The phrase is mostly used for the Duplex Drive variant of the M4 Sherman medium tank, that was used by the Western Allies during and after the Normandy Landings in June 1944.

The DD tanks were one of the many specialized assault vehicles, collectively known as Hobart's Funnies, devised to support the planned invasion of Europe.

In Britain, the Hungarian-born engineer Nicholas Straussler developed collapsible floats for Vickers-Armstrong that could be mounted on either side of a light tank to make it amphibious.

Trials conducted by the British War Office showed that such a tank, propelled by an outboard motor, 'swam' reasonably well.

In practice, there would be severe difficulties in transporting enough floats, even collapsed ones, to move a large unit of tanks across a body of water.

[4] In 1940, Straussler solved the problem by devising the flotation screen – a device which folded and was made of waterproofed canvas.

The screen covered the top half of the tank effectively creating a canvas hull, greatly increasing the vehicle's freeboard, and providing buoyancy in the water.

The reservoir had been the location where trials of the floating version of the World War I, Mark IX tank took place, 23 years earlier.

The base of the canvas flotation screen was attached to a horizontal mild steel boat-shaped platform welded to the tank's hull.

In combat, the flotation system was considered expendable and it was assumed the tank crew would remove and discard it as soon as conditions allowed.

A hydraulic system under the control of the driver could swivel the propellers; the commander from a platform at the rear of the turret, where he could see over the skirt, could contribute by operating a large tiller.

[18] The DD Sherman was used to equip eight tank battalions of American, British, and Canadian forces for the D-Day landings.

On the British Sword Beach, at the eastern end of the invasion area, the DD tanks worked well, as the sea was reasonably calm.

The 8th Canadian Brigade, at the eastern end of the beach, was forced to land without DD tanks because of rougher seas.

Twenty-seven out of twenty eight reached the beach but confusion caused by the massive smoke screen meant they landed around 2,000 yards (1,829 m) from their aiming point and saw some German opposition.

[21] Most of the crews were rescued, mainly by the landing craft carrying the 16th Regimental Combat Team,[23] although five crewmen are known to have died during the sinkings.

Research undertaken in 2000 by the Institute of Nautical Archaeology at Texas A&M University, in cooperation with Robert's Naval Historical Center's Underwater Archaeology Branch, found evidence that some had come close to safely landing: [T]he landing craft carrying them were drifting away from the target beach – forcing the tanks to set a course which put them side-on to high waves, thus increasing the amount of water splashing over and crumpling their canvas skirts.

Two tanks – skippered by men with enough peacetime sailing experience to know not to turn their sides to the waves – actually made it to the beach.

[3] The 756th had eight tanks that were launched 2,500 yards (2,286 m) from the beaches; one was swamped by the bow-wave of a landing craft and one sank after striking an underwater obstacle.

The 753rd battalion had 16 tanks, of which eight were launched at sea and successfully reached the shore, eight were landed directly on the beach later in the day.

The Staffordshire Yeomanry were converted to DD tanks after D-Day and trained initially at Burton-upon-Stather, near Scunthorpe, before moving with them to Belgium.

The DD Tanks' longest operational water crossing took place without casualties, but they had great difficulty in landing – 14 became bogged down in mud and only four were available for action.

Mats laid at the objective points (carried across beforehand by Buffalos) allowed the DDs to climb the steep, muddy banks of the river.

[25] The DD's last combat swimming operation was the Staffordshire Yeomanry's crossing of the River Elbe at Artlenburg on 29 April 1945.

There were no further swimming operations, but it was found that the folded flotation screen offered a large seating area, making the tanks useful troop transports.

The "T-6 Device" kit consisted of a structure of box-like, pressed-steel floats (pontoons) mounted on the front, rear and sides of a Sherman.

Compared with the DD, the floats were bulky and harder to stow, limiting the number of tanks that could be carried in a landing craft.

[27] During planning for an invasion of England in 1940 (Operation Sea Lion), the Germans also worked on developing amphibious tanks capable of directly supporting infantry during a beach assault.

Schwimmpanzer IIs were deployed from a specially modified landing barge (Type C) and could be launched directly into open water from a hatch cut into the stern.

Six other wrecks to be in the Moray Firth in Scotland[30] Two sunken Valentine DDs rest 3.5 miles (5.6 km) out of Swanage Bay, Dorset.