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M4 Sherman Duplex Drive Tank (DD)

M4 Sherman tank with Duplex Drive (DD) equipment installed, in a test launch from an LST on the English coast, circa March 1944
M4 Sherman tank with Duplex Drive (DD) equipment installed, in a test launch from an LST on the English coast, circa March 1944..

The M4 Sherman tank was the most widely used US tank of the Second World War. The Duplex Drive (DD) version was an adaptation for launching the tank in water during beach assaults or for crossing inland water obstacles. The British engineer Nicholas Straussler developed the DD tank concept in early 1941. He tested an early version on a British Tetrarch light tank in June 1941 and later with a heavier Valentine tank in early 1942. In April 1943, the equipment was adapted to fit the standard Allied medium tank, the US-built M4 Sherman. The M4 Sherman with the DD system was used by US, British, Canadian and other Allied forces.

Rear view of M4 Sherman with Duplex Drive (DD) kit, canvas screen fully deployed and propellers visible.
Rear view of M4 Sherman with Duplex Drive (DD) kit, canvas screen fully deployed and propellers visible.

The Duplex-Drive system consisted of a collapsible canvas screen that was fixed to a boat-shaped platform of mild steel, in turn welded to the hull of the tank at the sponson line. The screen was supported by horizontal metal hoops, erected by thirty-six rubber tubes filled from compressed air cylinders carried on the deck of the tank, held in place by lightweight jointed struts. When in the water, a properly rigged screen supported the tank in the water with the turret just above the water surface level. The device provided three feet of freeboard (just under one meter), with the screen extending 26 inches above the highest part of the turret. An evident drawback is that the screen prevented use of the tank's gun.

The tank hull was also waterproofed to the curtain base line and a bilge pump was installed to pump out water that may leak or splash into the tank. It took about 15 minutes to erect the screen and secure it properly; it could be collapsed in minutes to begin combat upon reaching shore.

Propellers of an M4 Sherman with Duplex Drive (DD), circa March 1944
Propellers of an M4 Sherman with Duplex Drive (DD), circa March 1944.

The DD tank was driven through the water at four miles-per-hour (6.4 kph) by two propellers that drew power through a simple transfer box from the tracks. Steering was via a hydraulic system that swiveled the propellers and through an auxiliary rudder controlled by the tank commander through a manual tiller. The tank commander had visibility over the screens by standing on the turret, while the driver was restricted to a periscope extension added to the normal periscope mounted on his hatch. For combat operations, the curtain is lowered by the use of a driver-operated system of hydraulic valves.

The advantage of the DD-system was that it could be adapted to most tanks with no need to design, prototype, test, and manufacture a purpose-designed amphibious tank. However, there were also many disadvantages to the system, the worst of which was its extreme vulnerability. Puncturing just a few of the air-tubes or even just a simple tear in the canvas skirt, would result in the DD tank taking on water. Given the tenuous buoyancy of the DD tank, a minor loss of flotation was sufficient to sink it. Testing of the DD tanks prior to D-Day proved that the fragile contraption would sink like a stone in anything rougher than a moderate sea. Waves greater than three feet would break over the skirt and easily swamp the DD. Furthermore, the boxy DD had a tendency to turn its side to the current putting its weakest canvas screens against the force of the water; broken ribs would then collapse the canvas and the DD was on its way to the bottom. The final recommendation from planning officers was that the DD not be launched in greater than 3 foot seas (1m), and no more than 4,000 yards/meters from the beach.

When introduced to US troops in training in England in the Spring of 1944, the Sherman DD was instantly unpopular. Called "Donald Ducks" they were not trusted to be seaworthy even in smooth waters, let alone choppy seas. Early reports from the training urged caution in their use, limiting use to calmer waters and launch-distance within 3,000 yards from shore. Higher echelons did not heed the recommendations.

The Results of the Employment of DD Tanks on D-Day

Testing an M4 Sherman with Duplex Drive (DD) for use on D-Day, 6 June 1944
Testing an M4 Sherman with Duplex Drive (DD) for use on D-Day, 6 June 1944.

Severe weather conditions in the English Channel caused a postponement of the Operation Overlord D-Day invasion from 5 to 6 June 1944. On D-Day, four to eight foot swells made problems for the smaller landing craft, swamping and sinking a number. The effect on the DD tanks was worse.

The 741st Tank Battalion at OMAHA Beach launched as scheduled 4,000 yards offshore. Some of the DDs sank immediately as they exited their landing craft, others advanced a few yards and then foundered. Of the 32 that were launched, 27 sank before reaching shore. Further west at OMAHA the 743rd Tank Battalion made the sensible decision to scrap the planned deployment of the DD tanks and launched them conventionally. Nevertheless, they lost eight tanks within minutes, four when a sea mine sank the LCT carrying them and four to German antitank guns.

At UTAH Beach, the 70th Tank Battalion lost five DD tanks of the 32 launched; unconfirmed reports say the cause was swamping. Finally, it is known that at least six of 18 DD tanks of the Canadian 1st Hussars were lost to swamping at Juno Beach. Other medium and light tanks, in addition to the DD tanks and conventionally waterproofed medium tanks of the 70th, 741st and 743rd Tank Battalions, were landed on the Normandy Beaches on D-Day. At Omaha Beach, congestion prevented the landing of D Company (the light tanks) of the 741st and 743rd Battalions until 7 June. However, the 745th Tank Battalion landed its battalion commander, reconnaissance platoon, and B Company (medium tanks) on 6 June. At UTAH Beach, D Company of the 70th Tank Battalion landed on the afternoon of 6 June and was attached to the 101st Airborne Division, which it joined on 7 June. Also the 746th Tank Battalion landed all of its medium and light tanks on D-Day. By the end of the day, it appears that at least 103 medium and 34 light tanks were ashore at UTAH Beach. And, despite the dreadful losses suffered by some of the DD tank units, about 90 medium tanks were ashore at OMAHA Beach.

Thus, on the two US beaches, 193 medium and 34 light tanks were ashore by the evening of 6 June. A total of at least 40 medium tanks had been lost, or 17.17 percent of the total of 233 mediums committed on that day. These also accounted for 21.39 percent of the 187 medium tanks lost by the US First Army from 6 June to 1 July 1944. Put in another way, the daily average of medium tanks lost for the 26-day period was 0.94 percent of those operational, the loss on D-Day was 18.27 times the daily average. During the same time period, 10.84 percent of the operational First Army light tanks were lost 0.42 percent per day although apparently none were lost on D-Day.

The results obtained by the use of the DD tanks on D-Day do not appear to have justified the extensive research, development, and training that went into deploying them. The DD tanks did not provide the needed fire support that was required by the first landing waves, and their fragile construction resulted in the needless loss of well-trained tank crews. In practice it was found that landing tanks directly on the beach from specialized landing craft was probably a more effective and safer means of entry than was the use of an amphibious tank. There appears little doubt that a lighter and less well protected vehicle would have been virtually useless in the circumstances that pertained on the Normandy beaches (and especially at OMAHA Beach). The fortified defense positions were such that only direct infantry assault or direct fire from a large-caliber weapon could have had any effect upon them. Furthermore, the weapons available to the defenders were fully capable of knocking out any lightly armored vehicle committed against them.

Just why it was felt that the DD tanks were necessary for the D-Day plan has never been adequately explained. By keeping the Sherman DD secret prior to D-Day, it may have been assumed that the DDs were such an unknown and inoffensive appearing object (looking much like a life raft) that the Germans would not fire upon them, giving the DDs the advantage of tactical surprise. With hindsight, that was clearly a poor assumption.

Employment of Sherman DD Tanks in Other Landings

M4A1 Sherman Tank with Duplex Drive (DD) amphibious adaptation. This tank is from the 756th Tank Bn., supporting 1st Infantry Regiment troops at Alpha Yellow Beach (Pampelonne), Operation Dragoon, Southern France, 15 August 1944
M4A1 Sherman Tank with Duplex Drive (DD) amphibious adaptation. This tank is from the 756th Tank Bn., supporting 1st Infantry Regiment troops at Alpha Yellow Beach (Pampelonne), Operation Dragoon, Southern France, 15 August 1944.

Notwithstanding problems with the M4 Sherman Duplex Drive at Normandy, the system was used in other landings. The US Army successfully used a small number of DD tanks during the landings in southern France (Operation Dragoon), and in some river crossing operations in cluding the Rhine on 24 March 1945. Sherman DD tanks were used at Salerno, the Po River and in several other operations on the Italian front after June 1944.

Deep Wading M4 Sherman Tanks

Marine Corps M4 Sherman landing at White Beach, Tinian, 24 July 1944.  Vertical vents were a Deep Wading kit so the air intake and exhaust could rise above water level when the tank was off-loaded far from the beach
Marine Corps M4 Sherman landing at White Beach, Tinian, 24 July 1944. Vertical vents were a Deep Wading kit so the air intake and exhaust could rise above water level when the tank was off-loaded far from the beach.

In addition to the M4 Sherman Duplex Drive (DD) tanks, several other systems for amphibious operation were developed during WW II. The most successful was the Deep Wading kit that adapted the M4 Sherman for operation in water up to turret depth but where the tank treads would rest on the bottom allowing the tank to be driven to the beach. For engine air, steel trunks were provided, rising above the rear deck, for both the intake air (forward) and exhaust air (back). A sealed hull and other modification completed the design. The Deep Wading kit could only be used when the water depth was well understood and the bottom was rather even. Nonetheless, deep waders were proven at Dieppe, Sicily, Salerno, Anzio, and in the Pacific. Compared to the Duplex Drive, landing tanks with Deep Wading gear directly from the LCTs was a much more reliable and timely method of delivering tanks in an amphibious assault. It had the important advantage of allowing the tank to join the fight immediately since the Deep Wading Kit did not block the main gun.

Material on this page adapted from "The Historical Combat Effectiveness of Lighter-Weight Armored Forces", 6 August 2001, Center for Army Analysis, Ft. Belvoir, VA, Contract Number DASW01-98-D-0058, Task Order 5, and other sources.

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