The Skink Anti-Aircraft Tank with Four 20 mm Autocannons

The Skink was a Canadian designed tank on a Sherman chassis during the Second World War, equipped with four 20 mm autocannons for use against low-flying aircraft. With a fire rate of 1,850 rounds per minute, it was soon found to have an overwhelming amount of firepower that was extremely effective against ground targets. While still in development it was put to the test in Europe, where it proved itself as a deadly weapon.

It was not without it flaws though, as while the Skink was very reliable, its ammunition was troublesome and the turret actually let in an alarming amount of metal fragments through gaps in its armor. Still, it worked well and troops were very fond of it.

In the end though it was its task of shooting down aircraft became its own enemy, as by the time it was ready the Luftwaffe was virtually wiped out, and an anti-aircraft tank was no longer required.

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Development

In early 1943 a Canadian committee laid out official requirements for a self-propelled anti-aircraft vehicle. It would provide cover for armored columns and other units against close air attacks. This vehicle was required to carry four 20 mm autocannons, have sufficient armor to defend against aerial attack, carry 600 rounds of ammunition and be capable of following targets travelling at up to 350 mph. This was not an easy set of requirements to meet, especially for a nation with rather minimal experience in designing armored vehicles.

In fact, no manufacturers produced any proposals, so the Canadian military had to do it themselves. The resulting vehicle was named Skink, after the species of lizard.

A design was soon drawn up which featured four Hispano-Suiza 20 mm cannons in a welded turret, fitted to the hull of the Grizzly (a Canadian-built version of the M4A1 Sherman). The advantage of using a tank hull for a mobile anti-aircraft gun is that it can keep up with the tanks it is supporting, and it can go through the same terrain they can.

The wooden Skink turret mock-up.
The wooden mock-up of the Skink’s turret, including the turret basket.

By September 1943 a wooden mock-up of this turret had been built by the Waterloo Manufacturing Company, and the design was considered promising enough for a steel turret to be ordered.

This was completed in December 1943, but its welded construction quickly proved to be difficult to make.

As a result, development switched to a cast turret instead. Dominion Foundries of Hamilton were contracted to produce a cast turret of a similar shape to the previous welded design. Once again though problems were encountered, as it proved difficult to properly balance a cast turret that rotated this fast. Eventually though this issue was resolved and the project would feature a cast turret from then on.

Looking down onto the Skink's welded turret.
The Skink’s initial welded turret, fitted with its four Hispano-Suiza 20 mm guns.

The tank fulfilled the requirements asked of it and briefly entered production at the beginning of 1944, however the project would suffer yet another setback when forces in Europe declared that they would no longer use the Hispano-Suiza 20 mm guns, as ammunition for this type was prioritised for aircraft.

Read More Germany’s Mighty Gepard – Dual 35 mm Autocannons

20 mm Polsten cannons were used instead of the Hispano-Suiza guns, requiring the turret to once again be redesigned to accept these new weapons. A Canadian report from 1945 says that the change in main armament “invalidated much previous design work”. While the potential for this change was apparently known, and the tank was at least somewhat designed with this expectation in mind, it still caused major problems for the project.

Skink AA tank cast turret.
The Skink’s cast turret. Note that it still has the Hispano-Suiza 20 mm guns.

The Hispano-Suiza weapons were fed by 60-round magazines with 200 round feeds planned, but the Polsten guns were only able to receive 30-round magazines. Despite the smaller magazines, the upper guns still had to be tilted sideways by 40 degrees to make room.

Attempts were made to develop a belt-fed system for the guns, which would have enabled a much longer times between reloads, but this pursuit ended in failure and was cancelled in July 1944. The 30 round magazines would have to do.

The Skink AA Tank

Many are likely itching that we keep referring to this tank as… well… a tank, as it should be classified as a self-propelled anti-aircraft gun (SPAAG). However, the Skink is only referred to as a tank in original reports and documentation. Its full designation was 20-mm, Quad, AA, Tank “Skink”.

It used the M4A1 Grizzly as its basis, a tank powered by the 400 hp R-975 radial and constructed with a cast hull. The Skink’s turret is, of course, the most significant aspect of the tank.

Skink document titles.
The Skink is referred to as a tank in its documentation.

Its shape is similar to that of the standard Sherman 75 mm gun turret, but the front is a unique shape designed around the four 20 mm Polsten cannons. Its armor was 57 mm (2 1/4 inches) at the thickest on the front, 50 mm on the sides (2 inches) and 38 mm (1 1/2 inches) on the rear – similar to a standard Sherman turret.

Protruding from the front of the turret was the guns, each mounted in their own mantlet. Three crew were in the turret, with the commander and loader/radio operator both sitting at the back, and the gunner sitting in the center of the turret. Each crew member had their own hatch with a Vickers-type periscope.

The gunner’s hatch was directly above him, in the center-portion of the turret. While he could fire the weapons buttoned up, to accurately target aircraft he had to operate with his hatch open, leaving him vulnerable.

Skink turret diagram.
A diagram of the Skink’s cast turret, with Polsten cannons.

Aiming was done with a US Navy Mark IX reflector sight. This was quite a basic piece of a equipment, with a single bulb illuminating a pipper in the sight. The gunner aimed the turret with grips on a control column. The turret was hydraulically powered, with the hydraulic pumps driven by an electric motor, and had 360 degrees of rotation and -5 to 80 degrees of elevation.

This powerful system was able to rotate the Skink’s turret an impressive 10 times per minute – a single rotation would take just 6 seconds, twice as fast as a standard Sherman’s turret.

Each 20 mm Polsten cannon had a rate-of-fire of 450 rounds per minute, for a combined output of 1,800 rounds per minute. If needed though the guns could be fired individually or in pairs. Sixty four 30 round magazines were carried in the Skink, stored in the hull and in the turret.

The Skink’s turret was planned to be distributed as a conversion kit for the Sherman, with the same 1,750 mm (69 inch) turret ring as that tank.

At first the Skink’s hull was virtually unchanged from the Grizzly – aside from modified stowage for the 20 mm ammunition – but as the project evolved the Grizzly’s standard electrical system was unable to provide enough power to the hydraulic pumps moving the turret.

Skink tank from the side.
The Skink turret could be mounted on other Sherman variants with some modifications.

As a result, the Grizzly’s two engine-driven electrical generators were modified with aircraft-grade components to provide almost 50 percent more power. The Grizzly’s standard electrical generators were designed so only one could charge the tank’s battery at a time, but in the Skink this was modified so both could charge the battery at the same time. With this, the Skink could actually charge its batteries during an engagement.

This latter change was reportedly made by off-the-shelf components, making the process of converting Shermans easier.

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A tougher change was regarding stowage of ammunition. The amount that could be carried varied depending on which variant of Sherman was being converted. As a result, a universal conversion kit was not immediately possible. Those in the field would have had to notify the supplier of their exact model of Sherman, and then be sent a kit specifically made for that tank.

Performance and Cancellation

Due to delays caused by redesigning the turret, by the time the Skink was ready the threat of the Luftwaffe in Europe had vastly diminished. In fact, German aircraft were nowhere to be seen, negating the need for a tank capable of shooting aircraft down altogether. As a result the production of the Skink went from an order of 300 to zero.

In total just three complete Skinks were built, with a further eight turrets ready for use on converted Grizzly tanks.

Interestingly though, instead of completely cancelling the project and storing or scrapping the produced vehicles, Canada believed they still had value and sent the Skink to Europe for testing. The exact movement of the three Skinks is unclear, but it appears that one was sent to the UK and one was sent to Europe for combat trials, while the third was kept in Canada.

In the UK a Skink was trialled in the latter stages of 1944 at Lulworth, Dorset. During firing trials the Skink was found to be an extremely effective weapon, with a number of good features. In particular, those testing the tank at Lulworth praised its interior layout of guns and ammunition. However, they also found it to struggle against fast moving targets and the controls had noticeable input lag. These contrasted with Canada’s own reports.

Skink tank used in the UK.
The Skink sent to the UK for trials.

Between February and March 1945 the Skink in North West Europe underwent combat trials. The Luftwaffe was completely absent during the trials so the tank never got to perform its intended role as an AA tank, but it was used to excellent effect against ground targets. The four 20 mm guns were found to put such an overwhelming amount of ammunition down range that the enemy would often surrender instead of retaliating.

This was amplified by the Skink’s high explosive incendiary tracer (HEIT) ammunition. These rounds were particularly good at setting buildings on fire – usually ones with defenders still inside. In many cases, having their shelter literally set on fire resulted in a prompt surrender. The report from the trial details that some buildings contained many defenders, but “when the buildings burned, however, they gave up quite easily”.

The Skink was shown to nearly all of the Canadian armoured regiments in the region, and all were very impressed with the tank. They concluded that it “was a valuable asset to an armoured regiment, but that its primary role should be as an anti-personnel weapon.”

Skink turret.
The Skink’s four guns had terrifying psychological effects that troops appreciated.

Troops considered it a much better vehicle than the Crusader AA tanks they had been equipped with previously. The Skink was in high demand for any operation that would encounter dug in infantry, but it had to be fielded alongside tanks with anti-armor capabilities in case enemy tanks were encountered.

The Skink also proved to be extremely reliable, at least in its frontline usage in Europe. In fact, no major breakdowns or mechanical issues occurred with any of the tank’s systems. The report makes a number of suggestions should the tank have been properly introduced, which included more reliable weapons, better gun sights and, importantly, improved splash protection around the four gun mantlets.

Overall though its service in Europe was positive, with the report stating that “an issue of 6 Skinks per armoured regiment would prove a valuable asset”, and that it would “become a vehicle of great merit and all-round usefulness.”

Skink tank during trials.
Although it had tremendous firepower against soft targets, the Skink had no anti-armor capabilities and had to be supported by tanks.

But, despite glowing reviews, the Skink would not enter production. After some more trials all hulls were scrapped, and the turrets were either scrapped or used as hard targets on firing ranges. Thankfully three empty turrets have survived.

The Skink stands as a fascinating development in the history of self-propelled anti-aircraft guns, becoming one of the best made during the war and perhaps one ahead of its time. In later years designs would emerge around the world that functioned on a similar basis to the Skink, with multiple autocannons in a fast-moving turret on a tank chassis. They had something the Skink didn’t though; radar and fire control systems.

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With these, the Skink would have been a truly excellent design. But hindsight is a cruel mistress, and the Skink remains as a interesting footnote in tank design.