The engineering paradox of making a 30-ton steel beast swim
In short: How do you make a heavy armored tank cross a body of water without it becoming a submarine? The British solved this during WWII with the "Donald Duck" tank. It combined canvas screens and propellers to turn heavy steel into a swimming machine, but the physics of buoyancy were a constant battle.
The challenge of amphibious warfare lies in the conflict between weight and buoyancy. During the First World War, engineers attempted to make the Mark IX tank float, but by the 1940s, the sheer mass of medium tanks made unaided buoyancy nearly impossible for heavy armor. While some nations, like Japan, used small, light amphibious tanks, the British sought a way to keep their heavy firepower intact. They turned to a "discardable" approach: adding external flotation devices that could be stripped away once the tank hit the sand.
The solution was the Duplex Drive (DD) system, developed by Hungarian-born engineer Nicholas Straussler. Instead of permanent floats, the DD Sherman used a waterproofed canvas screen supported by 36 vertical rubber tubes. This screen effectively created a canvas hull, providing about 3 feet of freeboard. To move through the water, the tank used a unique propulsion system: since the Sherman's gearbox was at the front, power was delivered to rear propellers via the tank's own tracks and sprocket wheels. This allowed the vehicle to swim at speeds up to 4 knots.
Despite the innovation, the DD tanks faced significant logistical and tactical hurdles. The screens were bulky, making it difficult to transport large units across water, and they were often too wide for standard landing craft. During the Normandy Landings in June 1944, the system saw mixed results. While they performed well on calm beaches like Sword Beach, rough seas at Gold Beach led to heavy losses. The tanks were famously remembered for their struggles on Omaha Beach, where sea conditions and mechanical failures hampered their ability to reach the shore and overpower defenses.
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- Summary by Intellect · source: DD tank
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