Author: Zoe Tolman, Assistant Archivist

As we're fast approaching the 80th anniversary of D-Day, we're taking a small break from anti-mine devices (we'll get back to them in another article, I promise). This article looks at the development of an excellent example of ingenuity used during the Normandy landings – the DD tank.

A Valentine DD tank being launched into the water.

Throughout military history, water has always been a massive barrier to military movements. Men (and horses) could swim a certain distance but machinery, especially armour, has always struggled to get across both seas and rivers. Without some ability to face the water themselves, tanks are dependent on other forces (infantry or airborne) to seize and hold bridgeheads for them. All this without the protection of armour and usually under heavy fire. That is assuming the bridges aren't blown up before they can take them.

Initial solutions to the water problem could be sorted into three main categories – amphibious tanks, tanks which could be temporarily turned amphibious, and devices which could ferry a tank across unaltered. In all cases however, the aim was for a tank whose land capabilities would be unaffected by the requirements for crossing and which could begin operation in that role as soon as it landed. Waterproofing for wading would ideally need to be removed as soon as possible before the engine overheated.

Amphibious Tanks

As early as 1917, the idea of an amphibious tank was proposed by Colonel Fuller as a means to cross the Rhine. He subsequently led the first test on the Welsh Harp (aka the Brent Reservoir) in 1918. Sadly, there doesn’t appear to have been any concerted effort on the idea in the inter-war years. This meant development wasn’t as far along as it could have been by the time the need for a water-ready tank resurfaced.

The main consideration for an amphibious tank is that it must be able to float. For some light tanks, this was achievable simply by ensuring the hull was watertight and using certain materials in the build. The heavier cruiser tanks required external floats to be attached to them. The first tank ‘designed to operate on land and to swim on water without the aid of any equipment not forming part of the basic design’ is reported to be a light tank in the Vickers-Carden-Loyd pattern, demonstrated in 1931. It was driven in the water by a propeller shaft from a power take-off on the gearbox which could be engaged and disengaged by means of a lever and dog-clutch. This allowed the propeller to run independently of the tracks or at the same time if required.

Vickers light tanks – note the propeller improvements.

A second variant with an improved rudder system was developed by Vickers-Armstrong. This had a distinctly boat-looking hull and could achieve speeds of 5mph in the water and 35mph on land. Although this saw official trials in 1940, emphasis seems to have shifted towards the use of external floats for tanks, both light and cruiser type. The Vickers Light Amphibious tank was never taken up into service.

Temporarily Amphibious Tanks

Attaching flotation devices moved the design solutions into category two – temporarily amphibious, albeit with some designed to stay on the tank semi-permanently. Floats for light tanks were designed by the Experimental Bridging Establishment (EBE) and took the form of 14ft platforms fitted on either side of the tank. With these attached, the tank could achieve a maximum safe speed of around 3.75 mph. EBE also went on to create a folding float prototype which could be lifted up to allow easier cross-country movement, particularly with the route to river-crossings in mind. These were ultimately not taken up either.

A light tank with EBE floats.

The floats for cruiser tanks were much larger and, quite quickly realised, impractical. They took too many man-hours to fit, took up too much room before they were fitted, and then made the tanks too wide for either cross-country movement or easy launching from craft. Since these were untenable, the third category came into play. A series of rafts, pontoons, and lighters were then designed to transport the cruiser (and infantry) tanks. All these options however appear to have been shelved after the first demonstration of what would go onto become the DD Tank.

A float on a cruiser tank – note the specially adapted tracks.

The DD Tank

In June 1940, a man called Mr Straussler, who had previously worked on detachable floats for Vickers-Armstrong light tanks, was handed specifications for an amphibious fighting vehicle which could float without the use of the external floats under development at the time. He then explained that he had in fact already been working on a system to instead allow any standard tank to float, regardless of weight, armour thickness, or armament, and without detracting from any of its land capabilities. Intrigued, the Ministry of Supply sent him to the Director of Naval Construction Department for approval and they promptly turned him down. However, after further discussions with the Ministry of Supply’s Tank Design Liaison, Major Reeves, permission was granted for a prototype to be built. According to records, the decision to go ahead with the trial was ‘due entirely to the insistence of Major Reeves’. It seemed that a device that subsequently proved itself incredibly useful at D-Day would have been otherwise lost without his efforts.

The DD tank (simply referred to as ‘Straussler-type flotation’ at the time) takes the form of a canvas screen attached to the deck of the tank. When the rubber columns inside the screen are inflated, the canvas forms a rigid hull from the tank deck upwards. This provides both buoyancy and a vastly increased freeboard (the distance from the waterline to the upper deck/point at which water can enter the vessel). Once the tank lands, the columns can be instantly deflated and the tank can then function as normal.

The air-tube frame on a Sherman DD tank.

The tank given to Straussler for his prototype was a Tetrarch and it was entirely unsuitable. It had a defective engine, a defective gearbox, the hull was so leaky that it took on around 3/4 ton of water in just 3-4 minutes on its first trial, and a separate onboard motor had to be used for propulsion since the hull design wouldn't allow for a propeller driven by the tank engine. Hundreds of man hours had to be sunk into the vehicle before it could actually demonstrate the flotation device. Once it did however, people were impressed. A report by the Department of Tank Design's representative even states that he hung his full body weight on top of the canvas screen when inflated and was 'unable to force it downwards'. Later trials also saw the screen exposed to prolonged small arms fire until it sank to ensure it would be able to stay afloat long enough to cross the 'average river' even if under attack. The Tetrarch was a proof of concept and, once suitably demonstrated, focus turned to fitting the Straussler system to the Valentine instead. Development was officially approved in December 1941.

The Tetrarch prototype with onboard motor.

The Valentine DD tank was heavily developed from 1941-44 but in 1942, the system was also applied to the Sherman tank. This was found to be much better suited. The Valentine ended up as more of a training and development vehicle. Improvements on the Sherman included replacing the compressed air bottles used to inflate the screen's columns with a compressor driven by the tank engine. This allowed for a much better rate of inflation, and generally improved the steering. Considering the Sherman can weigh up to around 32 tons, the fact that one could swim itself ashore and immediately provide support for the invading infantry is a remarkable feat and certainly an impressive development from the early designs.

Valentine DDs ready to launch (left) and in the water (right).

Information and pictures in this article are taken from E:05.0177.02 and E:03.0422.02.

Published in The Craftsman, May 2024.