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

right|thumb|Diagram of a flotation screen fitted to a Tetrarch tank, taken from Straussler's patent, , issued 1945

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.

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 (also known as Welsh Harp Reservoir) in north London 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

thumb|right|A Valentine DD tank being loaded onto a landing craft prior to a training exercise, Stokes Bay, Hampshire, January 1944

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 embarkation 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

thumb|right|Rear view of a Sherman DD with its screen raised, showing the twin propellers in their lowered position|upright

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 (gearbox at the front) 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 . 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

thumb|right|[[M2 Bradley Infantry Fighting Vehicle emerging from the water at Fort Benning (1983). This M2 uses a screen but is not DD]]

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 (e.g. tracks) for water propulsion also. These included the Swedish Stridsvagn 103 (S-Tank), 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

thumb|right|Men of [[British Commandos|No. 4 Commando engaged in house to house fighting with the Germans at Riva Bella, near Ouistreham. Sherman DD tanks of 'B' Squadron, 13/18th Royal Hussars are providing fire support and cover, 6 June 1944]]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 (LCT). These could normally carry nine Shermans, but could fit fewer of the bulkier DDs.

Omaha Beach

At Omaha Beach almost all of the tanks launched offshore were lost, their absence contributing to the high casualty rate and sluggish advance from that beach.

The first wave at Omaha included 112 tanks: 56 from each of the 741st and 743rd Tank Battalions. Each of these battalions had 32 DD and 24 other Shermans (including many Sherman bulldozers for clearing obstacles). Starting at about 0540, the 741st Tank Battalion put 29 DDs into the sea; 27 of these sank, with only two surviving the long swim to the beach. Some of the crews of the sinking tanks managed to radio back and warn following units not to launch so far out. The remaining vehicles of the 741st Tank Battalion, and all tanks of the 743rd Tank Battalion (except for the four aboard one LCT that was hit by artillery fire just off the beach), were landed directly on the beach, starting at about 0640.

DD Tanks were designed to operate in waves up to 1 foot (0.3 m) high; however, on D-Day, the waves were up to high. These were much worse conditions than the tanks had been tested in and hence they were swamped. Also, the tanks of 741st Tank Battalion were launched too far out: about offshore. These factors also exacerbated the inherent difficulty of steering a 35-ton "vessel" with a low freeboard. The crews were equipped with DSEA emergency breathing apparatus capable of lasting 5 minutes, the tanks were also equipped with inflatable rafts. Some sources claim that these life-saving measures were ineffective; this was contradicted by the testimony of survivors. although five crewmen are known to have died during the sinkings.

Some stayed afloat for a matter of minutes; according to the crews one tank swam for 15 minutes, another: "We weren’t in the ocean [sic] 10 minutes when we had a problem".

Operation Plunder, the Rhine crossing, began on the night of 23 March 1945. As well as the Staffordshire Yeomanry, DD tanks equipped the American 736th and 738th Tank Battalions and the British 44th Royal Tank Regiment. Some tanks were lost in the river, but the crossings were considered a success. The tanks were launched from points upstream from their objectives, to take account of the Rhine's strong current. Mats laid at the objective points (carried across beforehand by Buffalos) allowed the DDs to climb the steep, muddy banks of the river.

"T-6 Device"

An alternative to the DD was the "T-6 Device", developed by the US Army. Limited numbers of the "T-6 Device" were used by the US Army and Marines during the landings on Okinawa. 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. No propellers were fitted – propulsion was provided by the rotation of the tracks. The front and rear floats were discarded on the beach, some in the water. Explosive bolts were used. Side floats were removed from the tanks on shore when the tactical situation permitted. In an oral interview with former Pvt. Maurice Dean Derby 37699146, Browning Automatic Rifleman, Co. A, 17th Infantry Regiment, 7th Infantry Division, he related that "On the night of the landing (Okinawa, 1 April 1945) we were kept awake all night because the Tank Crews were beating on the pontoons with sledgehammers to remove them from the Tanks." The tanks were Co. B, 711 Tank Battalion.

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. However, the system was more seaworthy and had the advantage of allowing the Sherman to fire its main gun as it approached the beach. The Sherman's gyroscopic gun stabilization allowed accurate fire even when the tank was being pitched by waves.

German equivalent

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.

thumb|right|German Tauchpanzer III under test (1940); the [[crane ship , which was to support Tauchpanzer operations, is in the background]]

The Schwimmpanzer II was a modified version of the Panzer II which, at 8.9 tons, was light enough to float with the attachment of long rectangular boxes to either side of the tank's hull. The boxes were made of aluminum and filled with Kapok sacks to preserve buoyancy if water leaked into the pontoons. Motive power came from the tank's own tracks which were connected by rods to a propeller shaft running through each float. The Schwimmpanzer II could make 5.7 km/h in the water. An inflatable rubber hose around the turret ring created a waterproof seal between the hull and turret. The tank's 2 cm gun and coaxial machinegun were kept operational and could be fired while the tank was still churning its way ashore. 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. The Germans converted 52 of these tanks to amphibious use prior to Sea Lion's cancelation.

Instead of floating, the Tauchpanzer, a modification of the Panzer III and Panzer IV, drove on the sea-bed. A rubber hose supplied the engine and crew with air and gave the waterproofed tank a maximum diving depth of making it an extreme example of a wading tank. The Germans converted 168 Panzer IIIs and 42 Panzer IVs for use in Operation Sea Lion.

Surviving DD tanks

Valentine

thumb|right|Privately owned, Valentine DD at a military show (2010)

A DD Valentine, restored to running condition, is in private ownership in England.

The sunken wrecks of others are known to exist. With at lest seven having been identified in [[Studland|Studland

Bay]] in Dorset. Six other wrecks to be in the Moray Firth in Scotland

Two sunken Valentine DDs rest out of Swanage Bay, Dorset. These tanks are apart in of water.

Sherman

thumb|right|Surviving M4A2 Sherman DD in [[The Tank Museum, Bovington, including canvas flotation screen]]

  • The Tank Museum, Bovington in England has an M4A2 DD Sherman in working order, with its canvas flotation screens still intact.
  • Three of the DD Shermans lost on D-Day were salvaged in the 1970s. Two M4A1s are displayed at the Musée des Épaves Sous-Marine du Débarquement (Museum of Underwater Wrecks of the Invasion), a privately owned museum near Port-en-Bessin, in Normandy.

:One of these tanks, along with a variety of personal items recovered with it, has been purchased by an American company, Overlord Research, LLC, based in West Virginia. The owners intend to return the tank to the United States, preferably for public display in an American Museum, such as the D-Day Museum in New Orleans, Louisiana or the National Museum of the United States Army, to be constructed at Fort Belvoir, Virginia. Details of the repatriation of the DD Tank and its final location in the United States are still being determined. As the tank was submerged for many years, Overlord believes preservation in its current condition, which shows its loss at sea as it approached Omaha Beach, is an aspect that is more historically important than attempting a restoration.

  • A Canadian M4A4 recovered in 1971 is displayed as a monument at Courseulles-sur-Mer. In an effort led by Canadian ex-pat Léo Gariépy, who had commanded a 1st Hussars DD tank that landed near there in the first wave, the town took the initiative to retrieve from the water a DD tank ("Bold") which had sunk in front of Courseulles on D-Day. The tank was retrieved, restored, and installed in the Place Léo Gariépy.
  • In 2000, there was an unsuccessful attempt by the United States Navy to raise a sunken M4A1 DD Sherman, located near Salerno in Italy. It was eventually recovered on 18 May 2002. It has been restored and is on display in the Piana delle Orme museum, in Italy, near Latina, south of Rome.
  • A Sherman tank that was lost off the Devon coast in the UK, was recovered in the 1980s, largely due to the efforts of a beachcomber named Ken Small. It is now on display in the village of Torcross as part of a memorial to those who died on 8 April 1944 when an invasion rehearsal, Exercise Tiger, was attacked by E-boats. This Sherman is an M4A1 DD tank, this can be seen because of the specific gears to which the propellers were connected, under the rear deck. While the metal frame on which the flotation screen was fixed disappeared due to rust, some traces of it can still be seen around the hull.
  • An M4A2E8 HVSS Duplex Drive tank (possibly) the same tank unit was later displayed at the Museum Support Center in Anniston, AL, around 2013; A converted M4A2E8 was displayed at the World War II US Military Vehicle Museum in San Rafael, California, in 2016.
  • An M4A1 DD is part of the collection of the French tank museum, the Musée des Blindés. It carries a 76mm turret and main gun, which are not original to the tank when it was used during World War II.
  • An M4A2 DD tank is displayed in India, at the Cavalry Tank Museum, Ahmednagar, India.

See also

  • Deep wading (driving a tank on the seabed using a snorkel)
  • T-37A tank – World War II Soviet amphibious tank.
  • T-38 – World War II Soviet amphibious tank.
  • Type 2 Ka-Mi – World War II Japanese amphibious tank.
  • Type 3 Ka-Chi – World War II Japanese amphibious tank.
  • Allied Technological Cooperation During WW2

References

;Notes

;Bibliography

  • DD Tank Saving Private Ryan online encyclopedia, 14 April 2005
  • BBC online news 'The tanks that didn't land on D-Day' 15 April 2005
  • Juno Beach – Fort Garry Horse DD Tanks on D-Day
  • Juno Beach – 1st Hussars DD Tanks on D-Day
  • Vaughan, Don. Neptune’s Treasures: A survey of ships and other craft lost during Operation Neptune
  • Burton upon Stather Heritage Group are a voluntary organisation involved in the preservation and restoration one of the World War II testing sites for DD tanks on the banks of the River Trent. Their website contains interviews with those who worked on the then secret operations and those who lived in the area during the war. It also includes photographs of the testing site past and present.
  • Duplex Drive Tanks of D Day covers the development of the DD Tank and training in the UK at the various specialist wings (1943–48)