DD tank

The DD tank was required to provide infantry support on the beaches. By giving the tank the ability to float, it could be launched from a landing craft and make its own way onto the beach. Having tanks on the beaches would have created an advantage for the allies, as the German defences were designed to protect against troops and were not expecting mechanical weaponry. The tank was successful at Juno Beach but failed the Americans on Omaha Beach.

The DD tanks were one of the many specialised assault vehicles known as Hobarts funnies that were offered to American forces for the invasion, but the DD were the only ones adopted. It has been speculated that if the DD were better used, or some of the other vehicles had been used that American losses on the beaches of Normandy, particularly Omaha beach would have been far less.

There were other amphibious tanks, both before and after the Sherman used in World War 2, however the later ones were chiefly experiments as tanks were so heavy. The DD Tank was designed specifically for the Normandy invasion.

Development

In 1941 Nicholas Straussler solved the problems faced by other amphibious tanks by inventing the folding screen which was made of waterproofed canvas, this made tanks buoyant while not adding much size.

British Major General Percy Hobart applied Nicholas Straussler’s folding screen design to Valentine and Tetrarch tanks. Hobart first tested the folding screen with a Tetrarch tank in Portsmouth Harbour. The test was successful and production was started using the Valentine tank. The majority of the American, British and Canadian DD tank crews did their preliminary training on the Valentine DD and there were several training losses using the Valentine.

Weighing around 35 tons the DD Sherman lay low in the water. On one hand it wasn't very noticeable but on the other the top of the screen was barely above the water.

By 1944, it was clear that the Sherman tank was more suitable for amphibious use than the Valentine, one reason was that it could move in water with its gun forward ready to fire as soon as land was reached. The Valentine was also an older and generally inferior design. Modifications to the Sherman included; sealing of the lower hull, the addition of two propellors to the rear of the hull driven fom the engine gearbox and the addition of Straussler's flotation skirt around the hull.

DD Tanks could move at up to 4 knots with the propulsion provided by the pair of propellers. The tanks were steered in the water by directing the propellers and a rudder. The flotation screen was a canvas curtain supported by thirty-six rubber tubes. These tubes were inflated with air to give the curtain rigidity. The canvas was attached to a metal frame welded to the tank's hull. The screen could be inflated in 15 minutes, and quickly deflated once the tank reached the shore.

Designs were even made to give the Cromwell and Churchill tank the ‘DD’ treatment, but these were never finished. After the war the Centurion tank was tested with floatation screen and duplex drive. By the end of the 1950s development of swimming battle tanks had ceased, as they were becoming too heavy. Swimming armoured vehicles continued though in the floatation screen equipped British Abott self-propelled gun and the naturally bouyant Alvis Stalwart High Mobility Load Carrier (FV620).

Combat

In total 27 tanks sank at sea. It is believed the sea was too rough. The DD Tanks were designed to withstand waves up to 1 foot high, on that day the waves were up to 6 feet high. These were much worse conditions than they had trained in and they were swamped with water. Also, the tanks were released into the sea too far out (about 3 miles offshore), combined with the inherent difficulty in steering a 35 ton modified tank it is a tribute to the crews that they got as far as they did. The tank crews drowned at sea. Although they were equipped with emergency breathing apparatus capable of lasting 5 minutes this was not nearly enough in the turbulent sea. Some did manage to radio back and warn following units not to launch as far out.

Until very recently it was believed that the DD Shermans that were launched offshore of Omaha beach (from 741st Armored Battalion) were mostly sunk immediately, swamped by the seas that were much higher than the operators had practiced with. It was suggested that they were launched too far from shore. However tanks at the other four beaches suffered no such problems. New research suggests that the Omaha tanks were aiming for a church steeple on the visible horizon behind the cliffs. In order to maintain their line of sight it is believed that the tanks had to turn progressively away from the shore to combat the wavefronts pushing them down the beach, putting their sides virtually parallel with the waves/beach. This meant that the protective canvas flotation devices were easily swamped by the waves. If they had kept going directly forward with the front of the tank headed straight for the beach, they may well have reached it.

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