DD or Duplex Drive tanks, nicknamed "Donald Duck tanks", were a type of amphibious swimming tank developed by the British during the Second World War. The phrase is mostly used for the Duplex Drive variant of the M4 Sherman medium tank, that was used by the Western Allies during and after the Normandy Landings in June 1944. DD tanks worked by erecting a 'flotation screen' around the tank, which enabled it to float, and had two propellers powered by the tank's engine to drive them in the water. The DD tanks were one of the many specialized assault vehicles, collectively known as Hobart's Funnies, devised to support the planned invasion of Europe. History Early development Amphibious tanks were devised during the First World War; a floating version of the British Mark IX tank was being tested in November 1918, just as the war ended. Development continued during the interwar period. As tanks are heavy for their size, providing them with enough buoyancy was a difficult engineering problem. Designs that could float unaided were generally small and light with thin armour, such as the Soviet T-37. Heavier vehicles, such as the experimental, British AT1* had to be so large that the design was impractical. The alternative was to use flotation devices that the tank discarded as soon as it landed–the approach adopted by the Japanese with their Type 2 Ka-Mi and Type 3 Ka-Chi amphibious tanks. In Britain, the Hungarian-born engineer Nicholas Straussler developed collapsible floats for Vickers-Armstrong that could be mounted on either side of a light tank to make it amphibious. Trials conducted by the British War Office showed that such a tank, propelled by an outboard motor, 'swam' reasonably well. The system was unsatisfactory in other ways, due mainly to the unwieldy bulk of the floats that were big enough to float a tank – these were each roughly the size of the tank itself. In practice, there would be severe difficulties in transporting enough floats, even collapsed ones, to move a large unit of tanks across a body of water. Also, such floats made a tank too wide to launch from an off-shore landing craft, making their use in amphibious landings impractical. In 1940, Straussler solved the problem by devising the flotation screen – a device which folded and was made of waterproofed canvas. The screen covered the top half of the tank effectively creating a canvas