DD or Duplex Drive tanks, nicknamed "Donald Duck tanks" were a type of amphibious swimming tank developed by the British.
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.