DD Tanks for D Day – Operation Overlord

Sherman Tank with Duplex Drive preparing for D Day.
| June 6, 2019
Categories: Learnin'

DD Tank” does not stand for DDay Tank. Those two letter Ds are the acronym for Duplex Drive. They were the “amphibious” Sherman tanks intended to provide armored support to the Operation Overlord landings on all five beaches. Their use was a bitterly debated part of the invasion plans, for many reasons. Not least among them was the entire concept of the Normandy operation, which ran counter to amphibious warfare doctrine developed with years of bloody experience in the Pacific theater. The impact of the “swim tank” on the Allied attempt to secure a lodgement varied wildly from beachhead to beachhead. Read on to learn more. Read on.



Swim Tanks: Floating Armor

The Dubious Deployment of Duplex-Drive Tanks at Normandy

The necessities of war often bring about ingenious innovations out of sheer need. Some work out; some don’t. One of those innovations in the Second World War was the Duplex-Drive (DD) Tank. The DD Tank rose from the perceived need for an amphibious tank to swim ashore to support the initial waves of an amphibious landing.

The idea wasn’t anything new. The US military had tested an amphibious tank as early as 1924 when work was just beginning on what would become American amphibious doctrine as practiced in WWII. It quickly became clear that troops needed direct fire support against enemy emplacements as they landed on a hostile beach. Calls for a tank capable of swimming ashore with the troops continued throughout the 1930s but funding was never available, and no workable designs came to the fore.

US forces in the Pacific overcame the lack of a true amphibious tank by arming amphibious tractors (LVT or AMTRAC) which ferried many troops ashore. Depending on the configuration, the LVTs carried .30 and .50 caliber machine guns, 37mm cannons, and 75mm howitzers in turrets located forward of the cargo area. M4 Sherman tanks were brought ashore in dedicated tank landing craft called LCTs (Landing Craft, Tank) and were fitted with special extended exhaust ports and waterproofing so they could wade through shallow water (see my article, Island Hopping With Tanks).

Backing up the LVTs and Shermans were the guns of the US Navy, which not only saturated the target area with indirect bombardment, but with accurate direct fire using armor-piercing shells from battleships, cruisers, and destroyers. The Navy had worked out their basic fire support doctrine in the 1930s and perfected it by late 1943 in the landings in the Marshall Islands. By the time of the assaults on Kwajalein and Eniwetok, the Navy/Marine Corps team, and Army units following Navy/Marine doctrine had become an amphibious warfare machine. They needed no stinking tanks that could swim.

But LVTs were not available in Europe for the Normandy landings. Allied planners knew they needed something to fill the direct fire role, plus they felt that the presence of tanks on the beach would boost troop morale while hurting that of the defending Germans. That may have been true, considering that two of the three US divisions (the 4th and 29th Infantry Divisions) in the first wave would be receiving their baptism of fire at Normandy. Only the veteran 1st ID had seen combat before. Whether the defenders would have been affected is questionable at best. The German 352nd ID at OMAHA Beach was a top-quality unit with extensive combat experience. The US 4th ID landed against comparatively light opposition at UTAH Beach.

The DD Tank

There is no denying the DD Tank concept was an innovative, if flawed and misused, response to the need for direct fire support. The technical flaws are not difficult to explain. Simply put, the DD Tanks were M4 Shermans with attached propellers and a waterproof canvas curtain that allowed them to float. The tank commander steered by a gyroscopic direction-finder and a periscope. The curtain could be lowered in just a few seconds once the treads hit the sand, clearing the gun for combat. That’s basically it.

Duplex Drive Sherman Tank

The tanks could float, though they were far from stable, and they could make about 4 knots in calm seas, and therein lies the first problem. In testing off the coast of England, the tanks were blessed with calm water, and they performed as promised. But the English Channel on the morning of 6 June 1944 was anything but calm. The previous few days had been stormy, causing General Eisenhower to postpone the landings once, from 5 June to 6 June. Allied meteorologists detected a short break in the weather for 6 June, and Ike rolled the dice rather than postpone again. But it was windy and overcast, with waves in the three to four-foot range with some as high as to six feet. The DD Tank’s curtain was not designed for that kind of a beating, and the Army knew it. The DD Tanks’ training exercises were always canceled in bad weather or anything other than optimum conditions.

The second problem was with the engine exhaust. Testing had shown that there was a real danger of carbon monoxide poisoning to the crew since the gas from the exhaust had trouble rising out of the high curtain. The maximum range for the tanks to operate before posing a dire asphyxiation risk to the crew was 4,000 yards (about 2.3 miles). The Army and Navy were both aware of the limitations of the DD Tank. Officers from both services recommended that DD Tank units be guided by experienced navigators and that the decision to launch be made by an Army officer familiar with the capabilities and limitations of the tanks and their crews. The Army and Navy command structure ignored these recommendations.

The decision to launch was made by the DD unit commanders and the support flotilla commanders themselves, many of whom felt they should press ahead with their duty as they saw it. But due to the fear of German artillery, the LCTs were supposed to approach no closer than 6,000 yards (about 3.5 miles) off OMAHA. Because of the tanks’ slow speed in the water, they had to be launched ahead of the infantry so they could land at the same time. The infantry had to ride 11 miles in their landing craft in rough seas. So, the commanders had to decide. Many chose to go, likely with more than a few misgivings.

OMAHA Beach – the worst waters

Most of the DD Tanks designated for the eastern half of OMAHA Beach, in support of the 16th Regimental Combat Team of the 1st ID, sank. Many of their curtains collapsed as soon as they hit the water, causing them to sink immediately. Others went down more slowly, but still within sight of their LCT. Some managed to stay afloat and struck out for the beach. Several tanks launched even though they could clearly see the vehicles in front of them disappearing beneath the waves of the English Channel. We can only speculate that they felt it was their duty to do so, or perhaps they thought they could make it. Of the 32 tanks of Companies C and D of the 741st Tank Battalion, 26 sank. Of the remaining six, only two made it ashore under their own power. The rest were landed directly onto the beach by their LCT, whose ramp had malfunctioned, preventing it from launching at sea. Most of the tanks sank due to being swamped in the heavy seas, though it is possible that exhaust fumes overcame some of the crew during the trip to the beach.

The 743rd Tank Battalion, assigned to the western half of OMAHA in support of the 116th RCT of the 29th ID, fared better thanks to the support flotilla commander’s quick recognition of what was happening when the first couple of tanks from the 741st sank. It’s no coincidence that this man, Lieutenant Dean L. Rockwell, had been the officer who recommended caution to the Navy on the deployment of the DD Tank. He and the battalion commander quickly agreed to take the tanks to the beach on the LCTs.
The DD Tanks met with mixed results elsewhere. The conditions at UTAH were less severe than at OMAHA, and more tanks made it ashore and performed well. It likely helped that the DDs were launched only 1,000 yards (only about .6 miles) off UTAH. There were similar results on the British beaches. Some were lost, but not on the scale as at OMAHA. Some historians have claimed it was the employment by the American crews that was at fault. That is possible, considering the launch distance, but the British DDs off SWORD Beach were launched at 5,000 yards (2.9 miles) and 31 of the 34 made it ashore. Most likely, it was the conditions themselves combined with unproven technology, as the seas and currents off OMAHA were more severe than the other beaches. Their presence on the other beaches no doubt contributed to the success of the landings.

DD Sherman launching from an LCT

DD (Duplex Drive) Sherman tank launching from an LCT.

V Corps commander, General Leonard T. Gerow, wasn’t a fan of the DD Tank and argued against its use but was overruled by First Army commander General Omar Bradley and Eisenhower, who both liked the concept. After the war, Gerow admitted that more DD Tanks made it ashore than he would have ever believed. But their absence on Omaha was keenly felt.

The poor performance of the DD Tank at OMAHA was partially the result of an incoherent amphibious doctrine in the European Theater. As an amphibious warfare historian, I believe there should never have been a perceived need for something like the DD Tank. I don’t want to get too deep in the weeds, but I’ll explain what I mean. By 1944, US forces in the Pacific had honed their amphibious doctrine to a fine edge. The Navy/Marine/Army team was so good at getting ashore that the Japanese stopped bothering to defend the beaches after the Marianas fell. The ship-to-shore movement, logistical procedures, and fire support techniques were all but perfected. Only minor adjustments had to be applied to meet the needs of individual operations. Of course, the Navy and Marines had been working on that stuff since 1921. The Army units in the Central Pacific fell under Navy control in terms of carrying out the landings, so they benefited from that body of knowledge even though the Army had never put much effort into amphibious know-how.

ETO vs PTO Doctrine

Pacific Theater lessons vs European Theater misconceptions

The situation in Europe was just the opposite. Even though the US Navy possessed the best amphibious doctrine in the world, the Army called the shots. And the Army thought they knew better, despite having the services of Vice Admiral Kent Hewitt, one of the Navy’s top two practitioners of amphibious warfare (the other being Admiral Kelly Turner in the Pacific).

To be fair, the Army in Europe had a different mission than the Marine and Army units in the Central Pacific. The latter were specialized assault troops, especially the Marines. Getting ashore against heavily defended beaches was what they did, and brother, by mid-1944 they did it damned well. But the Army in Europe could not afford to have a specialized force like that. Their infantry units’ primary purpose was to prosecute a continental land campaign against the German Army. The divisions that landed in the first waves at Normandy received amphibious training, but they had other things to worry about too. The problem wasn’t in the troops themselves, but in the support they received.

Navy/Marine Corps doctrine employed massive amounts of naval gunfire in the form of preparatory bombardments and direct fire support against enemy emplacements. The Navy’s big guns would sometimes pound the Japanese for a week before the infantry set out for the beaches. They had dedicated fire support ships which were stocked with high explosive shells for antipersonnel missions and armor piercing rounds to use against enemy bunkers and pillboxes. HE rounds were also fired on the beach itself to explode enemy mines and destroy obstacles and wire.

Navy and Marine pilots were also highly skilled in the art of close air support. Marine close air support pilots had to be qualified infantrymen before they could fill that role. The Navy and Marines placed qualified pilots with the ground troops as spotters to talk to the pilots. Aircraft pounded the Japanese for days, or even weeks, before the landings and then operated with the infantry on the ground. One of the key elements of the Pacific doctrine was the last-minute airstrike against the beaches just before the first wave of the landing. The beaches were plastered for several minutes to keep the enemy’s head down right as the landing craft were coming in. The whole process was devastating. That’s what prompted the Japanese on Iwo Jima and Okinawa to fortify further back from the beach and only contest with artillery. They knew they couldn’t stop the infantry from getting ashore, and if the infantry could get ashore, so could the armor. All this was done in daylight to maximize the effectiveness of naval and air support.

Things were different in Europe. Even though naval gunfire had performed admirably in the North African, Sicily, and Salerno landings, the Army still believed it was not suitable to support daylight landings. Which was one reason the Army decided to land at night in all three of those operations. The other main reason was to achieve tactical surprise, which they did in North Africa, about halfway in Sicily, and not at all at Salerno. It had been the Navy’s big guns that were mostly responsible for breaking up the attack of the Hermann Gӧring Panzer Division against the Sicilian beachhead and that of the 16th and 29th Panzer Divisions at Salerno. The situation at Salerno was especially dire and could very well have pushed US forces off the beaches.



Navy air support was not available in Europe either. Pilots from the carrier Ranger provided outstanding support for Patton’s landing in Morocco and ol’ George begged for their support at Sicily, but it was not forthcoming thanks to the assertion that land-based Army aircraft had it covered. The problem was that Army pilots had little to no training in close air support. Again, the air forces in Europe had different priorities, like air superiority and bomber escort missions. But the Navy found it possible in the Pacific to execute air superiority and escort missions while also providing close air support. It was more a matter of the Army placing a very low priority on such missions than anything else.

Hewitt tried to bring Eisenhower and Bradley around to the Navy way of thinking, but they were having none of it. They even blew off Major General Pete Corlett, who was one of the Army’s top amphibious guys in the Pacific. Corlett had commanded the 7th ID at Kwajalein and had been sent to advise Eisenhower and Bradley for OVERLORD. Corlett was shocked that neither man wanted to hear what he had to say. They told him that Pacific operations were “bush league” and that European ops represented “graduate-level” amphibious warfare. Army command believed that the Normandy operation was unique in history and required unique solutions. It really wasn’t unique except in terms of complexity, but the basic amphibious doctrine was nothing new. They just made it harder than it had to be. A classic case of not seeing the forest for the trees. The command disregarded Corlett’s experience but gave him command of XIX Corps, which wasn’t activated until a week after the landings.

A British Sherman tank (DD) Royal Hussars pushing in from the beach.

A British Sherman tank (DD) Royal Hussars pushing in from the beach.

D Day

The result of all this was that the most important Allied (American and British) operation of the war was carried out with less than half of the naval gunfire support used in attacks against a single island in the Central Pacific and the only air support was a pre-landing strike by the B-17s of the Eighth Airforce. The DD Tanks were considered essential because landing them in LCTs would take away from the even dispersion of armored assets and risked their being destroyed or damaged in bunches if their LCT was targeted as it landed. They were also deemed essential because the Army had no faith in naval gunfire. Planners attempted to provide more fire support in the way of landing craft armed with guns and rockets. They could be effective in optimal conditions, but the chop off OMAHA Beach made it nearly impossible to aim the rockets, which were supposed to detonate the minefields. They failed to do so.

The story of OMAHA Beach has been told over and over. I won’t repeat it here. Suffice to say that the big airstrike by the B-17s didn’t go so well. The airmen insisted that the bombers come in perpendicular to the beach instead of parallel, for which Navy doctrine called. The perpendicular avenue of approach meant the release had to be absolutely perfect for the bombs to hit the target. Wellll…it wasn’t. The bombers flew at high altitude, and the overcast was at 2,000 feet. The pilots feared dropping the bombs short and hitting friendlies, so they overcompensated and dropped them well behind the beach. This was a bit of a problem since those bombs were supposed to help blow up the mines on the beach and help tear up some obstacles as well as demolish German emplacements. So that was a no-go. We’ve already seen how the DD Tanks were also a no-go.

Finally, the plan only allowed for a 25-minute preparatory bombardment to soften enemy defenses while Navy UDT teams and Army Engineers had the same amount of time to clear sixteen lanes through the obstacles for the landing craft. Here is where the supposedly “unique” character of Normandy reared its head. Landing in darkness on a falling tide was not ideal, but it had been done, and it’s almost certain the troops would have landed much more easily. There would have been little support from the Navy until the sun came up, but they wouldn’t have had to cross 400 yards of open beach in full view of the bluffs overlooking OMAHA Beach after landing in daylight at low tide. Landing in darkness would have reduced the demands on the demolition teams since the dangers of bunching up the landing force under fire would not have been as acute. Landing at night was a viable option.

DD tank with lowered curtain

“Swim tank”? A DD tank with lowered curtain. Most Duplex Drive tanks were Shermans, but there were British Valentine versions as well.

Another option would have been to land in daylight on a rising tide with the full force of the Navy behind the landings via a longer preparatory bombardment and, once again, more time for the demolition teams. Planners felt that a longer bombardment would sacrifice tactical surprise, giving the Germans time to react. This reason rings hollow given the damage done by the air force to the French transportation system in the previous months, as well as the fact that Allied minesweepers had been operating in sight of the beaches the previous evening. Not to mention the 23,000 or so paratroopers who had been sowing confusion behind the beaches all night.
So, instead of going with what had worked, albeit imperfectly, in the Mediterranean, or what was even then working in the Pacific, Allied planners went for the in-between. The result was stop-gap measures like the DD Tank to make up for the shortened fire support from the Navy. As it turned out, the Navy and the resilience of the US infantryman had to carry the day at OMAHA. US Navy destroyers closed to 500-800 yard to pour direct fire on German emplacements. Several units on the beach actually directed destroyer fire with hand signals, and a couple made radio contact. The battleship Texas hammered the Vierville exit with near-direct fire from its 12-inch guns, proving instrumental in opening a primary path off the beach.

Infantrymen for the 1st and 29th IDs began infiltrating between the German strong points at the beach exits when the plan had been to attack from the front with armored support. The strong points were eventually taken from the rear as US soldiers fanned out on the bluffs. Even so, it was a near-run thing. It was avoidable. The DD Tanks, as a concept, played no vital role, as they were too few in number and had too far to go through still-intact obstacles under enemy fire. They would have been more effective had they been landed from their LCTs under a proper naval fire support plan, as in the Pacific, or even landed at night.

In a perfect world, Marines would have been available to assault the beaches supported by the Navy, with Army maneuver units set to pass through and expand the beachhead. But those Navy and Marine assets were a little busy on the other side of the globe. The Army had to go with what they had. But that doesn’t mean they necessarily did it right. In the end, it was the men themselves who improvised, adapted, and overcame (sorry, I couldn’t resist that one, but it’s true). As we already know, the credit for eventual success on OMAHA belongs to them.

Duplex Drive Sherman tank with curtain raised

Duplex Drive Sherman tank with raised curtain. The DD.Tank might not have been the strangest of all WW2 tanks, but if not it’s certainly a contender.

Surprisingly, most of the 135 men who took a swim in the Channel when their tanks went down survived. The Army had equipped them with life vests and rafts, as well as an adaptation of what was called the Davis Lung. Thirty-three men drowned. Twenty-five were recovered over the next couple of days. An effort was mounted immediately after the war to recover the other eight bodies, but none were found. Another expedition was sent in 1987 after divers reported seeing remains in the hulk of a DD Tank on the bottom. Nothing was found. No further efforts have been forthcoming, and the sunken tanks remain in their watery graves.

Gordon Harrison, Cross-Channel Attack
Adrian R. Lewis, Omaha Beach, A Flawed Victory
Williamson Murray and Allan R. Millett, A War to Be Won, Fighting the Second World War
William F. Lawson, The Army Way; American Joint Operations in Europe in World War II, A
Doctrinal Analysis Master’s Thesis, American Military University



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