Sherman DD

The Sherman DD (Duplex Drive) was a Second World War M4 Sherman tank converted to swim ashore under its own power. The idea was first tried on Valentine tanks. The British 79th Armoured Division under Major General Percy Hobart, which handled most special armour projects, pressed for the Sherman to be used instead, since it was then the main medium tank of both British and US forces. General Eisenhower was impressed by a demonstration during invasion rehearsals, and more units were ordered to be equipped. British workshops could not convert enough tanks in time, so US factories converted some as well. British conversions were mostly of the M4A2 and M4A4 (Sherman III and V), and American ones of the M4 and M4A1.

Design

Buoyancy came from a tall, rubber-coated canvas screen fixed to a flange around the hull above the tracks. It could be raised in about 15 minutes by inflating 36 rubber pillars inside it. Three steel hoops and 13 hinged struts held it against the water pressure. The "duplex drive" was a pair of worm gears taken off the rear track sprockets, which drove two 26-inch propellers. The driver steered them with a hydraulic piston, or the commander used a tiller from a platform at the rear, since only he could see over the screen. On later river-crossing tanks the propellers could be swung up clear of the ground for land travel. Once ashore, the struts were broken hydraulically to collapse the screen so the turret could traverse and fire. Freeboard was only about 0.6-1.2 m, and the screen could cope only with small waves.

D-Day and after

DD tanks were meant to lead the Normandy landings on 6 June 1944, launched from landing craft offshore. Training took place in great secrecy, often at night, with tanks following a guide boat. The Canadian Fort Garry Horse landed a whole squadron of 18 safely because the Royal Navy carried them close to the beach before releasing them. In contrast, the Americans lost most of their DD tanks in the rough sea. DD Shermans were later used for river crossings, including the Rhine.