DD (Duplex Drive) Sherman Swimming Tank

The Duplex Drive

DD Sherman tank with float screen lowered, WWII

Duplex Drive Sherman tank with float screen lowered. UK government photo.

The goal in designing the Duplex Drive was to create a tank which could swim to an enemy held beach ahead of the infantry, instead of involving specialized landing craft for their delivery. These amphibious tanks could be unloaded a distance of approximately two miles from shore, where they would at first be barely visible to the enemy. Development on the DD tank began in 1941

Designing the Swimming Tank

The main difficulty faced in designing an amphibious tank was overcoming the problem of the weight of a heavy tank. After a variety of unsuccessful design attempts, a military engineer, Nicholas Straussler, (in Great Britain) devised a solution in 1940 which implemented a flotation screen (also called a skirt or buoyancy screen). The water-proof canvas screen was raised by filling rubber tubing with compressed air. The tubing could be inflated in about 15 minutes, but could be deflated and the screen lowered in a very short time once in shallow water. While in the water, the tank was suspended just below the surface by the float screen, and powered by two rear propellers.

Other versions of the DD were tried with the British Tetrarch and Valentine tanks, but eventually the American Sherman proved to be the most successful, with the ability to swim with its gun in a forward position. The DD Sherman was able to fire its main gun while approaching the shore and once the screen was deflated, it was ready for action as a conventional tank.

Tanks carried a crew of 5, and were armed with one 75 mm gun as well as two 0.3 inch machine guns. The maximum speed in water was about 4 knots and the tanks could manage up to 1 foot high waves. The unsuspecting Germans were in for quite a surprise when, what appeared to be rubber boats, began firing their main guns.

Difficulties during Water Operations

Sherman DD tank with screen raised. Imperial War Museum photo.

The difficulties during water operations were various. The low free-board (height of deck above the water) meant that the tank needed relatively calm water to avoid being swamped. Emergency gear was given to the crew which included an amphibious tank escape apparatus (ATEA) as well as inflatable rafts, in case the tank was swamped. Crews also had been trained to escape from a submerged tank. The tanks maximum water speed was quite slow, only four knots, and the DDs had inherent difficulties with steering while afloat.

Normandy Landings

The DD tanks were difficult to transport because they took more space than regular Sherman tanks, and were difficult to maneuver without damaging the screen. Tthe British and Canadian LCTs (landing craft tanks) carried just five DD tanks in a herringbone arrangement on the deck and the Americans transported only four of the DD tanks on the LCT(5)¹. Forming part of a multitude of specialty tanks known as 'Hobart's Funnies', these tanks were sent ahead to lead the D-day invasion of Normandy.

Ten tank battalions were distributed among British, Canadian, and American forces for the D-Day assault on Normandy. On Sword, and Utah Beaches, the majority of the DD tanks successfully swam to shore. On Gold Beach the tanks were brought directly to the shore by landing craft due to high seas. On Juno Beach, only some of the tanks were launched because of high seas. But on Omaha Beach, 27 of the 29 DD tanks sank at sea in six foot waves, after being launched three miles from the beach.

In the entire D-day operation, 290 DD tanks were used. Out of those, 120 were launched at sea, for which at least 42 sank. Approximately 140 DD tanks were launched in very shallow water or directly on the shore. The American DD tanks suffered 38% loss due to sinking, versus the British and Canadian which lost 31% due to sinking. The difference was that the American losses were all concentrated in one battalion.²

Over-all, the DD tank was considered the most successful of all the specialist tanks used during the Normandy landings. It was used used again in the invasion of southern France, the crossing of the Rhine in Germany, the crossing of the West Sheldt in the Netherlands, the crossing of the Elb in central Germany, and on the Italian Front.

Notes:
¹David Fletcher, Swimming Shermans: Sherman DD Amphibious Tank of World War II, New York: Osprey Publishing 2006, pp. 19-20.

²Steven J. Zaloga, US Amphibious Tanks of World War II, Long Island City, NY: Osprey Publishing Ltd, 2012, pp. 26 & 28.

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