A second variant on the Tetrarch design was the Tetrarch Duplex Drive ( " Tetrarch DD " ) . The Duplex Drive system was invented by Nicholas Straussler , and was designed to allow a tank to ' swim ' through water and participate in amphibious operations . The system functioned by erecting a large waterproof canvas screen around the tank above its tracks , which was supported by thirty @-@ six inflatable tubes and steel struts ; this gave the tank sufficient buoyancy 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 Tetrarch was the lightest light tank available at the time ; the converted tank was successfully tested on a number of lakes and reservoirs , allowing the Duplex Drive system to be tested on heavier tanks , such as the Valentine . The system would be used during Operation Overlord , when M4 Sherman medium tanks would land on the invasion beaches .