United States, where he was shown the plans for the new Medium Tank M3. On his arrival in Australia he was appointed Director of Design.

Watson's team produced a design that combined the final drive and gearbox from the M3 with a new hull and turret produced using materials available in Australia. Power was originally to be provided by Guiberson diesel engines, but the supply of these engines was limited (when tested in the M3 they proved to be somewhat unreliable). The Australian team then mimicked the Americans, and produced a tank engine by combining multiple commercial automobile engines. The first design used three Ford engines, but these were then replaced with three 115hp Cadillac engines arranged in a 'clover leaf' formation. Each engine kept its own crank shaft. The hull and turret were to use either cast or rolled armour, using metals available in Australia. At this stage the plan was to use the same vertical volute spring suspension as the M3.

Early in 1941 a wooden mock-up of the AC I was completed, but the plans were disrupted when the detailed design drawings for the M3 final drive arrived in Australia in April 1941. This revealed that the M3 drive was too complex to be built with the machinery then available in Australia. Machine tools were always one of the bottlenecks in wartime production, and none would be available from Britain or American for at least a year.

This shock led to the temporary suspension of work on the AC I. Instead work moved on the Sentinel AC II, which was to use truck engines and drives. This limited the weight of the tank, and forced the armour to be reduced. It also meant that no heavier guns could be used, and in September 1941 the AC II was abandoned.

Work on solving the problems with the AC I must have continued during this gap, as progress was now impressively rapid. The drive problem was solved by redesigning the final drive to only use components that could be built in Australia. The vertical volute suspension of the original design was replaced with a horizontal volute spring suspension system, based on a French Hotchkiss design. The first cast hull was built in October 1941, and the prototype was ready by January 1942.

Sentinel AC I from the right-rear
Sentinel AC I from the right-rear

The AC I hull had a sloped front, vertical sides and a gently sloped rear deck. There were six road wheels on each side, carried on three bogies, with the horizontal springs connecting the two wheels. The cast turret might have been problematic in combat, as it bulged out from the turret ring, creating a possible 'shell trap' between the turret and hull. The 2-pounder gun and a coaxial machine gun were carried in a wide mantlet, and a second machine gun was carried in a ball mount in the hull front. The turret carried three men - commander, gunner and loader - making it more flexible than many early British designs. The hull was fairly narrow, with no sponsons over the tracks.

A great deal of effort went into building production lines for the Sentinel. Construction of the Chullona Tank Assembly Shops, New South Wales, run by the New South Wales State Railways, began in January 1942. A second factory the Geelong Tank Assembly Shops, Victoria, were still under construction when the programme was cancelled. The Geelong plant would have been run by Ford (Australia).

The first production tank was completed at Chullona in August 1942. A total of 66 tanks were built, before all Sentinel production was cancelled in July 1943. By this point American production was reaching impressive levels, and enough American Medium Tanks were available to equip the 1st Australian Armoured Division. Eventually the Medium Tank Mk3/ Lee/ Grant became the most numerous tank in Australian service, with 752 delivered. The Sentinel was used as a training vehicle for units forming in Australia.

By the time the AC I entered production its 2-pounder gun was almost obsolete. The American Medium Tank M3/ Lee/ Grant had already entered combat in North Africa, with its 75mm hull mounted gun and 37mm turret gun, and the Medium Tank M4 Sherman was already in production. Even so the Sentinel programme was an impressive achievement, involving the largest and most complex metal castings yet to be produced in Australia, and going from the original specifications to production in just over a year and a half.

The Sentinel was also a good design, with good armour, a three man turret from the start and the potential to be upgraded. Two further versions reached the prototype stage during 1942.

The Sentinel AC III had a larger turret and turret ring, and carried a 25pdr howitzer. This model was accepted for production, but the programme was cancelled before this happened.

The Sentinel AC IV carried the 17-pounder high velocity anti-tank gun, and was an impressively modern looking vehicle.

One AC hull was used to test out a new torsion bar suspension system. If this had been combined with the 17-pounder the result would have been an impressively capable tank, but this modification didn't reach production.

Production: 66
Hull Length: 20ft 9in
Hull Width: 9ft 1in
Height: 8ft 5in
Crew: 5 (commander, driver, hull gunner, gunner, loader)
Weight: 62,720lb battle weight
Engine: Three Cadillac V-8 petrol engines, 117hp each, combined 351hp
Max Speed: 30mph road, 20mph cross country
Max Range: 200 miles road radius
Armament: One 2pdr OQF and coaxial .303in MG in turret, one .303in MG in hull front
Armour: 25-65mm

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How to cite this article: Rickard, J (19 July 2016), Cruiser Tank, Sentinel AC I ,

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