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Amphibious Encyclopedia · Military amphibians

Sherman DD (Duplex Drive) tank

DD (Duplex Drive) tanks were ordinary tanks made to swim by a folding canvas screen that turned the hull into a boat, with propellers for use in water and tracks for land [1][4]. Hungarian-born engineer Nicholas Straussler designed the system, and M4 Sherman tanks fitted with it led the assault on several beaches on D-Day, June 6, 1944 [1][4]. Their record that day ranged from success in calm water to heavy losses off Omaha Beach [2][4].

  • Military amphibians
  • 1943 to 1945 (Sherman DD in wartime use)
  • United Kingdom (design); conversions in the UK and the United States

Origins

DD stands for Duplex Drive, meaning the tank had two drive systems: propellers in the water and its normal tracks on land [4]. Nicholas Straussler, born in Hungary in 1891 and later a naturalized British subject, had designed collapsible floats around 1927 and 1928 but turned to a new approach by about 1934 [1]. Instead of adding floats, he raised a waterproof screen around the tank so that the screen and the tank together formed a flexible, boat-like hull [1][4].

A light Tetrarch tank was the first fitted with the screen and was tried on Brent Reservoir in June 1941 [4]. Valentine tanks followed in 1942, and by April 1943 the system had been adapted to the American-built M4 Sherman, which could swim with its gun facing forward [4]. In his own account, published by The Tank Museum, Straussler said about 2,000 tanks were converted in Britain and about 300 in the United States [1]. Wikipedia gives 693 Sherman DDs and 247 Valentine DDs built in Britain in 1944 [4].

How it works

A rigid deck ran around the tank just above the tracks. A heavy rubberized canvas screen attached to it was raised by inflatable tubes fed from compressed-air bottles and then locked by metal struts [1][4]. Straussler wrote that each tube pushed upward with about 600 lb (270 kg) of force [1]. Raising the screen took about 15 minutes and gave about 3 ft (0.9 m) of freeboard [4].

Two propellers at the rear, driven from the tracks rather than directly from the engine, pushed the tank at up to about 4 knots (7.4 km/h). They could be swung from side to side to steer, and the commander used a tiller from a platform behind the turret [1][4]. On reaching the beach, the crew collapsed the screen so the gun could fire [4].

D-Day

Eight American, British and Canadian tank battalions used DD Shermans on June 6, 1944 [4]. Results depended on the sea and on how far out the landing craft released them. At Utah Beach, where most tanks were launched about 1,000 yd (900 m) out, 27 of 28 reached the beach. At Juno, 21 of 29 launched by the 1st Hussars made it ashore [4].

Omaha Beach was the worst case. According to the U.S. Army's official account, Companies B and C of the 741st Tank Battalion were launched 6,000 yd (5.5 km) offshore, in waves averaging 3 to 4 ft (0.9 to 1.2 m) with some up to 6 ft (1.8 m) [2]. Struts broke, screens tore and water flooded the engines. Only 5 of the battalion's 32 DD tanks reached shore: 2 that swam in and 3 landed from a landing craft whose damaged ramp had kept it from launching them [2]. The officers of the 743rd Tank Battalion judged the sea too rough and carried their DD tanks to the beach instead [2][3]. The popular belief that every American tank at Omaha sank is therefore a myth [3].

Later use and survivors

DD Shermans were used again in the landings in southern France on August 15, 1944, and in the Rhine crossing of March 1945 [4]. On October 26, 1944, the British Staffordshire Yeomanry made the longest DD crossing, about 7 mi (11 km) across the Western Scheldt [4]. Development largely ended by the late 1950s as tanks became too heavy for flotation screens [4].

The Tank Museum at Bovington, England, keeps a Sherman DD that it describes as its only example with an intact canvas screen [1][4]. Several DD tanks lost off Normandy have been raised and are displayed in France [4].

Sources

  1. How to Invent a DD Tank, The Tank Museum (2024-05-24). tankmuseum.org/how-to-invent-
  2. Omaha Beachhead (6 June to 13 June 1944): D Day, The Landings, Historical Division, War Department, American Forces in Action series (HyperWar transcription). www.ibiblio.org/hyperwar/USA/USA-A-Omaha/USA-A-Omaha-3.html
  3. Did all the American amphibious tanks really sink at Omaha Beach?, D-Day Overlord (Marc Laurenceau). www.dday-overlord.com/?p=220359
  4. DD tank, Wikipedia. en.wikipedia.org/wiki/DD_tank

Last reviewed November 2026. Spotted something to correct or add? Tell us and include a source.

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