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DD Sherman tank with its flotation screen lowered.

DD tanks (for Duplex Drive) were amphibious swimming tanks developed during the Second World War. The phrase is mostly used for the M4 Sherman medium tanks used by the Allies in the opening phases of the D-Day landings in 1944.

The swimming tracked LVTs had already been used in the 1942-43 Solomons campaign in the Pacific, swimming 2 1/2 ton trucks in the 1943 invasion of Sicily, swimming Ford jeeps appeared in 1944, and the Soviets even developed swimming tankettes in the 1930s --but swimming medium tanks presented their own design problems.

The swimming tank idea arose when it was realised that the first waves of infantry that reached an invasion beach would be acutely vulnerable without the support of tanks. But if landing craft were used to carry those tanks, they themselves would be vulnerable to German heavy guns. The loss of too many landing craft would slow the movement of reinforcements from ships offshore and the invasion beaches would be choked with disabled and sunken landing craft. By giving tanks the ability to float, they could be launched from landing craft several miles from the shore and make their own way onto the beach.

The DD tanks were one of the many specialised assault vehicles, collectively known as Hobart's Funnies, derived to support the beach landings. All were extensively used by the British and Canadians, but the DD tanks were the only ones adopted by the American forces. It has been speculated that if the DD tanks were better used, or if some of the other specialised vehicles had been used, that American losses on the beaches of Normandy, particularly Omaha beach, would have been far less.

Amphibious tanks were tested during the First World War, but none worked well enough to use in combat. Development continued during the interwar period. Swimming tanks are generally of two kinds: ^[1]

• some with natural buoyancy, these were generally either too small to be useful or so large as to be impractical.
• others were normal tanks with floatation added on, but these were too big to fit onto landing craft.

In 1941 Nicholas Straussler solved the problems faced by other swimming tanks with the idea of a folding screen made of waterproofed canvas. This made tanks buoyant without adding much size, but was only useful in calm water.

Valentine DD tank with screen lowered and gun pointing towards the rear of the vehicle.

British Major General Percy Hobart applied Nicholas Straussler's folding screen design to Valentine and Tetrarch tanks. Hobart first tested the folding screen with a Tetrarch tank in Portsmouth Harbour. The test was successful and production was started using the Valentine tank. The majority of the American, British and Canadian DD tank crews did their preliminary training on the Valentine DD which incurred several losses.^[2]

By 1944, it was clear that the Sherman tank was more suitable for use with screen than the Valentine, one reason being that it could move in water with its gun forward ready to fire as soon as land was reached. The Valentine was also an older and generally inferior design. Modifications to the Sherman included sealing of the lower hull, the addition of two propellers to the rear of the hull driven from the engine gearbox, and the addition of Straussler's flotation skirt around the hull. DD Tanks could move at up to 4 knots (7 km/h) with the propulsion provided by the pair of propellers. The tanks were steered in the water by directing the propellers and a rudder. The canvas flotation screen was attached to a metal frame 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 with air to give the curtain rigidity. The screen could be inflated in 15 minutes, and quickly deflated once the tank reached the shore. In combat, the floatation system was considered expendable and it was assumed the tank crew would remove and discard it as soon as conditions allowed.

M2 Bradley Infantry Fighting Vehicle emerging from the water. This M2 uses a screen but is not DD.

Designs were even made to give the Cromwell and Churchill tank the DD treatment, but these were never finished. After the war, the Centurion tank was tested with floatation 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. Medium and light vehicles continued to be made amphibious by the use of floatation screens but without the DD. Instead, like the LVT they used their standard running gear (e.g. tracks) for water propulsion too. These included the Swedish Stridsvagn 103 (S-Tank), M551 Sheridan light tank, the British FV432 Infantry Fighting Vehicle, the Mark IV version of the Ferret armoured car and some versions of the American M2 Bradley Infantry Fighting Vehicle.

DD tanks on Utah beach

Men of 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 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. Limited numbers were also used during Operation Plunder, the British crossing of the Rhine on March 23, 1945.

The DD Sherman was used to equip ten tank battalions of British, Canadian and American forces for the D-Day landings. Tank landing craft or LCTs, each carrying four tanks, would launch the DDs from around two miles offshore. They would swim to the beaches and overpower German defences unprepared for attack tanks. In the event, the tank's record was a mixture of success and failure, although they are mainly remembered for their disastrous failure on Omaha Beach.

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 DD tanks were launched two and a half miles (4 km) from shore and 32 out of 34 tanks successfully reached the beach, where they covered the assault troops as planned. This contributed to the comparatively light casualties on this beach.

On Gold Beach, the sea was rougher and the LCTs were ordered to land the tanks directly on the beach rather than risk launching them at sea. Consequently, instead of being the first units ashore, the DD's landed at the same time as the infantry and the other specialist assault tanks. German anti-tank guns caused heavy losses in some sectors of the beach but the assault was successful.

On the Canadian Juno Beach, The Fort Garry Horse and the 1st Hussars were equipped with DD tanks, but only those of the 1st Hussars could be launched. They were assigned to the 7th Canadian Brigade, on the western end of the beach. Twenty-nine tanks were launched 800 yards (700 m) from the beach, twenty-one reached the shore. However, they only arrived after the first wave of infantry which consequently suffered heavy losses. Despite this, the tanks were able to destroy German defensive positions and Canadian troops soon advanced several miles inland. The 8th Canadian Brigade, on the eastern end of the beach, was forced to land without DD tanks because of rougher seas. They suffered heavy initial casualties, but were still able to make good progress.

On Utah Beach, four of the DD tanks were lost when their LCT struck a mine and sank. The remaining 28 arrived on shore successfully. However, on the way in, they were overtaken by the faster infantry landing craft and so only arrived 15 minutes after the initial infantry landing. Despite this, the DD tanks were one of several factors that contributed to the light American casualties on this beach.

At Omaha Beach almost all of the tanks launched offshore were lost, contributing to the high casualty rate and slow progress at that beach.

There were 112 tanks assigned to the first wave at Omaha Beach, with 56 tanks in each of the 741st and 743rd Tank Battalions. Each of these battalions had 32 DD Shermans and 24 other Shermans (including many Sherman bulldozers for clearing obstacles). Starting at about 0540, the 741st Tank Battalion put 29 DD Shermans into the sea, but 27 of these sank and only two made 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 as far out. The remaining tanks of the 741st Tank Battalion and all tanks of the 743rd Tank Battalion (except for 4 tanks aboard one LCT that was hit by artillery fire just off the beach) were landed directly on the beach, starting at about 0640.

The DD Tanks were designed to withstand waves up to 1 foot (0.3 m) high; however, on that day the waves were up to 6 feet (2 m) high. These were much worse conditions than the tanks had been tested in and thus they were swamped with water. Also, the tanks of 741st Tank Battalion were released into the sea too far out,^[3] about 3 miles (5 km) offshore. Considering the inherent difficulty in steering a 35 ton modified tank, it is a tribute to the crews that they got as far as they did. The crews were equipped with emergency breathing apparatus capable of lasting 5 minutes, and the tanks were also equipped with inflatable rafts.^[4]. Although some sources claim that these life-saving measures were ineffective,^[5] this view is contradicted by the testimony of survivors.^[6] In fact, most of the crews were rescued, mainly by the landing craft of the 16th RCT (Regimental Combat Team)^[7] although five crewmen are known to have died during the sinkings.^[8]

Until very recently it was believed, despite testimony of survivors to the contrary,^[9] that most the DD Shermans of 741st Tank Battalion were sunk immediately, swamped by the seas that were much higher than the operators had practiced with. However, tanks at the other four beaches suffered no such problems. New research suggests that the Omaha tanks were aiming for a church steeple on the visible horizon behind the cliffs. In order to maintain their line of sight it is believed that the tanks had to turn progressively away from the shore to combat the wavefronts pushing them down the beach, putting their sides virtually parallel with the waves/beach. This meant that the protective canvas flotation devices were easily swamped by the waves. If they had kept going directly forward with the front of the tank headed straight for the beach, they may have reached it.

Others believe that the error was on the part of the commanders aboard the ships from which the tanks were launched. They simply gave the order to launch too early, possibly to avoid getting too close to the battle themselves.

The Operation Dragoon landings took place on 15 August 1944. The invasion took place between Toulon and Cannes.

A total of 36 DD-tanks were used by three American tank battalions, the 191st, the 753rd and the 756th. The 756th battalion had eight tanks that were launched 2500 yards (2300 metres) from the beaches; one was swamped by the bow-wave of a landing craft and one sank after striking an underwater obstacle. The twelve tanks of the 191st battalion were all landed on or close to the beach. The 753rd battalion's had 16 tanks, of which eight were lauched at sea and successfully reached the shore and eight were landed directly on the beach later in the day

Deep Wading Gear on a Sherman on Tinian in the Pacific in 1944.

Although Duplex Drive allowed the landing craft to release the tank farther from shore, the alternative deep wading gear allowed a tank to drive partially or wholly underwater on the sea floor rather than swim. Deep wading Churchills took part in the 1942 Dieppe raid^[10] and deep wading tanks operated on D-Day also. Allied tanks were given waterproofed hulls and air intake and exhaust trunking to allow them to come ashore from shallow water. Tall ducts extended from the engine deck to above the turret top and these needed to stay above water. The front duct was the air intake for the engine and the rear duct vented the exhaust. This device saw use in many amphibious invasions, and was used on light tanks and tank destroyers as well. The US had similar devices for trucks and jeeps.^[11]

During the planning of Operation Sealion the Germans also developed tanks with the same purpose as the Sherman DD; to provide armoured protection to infantry during an amphibious assault. The Tauchpanzer III was a modified version of the Panzer III and, like the Sherman DD, was dropped from a landing craft around a mile off-shore. However, instead of floating the Tauchpanzer III 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 15 metres (50 feet.) making it an extreme example of a wading tank.

Many modern vehicles use similar devices (See Snorkel).

Front portion of Surviving DD tank in the Bovington Tank Museum including canvas flotation screen, Dorset

The Bovington Tank Museum in England has a complete DD Sherman with its canvas flotation screens still intact. A DD Valentine is still in running condition in private ownership in Wolverhampton, England.

Three of the DD Shermans lost on D-Day were salvaged in the 1970s. One is 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. Another is displayed at the Juno Beach Centre, a museum dedicated to the Canadian contribution to D-Day, near Courseulles-sur-Mer.

In 2000, there was an unsuccessful attempt by the US Navy to raise a sunken DD Sherman, located near Salerno in Italy. It was eventually recovered successfully on 18 May 2002. It has been restored and is on display in the Piana delle Orme museum near Latina, Italy.

A Sherman tank that was lost off the coast of Devon, UK was recovered in the 1980s, largely due to the efforts of a beachcomber called 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. Images on the shermantank.co.uk website^[12] imply that this was a DD tank, but images of the actual vehicle on the same website show it to be a standard model with no DD modifications. According to Charles B. MacDonald,^[13] the tank in question was lost in 1943; that is, before the DD Sherman was developed.

Wikimedia Commons has media related to:

Category:DD tank

• T-37 tank - pre-World War 2, Soviet amphibious tank.
• T-38 tank - World War 2, Soviet amphibious tank.
• Type 2 Ka-Mi - World War 2, Japanese amphibious tank.
• Type 3 Ka-Chi - World War 2, Japanese amphibious tank.
• Allied Technological Cooperation During WW2

• http://www.sproe.com/t/tank-dd.html 14 April, 2005
• BBC online news 'The tanks that didn't land on D-Day' 15 April, 2005 A new theory on the sinking of the floating Sherman tanks on D-Day
• http://www.ospreypublishing.com/content1.php/cid=98
• Juno Beach - Fort Garry Horse DD Tanks on D-Day
• Juno Beach - 1st Hussars DD Tanks on D-Day
• The First Hours of D-Day on Omaha Beach. Excerpted from Omaha Beachhead (6 June-13 June 1944) American Forces in Action Series. Historical Division. War Department (Washington, D.C.: Center of Military History, Facsimile Reprint, 1984), pp. 35-87. Available on-line at the Sam Houston State University website; also at the the official US Army website (www.army.mil).
• Vaughan, Don. Neptune’s Treasures: A survey of ships and other craft lost during Operation Neptune. Available on-line at the Military Officers Association of America (MOAA) website.