 A second variant on the <unk> design was the <unk> <unk> Drive ( " <unk> DD " ) . The <unk> Drive system was invented by Nicholas <unk> , and was designed to allow a tank to ' swim ' through water and participate in amphibious operations . The system functioned by <unk> a large <unk> canvas screen around the tank above its tracks , which was supported by thirty @-@ six <unk> tubes and steel struts ; this gave the tank sufficient <unk> to float , and was then propelled along by a small propeller powered by the tank 's engine . The screen could be collapsed by using a small explosive charge once the tank reached land . The system was fitted during June 1941 , as the <unk> was the lightest light tank available at the time ; the converted tank was successfully tested on a number of lakes and reservoirs , allowing the <unk> Drive system to be tested on heavier tanks , such as the Valentine . The system would be used during Operation <unk> , when M4 Sherman medium tanks would land on the invasion beaches . 
