noun - Sherman DD or 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 a propeller powered by the tank's engine to drive them in the water.
The DD tanks were one of the many specialised assault vehicles, collectively known as Hobart's Funnies, devised to support the planned invasion of Europe.
Flotation was provided by thirty-six rubber air tubes/pillars. The tubes were attached to a collapsible canvas screen which, once air was pumped into the system via a pneumatic pressure system, would expand to provide the necessary buoyancy. The screen itself was attached to a boat-shaped metal framework welded to the tank's hull. This system could be inflated in 15 minutes, and could be quickly deflated when the tank reached a depth of 5 feet or less. Once the screen was deflated the tank had full use of its turret and could operate as a normal tank.