DD tank
DD or duplex drive tanks, nicknamed "Donald Duck tanks", were a type of amphibious swimming tank developed by the British during the Second World War. 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.
DD tanks worked by erecting a canvas 'flotation screen' around the tank, which enabled it to float in water. 'Duplex drive' refers to the fitted propellers allowing propulsion through water, which supplemented the usual track propulsion used when the flotation screens were lowered upon landing to fight as an ordinary tank.
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.
History
Early development
Amphibious tanks were devised during the First World War; a floating version of the British Mark IX tank was being tested in November 1918, just as the war ended. Development continued during the interwar period.As tanks are heavy for their size, providing them with enough buoyancy was a difficult engineering problem. Designs that could float unaided were generally small and light with thin armour, such as the Soviet T-37. Heavier vehicles, such as the experimental, British AT1* had to be so large that the design was impractical.
The alternative was to use flotation devices that the tank discarded as soon as it landed–the approach adopted by the Japanese with their Type 2 Ka-Mi and Type 3 Ka-Chi amphibious tanks. 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.
The system was unsatisfactory in other ways, due primarily to the unwieldy bulk of the floats that were big enough to float a tank – these were each roughly the size of the tank itself. 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. Also, such floats made a tank too wide to launch from an off-shore landing craft, making their use in amphibious landings impractical.
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. When collapsed, it would not interfere with the tank's mobility or combat effectiveness.
The first tank to be experimentally fitted with a flotation screen was a redundant Tetrarch light tank provided to Straussler. Its first trial took place in June 1941 in Brent Reservoir in north London in front of General Sir Alan Brooke. 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. Satisfactory sea trials of the Tetrarch took place in Portsmouth Harbour.
A prototype of a Duplex Drive Valentine tank began trials on 21 May 1942, although it subsequently sank. In June 1942, permission was given by the Ministry of Supply for the manufacture of 450 Valentine DDs.
It later became clear that the Sherman was more suitable for use with a screen than the Valentine and the DD screen was adapted for the Sherman by April 1943. One reason for this was that the Sherman could move in the water with its gun forward ready to fire as soon as land was reached. The Valentine was also an older and generally inferior design.
247 Valentine DD and 693 Sherman DD were built by the United Kingdom in 1944.
Training
Valentine DDs were used for training and the majority of the US, British, and Canadian DD crews did their preliminary training with them. Crews learned elementary phases of the DD equipment at Fritton Lake, on the Norfolk/Suffolk border. Here they learnt to waterproof and maintain their tanks, use Amphibious Tank Escape Apparatus, launch from mock up LCT ramps and navigate around the two and a half mile lake. After two weeks of training at Fritton, the crews moved to Stokes Bay, Gosport, Hampshire for three weeks of intensive training from landing craft. The DDs would be loaded on the hards at Stokes Bay, and launch into the Solent. They would form up in echelon, cross of water and land at Osborne Bay on the Isle of Wight. The regiments would then move to Combined Training Centres, such as at the Moray Firth in Scotland and Barafundle Bay in Wales to train with other elements and units, during which period crews incurred several losses.On 4 April 1944, Operation Smash was held at Studland Bay in Dorset with the Valentine DDs. The trial run of the tanks ran into difficulty when a change in the weather adversely affected the sea conditions. Six tanks sank with the loss of six crew members.
The sunken wrecks of at least 10 tanks, lost during training, are known to lie off the British coast. Another sunken DD tank remains at the bottom of Fritton Lake.
Sherman DD
Modifications to the Sherman included the sealing of the lower hull, the addition of the propeller drive and the addition of Straussler's flotation screen around the hull, together with its inflation system. The base of the canvas flotation screen was attached to a horizontal mild steel boat-shaped platform welded to the tank's hull. The screen was supported by horizontal metal hoops and by 36 vertical rubber tubes. A system of compressed air bottles and pipes inflated the rubber tubes to give the curtain rigidity. The screen could be erected in 15 minutes and quickly collapsed once the tank reached the shore. In practice there was about of freeboard. 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. In practice, some units kept the flotation equipment and their tanks were used in several amphibious operations.A pair of propellers at the rear provided propulsion. One problem presented by the Sherman was that the configuration of the transmission made it impossible to take a drive-shaft directly from the gearbox to the propellers. The solution to this was to have sprocket wheels at the rear of the tank so power was delivered to the propellers by the tank's tracks. DD Tanks could swim at up to. Both the commander and the driver could steer in the water, although with different methods. 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.
The first DD Shermans produced by the British were used by both British and US units. Later production was by both the US and the UK. British Shermans were Sherman III and Sherman V conversions. The US used the M4A1 only for their conversions.
Experience from D-Day led to an improved, Mark II version of the DD Sherman. The screen was extended and strengthened by fixing to the turret, a new type of bilge pump fitted and a second set of hydraulic steering controls was fitted at the commander's station, although his tiller was retained. An air compressor replaced the air cylinders that provided the pressurized air to erect the screen. After D-Day, US Army interest decreased, looking for other options.
While the US Army in Europe used the Sherman DD design, in the Pacific LVTs were equipped with armor and guns to support landings up to the sea line; from the sea line, tanks were supposed to support infantry.
Later flotation screen use
Designs were made to give the Cromwell and Churchill the DD treatment, but these were never completed. A floating, flame-thrower equipped version of the Universal Carrier was tested, as was a flamethrower-equipped DD Sherman. This towed an armoured fuel trailer, like those used by the Churchill Crocodile. The trailer, in the water, was supported by an inflatable flotation device.After the war, the Centurion was tested with a flotation screen and duplex drive. By the end of the 1950s, development of DD tanks had ceased, partly because main battle tanks were becoming too heavy to be practically made to swim – although experiments were carried-out in the mid-1960s with a floating Centurion that used a similar system, but with rigid panels instead of a flexible screen.
The 38 tonne Vickers MBT was fitted with a flotation screen that allowed it to swim.
Medium and light vehicles continued to be made amphibious by the use of flotation screens into the 1980s, but without the DD. Instead, they used the movement of their standard running gear for water propulsion also. These included the Swedish Stridsvagn 103, the American M551 Sheridan light tank, the British FV432 Armoured personnel carrier, the Mark IV version of the Ferret armoured car and early versions of the American M2 Bradley Infantry Fighting Vehicle. Of these, only the FV432 and the Bradley remain in service and current versions lack flotation screens.
Combat
The main use of DD tanks occurred on D-Day. They were also used in Operation Dragoon, the Allied invasion of southern France on 15 August 1944; Operation Plunder, the British crossing of the Rhine on 23 March 1945, and in several operations on the Italian Front in 1945. DD Tanks were sent to India; the 25th Dragoons were trained in their use, but planned operations against the Japanese in Malaya never occurred.D-Day
The DD Sherman was used to equip eight tank battalions of American, British, and Canadian forces for the D-Day landings. They were carried in Tank Landing Craft, also known as Landing Craft, Tank. These could normally carry nine Shermans, but could fit fewer of the bulkier DDs. British and Canadian LCTs carried five tanks, the Americans carried four as their LCTs were shorter at about.The DDs would typically be launched around from the shore, swim to the beaches and overpower the German defences. The tank's record was a mixture of success and failure, although they are mainly remembered for their disastrous performance on Omaha Beach.