Australian Cruiser Tank, AC Mark III Thunderbolt

Australian Cruiser Tank AC III. The hull consists of four main castings of armour plate: the nose (which houses the front axle drive assembly, braking components final drive and gear box pinion) the main hull body, the turret and the power unit cover plate. The nose is bolted onto the machined face of the main hull body. The suspension system is a Horizontal Volute Suspension System (HVSS) copied from the French Hotchkiss design of the 1930's. It consists of three double wheeled bogies and three track return rollers on each side. The U.S style drive sprockets are mounted at the front of the vehicle with the idler wheels at the rear. The tracks are a double pin rubber block type similar to the M3 Lee/Grant track. Affixed to the front hull is a brass plaque which credits the Bonython family for raising 25,000 pounds towards the purchase of a single cruiser tank. The powerplant consists of a trio of joined Cadillac, 75 V-8 petrol engines, with a common crankcase. This was described as a Perrier-Cadillac after the designer and developed 397 hp. The main armament is a 25 Pounder field gun converted to fit into the modified turret. Secondary armament is one coaxially mounted .303 Vickers medium machine gun. The vehicle is painted overall dark green. This coating is patchy with extensive areas of loss. Minor fittings show evidence of corrosion.

The Cruiser Tank project resulted from an Australian Government decision in May 1940, appreciating that the supply of tanks from Britain would be difficult due to the deteriorating war situation, to design and produce an Australian tank. Considering that at the time Australian industry had little heavy engineering plant, this was an ambitious project.

After a fact finding mission to the USA on tank design and the despatch of a design specialist from England, a specification was drawn up to manufacture the main hull, forward transmission housing, turret and mantlet as cast units. It was hoped to use as many off-the-shelf automotive components as possible in the new tank design.

By the end of 1942 the ACI had been trialled and weaknesses modified to suit changing war conditions. The improved AC3 design was well advanced. However by mid 1943, the threat of Japanese invasion had passed and with the supply of American and British tanks exceeding requirements, the AC series became unnecessary and an extravagant use of Australia's limited resources. Of the AC tanks produced only a small proportion were provisionally accepted by Army, and these subject to further modifications and improvement.

The Memorial's ACIII variant is an upgunned variation of the ACI, and utilises a modified 25pdr field gun for increased firepower. This necessitated considerable design modification, mainly in the provision of a larger turret and turret ring and deletion of the hull machine gunner's position. This Mark also included the redesigned and compact engine installation featuring a common crankcase; allowing room for extra fuel tanks.

The ACIII was delivered to the Army on 12 June 1943. The ACIII with Perrier Cadillac engine was given the name Thunderbolt. The ARN is 8066. This was the only AC III to be built although another may have been started, however with the cancellation of the AC programme in August 1943 no more were completed.

Special features of the tank are: Cast hull. HVSS suspension copied from French Hotchkiss design in place of the M3 type vertical volute suspension as first planned. Crew of four (as opposed to the five carried in the ACI). Very low, stable, fast vehicle, with good armour protection and development potential. Tracks are American rubber block type.