This equipment was devised by the British as a method of converting a normal combat tank into a temporary amphibious tank for sea or river crossing. Developed during April 1943, Sherman DD tanks were used in the Normandy landings. For conversion the tank was waterproofed and fitted with a collapsible screen and 36 rubber air-tubes or pillars. This assembly was attached to a deck that had been welded round the hull of the tank. The rubber pillars were filled by compressed air, carried in two cylinders on the hull of the vehicle, on being inflated the tubes raised the canvas screen which was then locked into position by struts. On entering the water the screen acted as a flotation device. Two small screw propellers, driven from the bevel drive of the vehicle, propelled the tank at approximately 4 knots. On reaching the shore the air was released from the air tubes causing the screen to collapse and the vehicle reverted to its normal combat role. Various marks of Sherman DD equipment existed, later marks having metal framework for the screen.
Sherman III and V DD: These were the original conversions using similar screens and fittings to the Valentine DD.
Sherman III and V DD Mk I: Improved conversion for issue to troops with strengthened top rail and inter-locking struts.
Sherman III DD Mk II: DD Mk I with detail improvements.
Sherman III and IIIAY DD Mk III: American conversions for British and (limited) American use with detail changes. Latter vehicle had HVSS and 76mm gun. In British service late 1945.