The amphibious variant of the M4 Sherman tank used on D-Day was the Sherman DD, meaning Duplex Drive Sherman.
The system was developed by Hungarian-British engineer Nicholas Straussler and adapted for Sherman tanks by fitting a collapsible canvas flotation screen around the hull. When inflated and raised, the screen gave the tank enough buoyancy to swim. A pair of propellers at the rear provided water propulsion, while the tank’s tracks remained available for movement once the vehicle reached land.
Sherman DDs formed part of the specialized vehicles known as Hobart’s Funnies, assembled to help Allied forces overcome Normandy’s beaches and coastal defenses. On D-Day, British and Canadian units launched many of their tanks from offshore landing craft. Rough seas and long distances from shore caused severe losses among some American-launched DD tanks, while others successfully reached the beaches and supported the assault.
“DD” does not mean “D-Day”; it refers to “Duplex Drive.” The tank was still fundamentally an M4 Sherman, not a separate Sherman model like the heavily armored Sherman Jumbo, the later “Easy Eight,” or the British 17-pounder-armed Firefly. The flotation equipment was intended to be discarded after landing.