Austarmycrest.gif (8966 bytes)Australian Military Vehicles Research


Australian Sentinel Tank by Michael Koudstaal (Revised 2005)

Australian Cruiser Tank Mk I "Sentinel"
With the rapid advance of Germany in Europe placing Britain under pressure and the possibility of Japan going to war to seize territory in the Pacific it became clear that Australia could not rely on supplies from elsewhere and would have to manufacture most of itís own military equipment, including it's own tanks.

    The rebuilt AC1 Sentinel at the Melbourne Tank Museum. Note this vehicle is in fact fitted with a 37mm barrel and not the 2 pounder it should carry.

The Australian Cruiser Tank Mark I (or AC1) began as a General Staff specification issued in November 1940. The Army needed a tank capable of a top speed of 56km/h (35 miles per hour) with a range of 240 km (150 miles), armaments consisting of a 2 pounder anti tank gun and a Vickers machine gun, a crew of 4, and armoured to provide protection equivalent to 50mm (2 inch) of armour plate. Following the British practice 1 in 8 were to be armed with a weapon capable of firing smoke rounds and if possible high-explosive, this weapon to be fitted in place of the 2 pounder. A second machine gun and gunner to operate it were to be included in the hull if practical.

Initially a locally produced tank based on the United States M3 medium tank was considered as a possible solution. This tank would have the lower hull, suspension, engine, and transmission of the M3 combined with a new upper hull and cast turret. This would have resulted in a tank broadly similar to the Canadian "Ram" which was also based on the M3 design. However this raised several problems not least of which was that the M3ís engine was not available in Australia, and all machinery capable of producing rolled armour plate of the required thickness were already occupied with other projects.

Realising that the M3 design could not be produced within Australia design work then began on what would become the AC1. This design was based around what the available industry in Australia could produce while maintaining where possible as much in common as possible with other allied tanks. The power unit of the M3 was replaced by three petrol V8 Cadillac automobile engines coupled together. The M3ís synchromesh gear box was one of the few original components retained in the new design and it proved impossible for Australian industry to produce.

Deciding to try and avoid such apparently insurmountable problems, during May 1941 the AC1 design was suspended and work began on a simpler, lighter design largely based on imported parts and assemblies, including power and transmission units designed for heavy trucks. Given the title Australian Cruiser Tank Mark II (or AC2) this design was not liked by the Army who considered it to be too light for most purposes, but accepted it on the basis of rapid production, and that it would be better than nothing. The AC2 design quickly ran into problems with the very thing that had made it an attractive option in the first place, the imported components. These were continuously delayed. As the design emerged the projected weight was calculated and found to be well above the maximum limit of the transmission units. The only way to reduce the weight was by removing armour, which was totally unacceptable and so in September 1941 the AC1 design was revived and the AC2 was abandoned. Work on the M3 gearbox and final drive that continued during the AC2 project had by this time arrived at a design that did away with the synchronising mechanism and roller bearings and used straight cut teeth reverting to a "crash" type instead. Such a unit was within the capacity of the machinery then in Australia and coupled with the imported engines formed the complete drive train for the tank.

The armour of the AC1 provided no less of a challenge, not being able to use rolled armour left only one option, and that was cast armour. Initially the design called for multiple castings bolted together to form the hull, but the firm of Bradford Kendall Ltd. who were responsible for the main armour castings believed that it should be possible to cast the entire hull in one piece. This would make the hull seamless, improve rigidity and eliminate the need to machine many surfaces, simplifying production and saving both time and manpower. The turret was cast as one piece with a bolted on front plate which would have allowed easy upgrading of the armament.

The AC1's middle HVS suspension bogie unit and emergency escape hatch.

The suspension system was originally identical to that of the M3 medium however after a bad report on these they were changed to horizontal volute spring similar to Hotchkiss type. The AC1 road wheels were of a plain dished design for the E series, with four, six, or eight holes drilled in the side for production vehicles although they were otherwise the same dimensions as the M3 as were the drive sprocket, tensioning idler and track return rollers, allowing the use of any track type that could be fitted to the M3 and most Sentinels were fitted with imported American T51 tracks. These tracks were a dual pin rubber bushed design which had a pitch of 152mm (6 inches) and were driven by a 13 tooth drive sprocket. A locally produced steel track given the designation Steel Track, Mark VI, Australian Pattern, the cast Manganese Steel links had a 102mm (4 inch) pitch, were 40.6cm wide (16 inches) and used a 20 tooth drive sprocket. Strangely the pins were retained by lead plugs, the same method as Universal Carrier tracks. The design for this track was based on British Cruiser Mark VII tracks modified to have two outer guide horns so as to be interchangeable with M3 tracks. The end result was similar in appearance to CDP (Canadian Dry Pin) tracks and offered better traction and control than the rubber tracks.

By 1942 the General Staff, still with an expectation of fighting German tanks, had changed the specification increasing the armour basis first to 65mm (2 Ĺ inch) and then later to 75mm (3 inch) although the second increase seems to have been ignored. Three pilot models the "E series" were assembled to the revised specification. The first "E1" was ready in for automotive tests, the second "E2" for gunnery trials, and the third "E3" as a pilot model for mass production. E3 possibly became AC1 8001. The automotive model performed well achieving a top speed of 64 km/h (40 mph) on roads and 52 km/h (32 mph) cross country.

By August 1942 the first production AC1s were completed and given the name "Sentinel". The hull front varied in thickness from 65mm (2 Ĺ inch) to 45mm (1 ĺ inch) depending on the angle. The sides and rear specified as 45mm (1 ĺ inch) thick with an armoured overhang covering air vents that ran along the top sides of the tank. An oval escape hatch was placed between the first and second bogies on both sides. The one piece axel housing and final drive cover is bolted onto the front of the hull. Armoured to a maximum thickness of 65mm (2 Ĺ inch) thinning to around 50mm (2 inch) as it curved away from the vertical. The turret armour was 65mm (2 Ĺ inch) thick thinning to around 50mm (2 inch) on the sloped parts and around 35mm on the top. Like most countries tolerances were made for the armour castings to be thicker than specified in the design. For ballistic traceability on every major cast component in raised lettering is the manufacturer code, part type and the number of the casting. The clearest example is found on the front of the axel housing. An example would be BK AH 31, indicating it was cast by the firm of Bradford Kendall Ltd. and is Axel Housing number 31. The numbers on the rear of the hull were ground off to allow the spare track length to sit flush on the rear of the hull, and on the AC1 the turret number is usually concealed by the turret bin. The only rolled plates used in the tank were the 16mm (5/8 inch) floor plate and the 23mm (7/8 inch) engine cover plate. The amour castings for the first 13 tanks had not gone through the heat treatment process and so were classed as unarmoured, making them suitable for training only.

The three Cadillac engines were placed two abreast with the third located behind and between the others. Each engine was connected via a clutch to a three-in one-out transfer box mounted under the turret. The output shaft ran though the main clutch to the gearbox and the Cletrac type controlled differential similar to that used in the US M3/M4 medium tanks. This unusual arrangement allowed the AC1 to be driven on only two or even just one of its engines, which could be started independently or symultaneously. Using 70 Octane fuel the combined output of the engines was around 246 kW (330 bhp). The radiators and fuel tanks were fitted around and behind the centrally mounted rear engine. Internal fuel tank capacity was around 130 imperial gallons, with an external fuel tank capacity of around 44 gallons. Maximum speed was limited by engine governors to around 48 km/h (30 mph), average road speed 45 km/h (28 mph), average cross country speed 34 km/h (21 mph).

The driver sat on the right hand side and the hull machine gunner on the left separated from the driver by the water cooled Vickers machine gun and the gearbox. There were no sighting arrangements for the hull machine gun other than the gunner's periscope.

The turret was traversed by hand or power traversed by a 40 volt motor supplied by a generator attached to the three in one out transfer case. Operating any of the engines immediately provided power to the turret traverse, and compressed air for the power assisted brakes. Stowage was provided for 130 rounds of 2 pounder ammunition mostly in racks in the hull around the turret basket, like British tanks of the era only AP shot to be carried although HE was a possibility as used in the Australian Matildas. The loader/radio operator was in the right hand side of the turret, the radio sitting in the turret bustle. The 2 pounder was centrally mounted with the water cooled Vickers machine gun on the right and the gunner to the left. Elevation was, like most British tanks armed with 2 pounders, manually controlled directly by a shoulder brace. The tank commander's station was located behind the gunner and equiped with a fully rotating cuploa with two periscopes, and a Bren LMG for anti-aircraft fire.

The AC1, like all tanks was not without problems. The original liquid cooling system was found to be inadequate and to allow an unequal flow to the three engines, resulting in overheating to the point of cracking some engine blocks and seizing others. The tyre bonding was poor causing tyre delamination at low mileage. The turret drive was found to be underpowered and the turret was unbalanced which meant that it would not always operate satisfactorily when the tank was on a steep enough incline. All problems were rectifiable by various means. The tyres and road wheels were interchangeable with M3 tyres and all steel road wheels were trialled as another solution. A redesigned cooling system and new fan solved the cooling problems on one tank. But the programme was cancelled before these could be fitted to all AC1 tanks.

All Sentinels were paint with a two tone disruptive camouflage scheme consisting of Australian Standard Camouflage (ASC) J Khaki Green, and ASC W Light Earth. The only markings carried was the serial number in white numbers on both sides of the driverís compartment and on the back of the hull on the right hand side under of the rear deck overhang. The exception being The only AC3, which was also finished in a two tone camouflage scheme, but only carried its number only on the left hand side of the drivers compartment and on the centre of the rear engine deck overhang.

Even before the first AC1 left the production line it was recognised that the 2 pounder was inadequate as a tanks main armament. The Army requested that heavier armament, the 25 pounder, be fitted to production vehicles but a number of 2 pounder mounts had already be manufactured and changes in design would entail a delay of several months. It was decided to limit the production of the AC1 to 65 vehicles and change over to a new design with a heavier main armament as soon as possible. No AC1 ever saw combat, although with a few superficial modifications some were used as "German Panzers" in the 1944 film "The Rats of Tobruk".

Australian Cruiser Tank Mk III "Thunderbolt"
The Australian Cruiser Tank mark III (or AC3) was a redesign and greatly improved over the AC1. The AC1 from the beginning was designed to accommodate the larger 6 pounder anti-tank gun, but the pressing need for these guns meant that none could be spared. Heavy tanks being fielded by Germany at his time were immune to all but the luckiest of shots from the 2 pounder anti-tank gun.

A variety of alternative armaments were considered to increase the fire power of the Australian cruisers, including the 6 pounder anti-tank gun, the obsolete 18 pounder field gun, the 25 pounder field gun / howitzer, the 3 inch 20 cwt anti-aircraft gun, the 3.7 inch anti-aircraft gun, and the 17 pounder anti-tank gun. The 87.6mm (3.45 inch) 25 pounder was selected for the main armament, as it had excellent high-explosive capability while maintaining adequate armour-penetration performance. To test whether the Australian Cruiser could cope with the recoil one of the development vehicles, AC1 E2 was fitted with a 25 pounder field gun/howitzer in a new turret equipped with a redesigned overhead short recoil system. The tests were successful and a decision was made to mount this weapon on a production basis.

Other major changes included the removal of the hull machine gun and gunner to allow for storage of the larger ammunition, the front hull being changed from the AC1ís quite complex front to a smooth flat plate approximately 51mm (2 inch) thick sloped back to 24 degrees from horizontal. The top of the armoured cowl that runs along the sides and back of the hull for air to be drawn in and exhaust to be expelled was raised so the turret ring was protected from all sides. The engine access hatches were changed for the Perrier-Cadillac and the cast tail of the AC1 rear engine deck overhang was made integral to the hull casting. The AC3 had the same provision for an external fuel tank as the AC1.

The 25 pounder was mounted in the turret with a coaxial 0.303 Vickers water cooled machine gun. The AC3 was of similar dimensions to the AC1 and used the same 137cm (54 inch) turret ring, this in particular meant that the AC3 turret was somewhat cramped particularly for the loader who had to be careful to keep out of the recoil path of the gun. To swing the larger turret the 40 volt electrical traverse motor of the AC1 was replaced by a 110 volt motor. Stowage was provided for 120 rounds of ammunition, 60 High Explosive/Smoke shells and 60 of the shorter 9kg (20 lb) Armour Piercing shot, and 2500 rounds for the coaxial Vickers, stored in 250 round boxes. As the 25 pounder used 2 part ammunition 120 cartridges were to be carried consisting of charge 3 and charge Super. Despite the two part ammunition and the cramped turret the rate of fire showed that the tank was far from unusable, in tests with the commander assisting the loader by ramming the projectile the crew were able to fire 8 rounds in 73 seconds. The tank mounting of the 25 pounder proved to be excellent, unlike the towed version multiple rounds could be fired without having to relay the gun on the target.

The "Perrier-Cadillac" power unit was completely new, the three V8 Cadillac engines for the AC1 design were kept but instead of the two abreast and one behind "clover leaf" layout the three engines were mounted around a common crank case to form a single 24 cylinder unit. This saved space and eliminated the three in one out transfer box, resulting in reduced losses through the drive train. This and the use of 80 octane fuel boosted the output of the power unit to 296kW (397bhp).

Only one AC3 was fully assembled for tests before the programme was shutdown, serial number 8066. During May '43 the hulls of serial number 8066 and one other Mk.3 had completed road trials without their turrets. Work had begun on the assembly of 25 others and various parts and assemblies were being produce in large numbers, including around 150 cast hulls and turrets. Overall the AC3 was shorter, faster, more heavily armoured with the exception of the turret front, and carried a larger gun than the basic M4 Sherman.

Australian Cruiser Tank Mk IV
The AC4 design was a further refinement of the AC3, with a 178cm (70 inch) turret ring providing more room for gun and crew. The General Staff specification called for armament of a 17 pounder anti-tank gun and coaxial Vickers machine gun with elevation of 20 degrees and depression of 10 degrees. Stowage for 50-60 rounds of 17 pounder ammunition comprised of a mix of HE and APC and 2500-4000 rounds of machine gun ammunition. Armour basis was still the same as the AC1 and AC3.

The weight of the tank had increased to around 32 tonnes but nominal ground pressure was lower than the AC3 due to increased track contact area. Top speed was to be increased to 56km/h (35mph) by planned changes the gearbox gear ratios. The reintroduction of synchromesh gears was requested if it had not already been included in the AC3. The Perrier-Cadillac was to have been used but a power plant of four Gipsy Major air-cooled inline aircraft engines were considered as an alternative, the combined output of these engines would have provided more power than the Perrier-Cadillac and performance would have improved.

Hull design was similar to the AC3ís with the drivers door moved to the side of the hull, the engine bulkhead shifted to the rear slightly, the air intake located across the top of the hull behind the turret was removed and the air vents on the side of the fighting compartment enlarged to compensate.

Consideration was given to using US turret traverse equipment and M4 suspension units instead of the AC1 and AC3 HVSS units in order to move to a more standardised tank.

By this time British experience had shown that tanks faced two major threats on the battlefield, Anti-tank guns mounted in tanks, and the more numerous towed Anti-tank guns. To combat these threats tanks needed either a powerful anti-tank gun or a powerful high explosive weapon respectively. As a result two interchangeable turret fronts were designed to enable the AC4 to mount either the 17 pounder anti-tank gun or a 25 pounder field gun/howitzer as fitted to the AC3. The 25 pounder armed variant was to replace part of the AC3 order but not all, as a large number of parts and assemblies had already been manufactured for the AC3.

The design for the AC4 was not finalised before the Australian Cruiser Tank Programme was terminated in July 1943.

The end of the Australian Cruisers
By 1943 for Australia the war had changed no longer fighting in North Africa or Europe against Germany and Italy, but in the jungles and on the islands of the Pacific against the Japanese. With the USA now fully geared up for war and likely to be able to supply large numbers of tanks should they be required and surplus American M3 Lees and Grants, M3 Stuarts and British Matilda IIs already in Australia to equip all armoured formations, the pressing need for tanks of any description had been filled. Although the need for a tank like the AC3 or AC4 with heavy armour, and fully traversable armament never really went away nor was it filled.

Lend Lease authorities in the United States had been exerting pressure on Australia for some time to standardise itís tank production on the M4 or T20 in order to streamline supply and maintenance. Unless Australia agreed to switch over to a US design it would receive no more assistance in this programme in the way of components or machine tools from the US.

Great Britain had shown greater interest and support for the Australian programme, first lending one of it's own tank designers, Colonel W.D. Watson, to the project and then supplying first a Crusader, purchased in 1941 as an example of a modern cruiser tank, and later a Cromwell, purchased in September 1942 to assist with the up gunning of the ACs, although it didn't arrive until after the program had already been terminated. Impressed with the reports brought back by Watson in late '42, the War Office even asked that three ACs be sent at the earliest possible time, the first to mount a 17 pounder the other two with 25 pounders. Consideration was given to sending preproduction examples but this request ultimately went unfulfilled.

The Australian Government had agreed in late '42 in principle, like it's allies, to the production of a standard tank but when faced with the fact that both the M4 and T20 were significantly more complex than any of the Australian Cruisers, would require twice the man-hours to manufacture, used armaments and ammunition that were not in production in Australia and engines that would have to be imported the programme was terminated, allowing the diversion of manpower to other tasks.

The 65 completed AC1s, the two prototypes and the only complete AC3 were handed over to the army at the end of the programme and spent the rest of the war in storage. They were too different to the M3s and Matildas to be used for training and would require modifications and manufacture of spares if they were to be used as special purpose vehicles. In 1945 at the end of the war of the 68 ACs three were selected to provide "a physical record" for preservation in war museums in Australia and Britain, the rest were dismantled, the hulls sold off for conversion into bulldozers, or other heavy machinery, their turrets and other small parts for scrap metal. Some or possibly all the AC3 hulls and turrets produced but never used were setup as hard targets on live fire ranges. The three complete vehicles saved for war museums were the two most complete AC1s, serial numbers 8030 and 8049, and the only AC3 serial number 8066. AC1 8049 was shipped to Britain in 1946 were it was placed in storage and exists today in the Bovington Tank Museum in Dorset and is probably the most complete example, although neither it nor any of the others have an original or correct camouflage scheme. AC1 8030 survives in a less complete but restored to running condition at the Royal Australian Armoured Corps Tank Museum in Victoria. AC3 8066 survives in an unrestored state at the Australian War Memorial in Canberra.

A composite vehicle at the Melbourne Tank Museum's junk yard, built from an ex-dozer AC1 hull, an AC3 turret recovered from a firing range, and 2 pounder anti-tank gun. Despite being roughly the same weight as the M3 medium to it's left, the Sentinel presents a frontal target area only about the same size as the M3 Light to it's far left.

Trials, Tests and Experiments

25 pounder field gun/howitzer armed test vehicle
The 25 pounder armed test vehicle was used to trial the 25 pounder as a tank armament for the AC3 design. Proofing tests were held on the 29th of June 1942 in a prototype turret mounted on development hull E2 by a 137cm (54 inch) turret ring. Gunnery trials were held shortly after on the 27th of July. The tests were successful and the AC3 was ordered to be produced immediately after AC1 production had finished.

Double 25 pounder field gun/howitzer armed test vehicle
The twin 25 pounder armed tank was a test vehicle only. As a 17 pounder was not available at the time two 25 pounders were fitted into a turret mounted on the development hull E1 by a 163cm (64 inch) turret ring. The intent was to test the recoil system and turret ring by significantly exceeding the recoil force of a single 17 pounder anti-tank gun. The pair of 25 pounders fired together produced approximately 120% of the recoil force of a single 17 pounder anti-tank gun. The tank survived and was subsequently armed with a 17 pounder.

17 pounder anti-tank gun armed test vehicle
The 17 pounder anti-tank gun armed test vehicle used to test the armament for the AC4 design. The design for an experimental 17 pounder armed version of the AC had started back in February 1942. Development hull E1 was mounted with a 17 pounder anti-tank gun in a turret with a 163cm (64 inch) turret ring. Firing trials in January 1943 were successful and the weapon was selected for the AC4 design.

Suspension test vehicle
One hull was modified to test a different suspension system of five large road wheels with three track return rollers. Each road wheel had a vertical volute spring semi-recessed into the hull, similar to "Christie" suspension. Probably inspired by the suspension of the British "Crusader" Cruiser tank that had been imported to assist with the design and development of the Australian Cruisers. The concept was not adopted for production.

Armour Basis - Armour Basis for AC1, AC3 and AC4. The armour basis here is defined as providing protection equivalent to a vertical plate of the specified thickness. So if the armour is sloped so as to present an angled face to an incoming projectile it can be of a reduced thickness and still provide the same level of protection.

65mm (2 1/2 inch) Hull Front
45mm (1 3/4 inch) Hull sides and rear
23mm (7/8 inch) Hull top
16mm (5/8 inch) Hull floor
65mm (2 1/2 inch) Turret all round
25mm (1 inch) Turret roof

Initially the specification called for 50mm (2 inches) of armour which was considered proof at close range against anti-tank guns of performance similar to that of the 2 pounder. Experience against German tank and anti-tank guns proved that this was inadequate and the specification was raised first to 65mm (2 1/2 inches) and then 75mm (3 inches) although the second increase seems to have been ignored.

Australian Zirconium-Alloy Cast Armour specification
Brinell Hardness: 230-240.
Izod Impact Strength: 20ft/lb rising through production to 52ft/lb.
Ultimate Tensile Strength: 756MPa (49 T/sq inch). Purposely kept low to reduce spalling.

This armour was developed specifically for the Australian Cruiser Tank, it's formulation was based on the Australian Bullet Proof Plate armour as used for Australian built Universal Carriers, the Dingo scout car, and other projects. Unlike armour produced in other countries the Australian cast armour contained no nickel as there was no source for this in Australia. Similar to British IT90 cast armour it was slightly softer but much tougher, resulting in less spalling from hits. When an AC1 turret was compared against a specially imported M4 turret the armour proved at least as good the American armour if not better.

Australian Cruiser Tank designs and projects

Australian Cruiser Tank Mark I "Sentinel"
Armament: one 40mm 2 pounder anti-tank gun, two Vickers machine guns, one anti aircraft Bren LMG, one Thompson SMG.
Ammunition: 130 rounds 2pdr, 4250 rounds Vickers machine guns, 900 rounds for Bren, 300 rounds for Thompson, 6 hand grenades, 1 Verey signal pistol and 12 flares.
Armour: Lower hull front 65mm, upper hull front 65-45mm at varied angles, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 35mm.
Turret ring: 137cm (54 inch)
Traverse Rate: 20 degrees/second.
Crew: 5
Weight: 27.5 tonnes.
Height: 2.56m (8ft 5 inches)
Length: 6.35m (20ft 10 inches)
Width: 2.8m (9ft 2 inches)
Ground Clearance:0.4m (1ft 4 inches)
Ground Pressure: 92 kPa (13.4 psi), at 76mm (3 inch) sinkage 79kPa (11.5 psi)
Engine: 3 V8 Cadillac engines.
Power output: 246 kW (330 bhp) at 3050 rpm.
Gearbox: Simplified M3. 5 forward gears, 1 reverse.
Steering: Cletrac type controlled differential.
Turning circle diameter: 17.7m (58ft)
Tracks: American T51 or Mark VI, Steel track, Australian Pattern.
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Maximum grade: 35 degrees
Fuel: 590L (130 imp. gallons). External tank 200L (44 imp. gallons)
Range: 175 km (110 miles) without external tank.
Maximum permissible speeds: 1st 5km/h (3 mph), 2nd 10km/h (6 mph), 3rd 20km/h (12.5 mph), 4th 35km/h (22 mph), 5th 48km/h (30 mph), Reverse 6km/h (3.5 mph)
Speed: maximum 48 km/h (30 mph), average road 45 km/h (28 mph), average cross country 34 km/h (21 mph)
Serial numbers: 8001 to 8065
Status: 65 units produced.

Australian Cruiser Tank Mark IA
Armament: one 6 pounder anti-tank gun and one Vickers machine gun.
Turret ring: 137cm (54 inch)
Engine: 3 V8 Cadillac engines.
Status: Designs completed, not intended for production.

Australian Cruiser Tank Mark IB
Armament: one 25 pounder field gun / howitzer and one Vickers machine gun.
Turret ring: 137cm (54 inch)
Engine: 3 V8 Cadillac engines.
Status: Designs completed, not intended for production.

Australian Cruiser Tank Mark II
Armament: one 6 pounder anti-tank gun and two Vickers Machine guns.
Engine: two diesel engines.
Status: Abandoned Design Project.

Australian Cruiser Tank Mark III "Scorpion"
Armament: one 2 pounder anti-tank gun and two Vickers machine guns.
Turret ring: 137cm (54 inch)
Engine: 400HP Single row Pratt and Whitney Radial
Status: Planned variant of the AC1 with a different power plant. None produced.

Australian Cruiser Tank Mark III "Thunderbolt"
Armament: one 25 pounder field gun / howitzer, one Vickers machine.
Ammunition: 120 rounds 25pdr (60 AP, 60 HE and Smoke), 2500 rounds Vickers machine gun.
Armour: Lower hull front 65mm, upper hull front 50mm at 24 degrees, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 35mm.
Turret ring: 137cm (54 inch)
Crew: 4
Weight: 29 tonnes
Ground Pressure: 99 kPa (14.4 psi)
Engine: Perrier-Cadillac
Power output: 296 kW (397 bhp)
Gearbox: Simplified M3. 5 forward gears, 1 reverse.
Steering: Cletrac type controlled differential.
Turning circle diameter: 17.7m (58ft)
Tracks: American T51 or Mark VI, Steel track, Australian Pattern.
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Maximum grade: 35 degrees
Fuel: 840L (185 imp. gallons). External tank 200L (44 imp. gallons)
Range: 300 km (180 miles) without external tank.
Maximum permissible speeds: 1st 5km/h (3 mph), 2nd 10km/h (6 mph), 3rd 20km/h (12.5 mph), 4th 35km/h (22 mph), 5th 48km/h (30 mph), Reverse 6km/h (3.5 mph)
Serial numbers: 8066 to 8265
Status: At least one unit produced. 200 ordered.

Australian Cruiser Tank Mark IIIA
Armament: one 25 pounder field gun / howitzer and one Vickers machine gun.
Turret ring: 178cm (70 inch)
Engine: Perrier-Cadillac
Status: Proposal only.

Australian Cruiser Tank Mark IV
Armament: one 17 pounder anti-tank gun and one Vickers machine gun.
Ammunition: 50-60 rounds 17pdr, 2500 rounds Vickers machine gun.
Armour: Lower hull front 65mm, upper hull front 50mm at 24 degrees, top 25mm, remainder 45mm. Turret all round 65-50mm at varied angles, top 35mm
Turret ring: 178cm (70 inch)
Crew: 4
Weight: 30 tonnes
Ground Pressure: 96 kPa (14 psi)
Engine: Perrier-Cadillac
Power output: 296 kW (397 bhp)
Steering: Cletrac type controlled differential.
Turning circle diameter: 17.7m (58ft)
Tracks: American T51 or Mark VI, Steel track, Australian Pattern.
Vertical Step: 1m (3ft 6inch)
Fording depth: 1m (3ft 6inch)
Trench crossing: 2.4m (8ft)
Fuel: 840L (185 imp. gallons). External tank 200L (44 imp. gallons)
Range: 300 km (180 miles) without external tank.
Maximum permissible speeds: 1st 6km/h (3.5 mph), 2nd 12km/h (7.3 mph), 3rd 23km/h (14.3 mph), 4th 42km/h (26.3 mph), 5th 56km/h (35 mph), Reverse 6km/h (4 mph)
Serial numbers: 8266 to 8775 (Including Mark IVA)
Status: Intended for production. Design incomplete. 400 ordered.

Australian Cruiser Tank Mark IVA
Armament: one 25 pounder field gun / howitzer and one Vickers machine gun.
Turret ring: 178cm (70 inch)
Engine: Perrier-Cadillac
Status: Intended for production. Design incomplete. 110 ordered.

Sources
Bellona Military Vehicle Prints Series Twelve, UK, Merberlen, 1967.
AFV Weapons Profile Series Two, Number 31, Profile Publications Limited, England.
The Role of Science and Industry, D.P. Mellor.
Records held by The National Archives of Australia. www.naa.gov.au
Paul D. Handel's Crusader Down Under and Cromwell Down Under
Mr Richard Simmie, with out whom this would have been impossible.

Revised August 2005, and it's still not done.


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