Duplex Drive Shermans
Due to the limited nature of available reference sources, some of the information presented here must be considered as "educated guesswork".


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The Sherman based Duplex Drive swimming tank was equipped with a canvas floatation screen which was supported by a framework consisting of inflatable air pillars and metal struts. The screen was pneumatically controlled and could be raised in a matter of minutes and dropped in a matter of seconds. Propulsion was provided by twin propellers driven by the tank's rear idlers. It was thought that the highly classified DDs, dispersed in the water, would appear to be "oversized row boats" to the enemy, whose shore batteries might ignore them in favor of larger targets.


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The Duplex Drive concept was conceived in 1941 by the British firm of Messrs. Nicholas Straussler & Co. Tests were initially conducted in Summer 1941 using a Tetrarch Light Tank. A year later, the design was adapted to the more suitable Valentine Tank (above left). Successful trials led to a production contract, and the first 20 conversions were completed in March 1943. However, by mid 1943, the Valentine had come to be considered obsolete. Nonetheless, Valentine conversions continued well into 1944, even after it was decided to shift DD production to the Sherman. It is somewhat surprising that British production of Sherman based DDs, actually started later than US production. As the table above (courtesy of Peter Brown) shows, the first 45 units were not completed until March 1944.


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293 of the British conversions were based on the Sherman III (M4A2), and 400 on the Sherman V (M4A4). The US converted M4A1(75)s exclusively. Above, this Sherman III is the only surviving DD with its original floatation screen intact. It is preserved at the Tank Museum at Bovington. Photo courtesy of "Megashorts".


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A British Sherman V (M4A4) DD with its flotation screen lowered. The Sherman V was a foot longer than the Sherman III, and as a DD afloat, provided about a foot more freeboard. It was stated that the DD could swim 1000 yards in 7 minutes at an average speed of 4 1/2 MPH. Note that the tank's headlamps (items 1) were extended up about 14 inches in order for the light beams to clear the screen when it was in the lowered position. The navigation lights (items 2) were for training only and NOT to be used in combat conditions.


The US Duplex Drive conversions



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In the early stages of planning for D-Day, the US Army envisioned the use of wading tanks (above), which had already proven their worth in several amphibious operations. The British demonstrated the DD to US planners in November 1943. Soon after, it was decided that the US would also employ DDs for the invasion, scheduled at that time for May 1944. Since British production could not meet US requirements, the initial plan called for the manufacture of conversion kits in the States, which would then be applied to US Army Shermans already in the UK. However, the complex nature of the design quickly led to the conclusion that it would be preferable to do the complete conversions in the US.


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In January 1944, the most technologically advanced tank in the US arsenal was the M4A1(76), which was just beginning to roll off the assembly lines. These were wanted for the DD project until it was found that the first 160 were "already earmarked for another project". (They were being sent to the UK for troop familiarization). The Stock Control Branch advised the Army that 350 unused M4A1(75)s could be made available, and a Production Order (above left) was placed. The project was classified as "Secret", and therefore much of the documentation was not preserved. The "now it can be told" clipping above was from the September 22 1945 edition of "Bombshell", the newsletter of the Cleveland Ordnance District.


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During the course of work on the various US swimming tank experiments, one officer observed that the rounded contours of the cast hull Sherman gave it better "hydrodynamic properties". Perhaps this played into the US decision to convert only M4A1s? The tanks that were provided were unallocated, late 1943 models. Many were of December Pressed Steel Car production, and had such late features as "large" drivers' hatches and high bustle turrets. Two such examples were salvaged from the sea, and are on display in Torcross, England (above left, courtesy Alf Adams) and Port-en-Bessin, France (above right).


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The use of the M4A1(75) required the relocation of the air cleaners to the inside of the engine compartment. In their original positions, they interfered with the application of the DD frame, as well as the proper operation of the propellers. This change required 50 to 75 man hours, and was one of the reasons it was decided to do the conversions in the US. The right side photo shows a Duplex Drive with the propellers dismounted.


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The conversion program commenced in January 1944. Due to the secret nature of the project, assembly areas were cordoned off, and the finished units were shipped completely encased in plywood boxes. Also starting in January, Major William Duncan of the 743rd, and most members of the DD Tank Battalions received a month's training on Valentine DDs at the British Assault Training and Development Centre. Duncan was appointed Commandant of the US DD Tank School (Camp MacDevon) which was located at the English resort town of Torcross. Training began on March 15 with the arrival of the first 4 M4A1 DDs. Remarkably, 348 were delivered to the UK by the end of April. (The two pilot models, Serial Numbers 37844 and 37851, were ordered placed in secure storage at Lima Tank Depot "for possible future development work.") The above shows several of the highly classified units at the DD School, "hiding" in a wooded area, with their fragile canvas screens perilously close to the trees.

The US Army had determined that two companies of the 70th, 741st and 743rd Tank Battalions would use DDs on D-Day. Each company would be equipped with 16 units. This required 96 tanks for training, and, because of the fragility of the design, a further 96 unused DDs for the actual invasion. 80 of the US DDs were provided to the British, since their conversion program had fallen behind schedule. This left 76 M4A1(75) DDs in the UK "with no operational need at this time".


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Major Duncan sent out several progress reports in the Spring of 1944. On April 30th he wrote, "The craft can be navigated by periscope with all the crew buttoned up and can land, and deflate with all crew members under armor." To enable vision over the raised screen, extended periscopes of 93 and 63 inches were provided for the driver and tank commander respectively. The driver's periscope was fixed forward, while the commanders' could be rotated. The authors could not find a single photo showing a buttoned up Sherman DD underway, or with both extended periscopes in use. Above are a pair of images from a British Report that show the driver's periscope fully extended.


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Crews were instructed to "Remove and repack periscope extensions" as soon as possible after landing. The US DD on display at Piana Dell Orme, Italy and the large hatch DD at Port-en-Bessin appear to have the section of the extended periscope shown in pink (above right), attached to the driver's auxiliary periscope housing.


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Crew accounts provide anecdotal evidence that the tanks did not swim in "buttoned up". For instance, Sgt. Turner Sheppard, who commanded one of the only 2 DDs of the 741st that succeeded in swimming in, stated that "I was on deck steering", and later, "we deflated as soon as we were out of the water. I was then able to get into the turret for the first time". Lance Corporal Patrick Hennessey of the 13/18 Hussars wrote : "Slowly, we began to make headway. The crew were all on deck apart from Harry Bone who was crouched in the driving compartment intent on keeping the engine running".

The few photos and training films available invariably show men "on deck". The driver was, of course, seated at his station. Steering could be passed from the driver to the pilot, and it is thought that, under most combat circumstances, the tank commander served as pilot and steered the tank from the platform erected at the rear of the turret. Two crew members may have remained outside of the tank on the look out for problems, while one man may have taken a position in the turret, serving as radio operator.


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Major Duncan and his staff reported on several problems encountered with the DDs during training. It was found that sand became packed between the track and the drive sprockets attached to the idler wheels (above, left side photo). This put the sprockets out of phase with the tracks, and led to broken tracks and other problems. Extensive testing showed that the rear drive sprockets were not needed, since the propellers could "be driven by friction only". As a result, the rear sprockets were removed from all of the US converted DDs.


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US DDs were equipped with canvas fume screens (above left, item 1). These were supposed to direct the engine exhaust away from the crew. Duncan's reports noted that the screen was not very efficient, and became scorched and presented a fire hazard. Carbon Monoxide poisoning was not seen as a problem until crews made runs of 4000 yards or more. As a result, the school had exhaust extensions made locally. These were effective in directing the engine fumes over the top of the canvas frame. They were "braced from the turret and attached so that when the turret revolves the extension will automatically be removed." The few photos available show these "chimneys" only on US DD conversions, and only on D-Day. Above right, a movie frame showing the exhaust extension on a "large hatch" DD. This is thought to be one of the only 5 DDs of the 741st Tank Battlion that actually landed on Omaha Beach.

Major Duncan recommended that all future units be equipped with large drivers' hatches, loader's hatches and pistol ports. That would have been the case had the planners been able to procure the M4A1(76)s they had originally requested. While the authors estimate that there could not have been more than 100 large hatch M4A1(75) DD conversions, Col Morton of the 10th Canadian Armoured Regiment "Ft Garry Horse" (equipped with Sherman V DDs) must have seen a few, since he wrote, "Larger hatches for the Dvr a Co Dvr are reco[mmended] as in the Radial engine version of the Sherman".

Major Duncan believed that the swimming tank had the advantage of presenting a small target in the water, and upon "becoming" a tank on the beach, "will be a psychological shock to the enemy". Duncan's final recommendation was that DDs should be launched on D-Day, but not more than 4000 yards from shore, and in sea conditions not exceeding Force 3. If conditions were adverse, he recommended that the tanks be landed directly on the beach.


D-Day

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It was planned that the DDs would be launched about 3 miles (approx. 5000 yards) off shore, and would hit the beaches 10 minutes before H-Hour. They would use their firepower to eliminate or suppress enemy strong points, just before the assaulting infantry landed.


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Commonwealth DD regiments were each equipped with either 38 or 40 DDs. Their LCTs could carry 5 DDs as opposed to 4 for US LCTs. All of the British produced conversions used on D-Day are thought to have been based on the Sherman V (M4A4), pictured above. The 4/7 Royal Dragoon Guards and the Nottinghamshire (Sherwood Rangers) Yeomanry, which were both part of the 8th Armoured Brigade, are thought to have been equipped mostly or entirely with the 80 M4A1 DDs provided by the US.

It is to be noted that the sea conditions on D-Day were variously described as "Force 4" or "not ideal" or, by many Commonwealth and US crews, as beyond anything they had experienced during training. Under these circumstances, it is somewhat surprising that any of the fragile "craft" were launched. However, a number of reports note that, after all the training, the officers and men of the DD units were anxious to demonstrate their abilities, and this may have played a part in some of the decisions to launch. The June 21st "post mortem" report by Lt Col Morton of the Ft Garry Horse has an almost regretful tone..."I do not consider that the D.D. equipment or training was wasted by this Regt launching so close to the beach, without a swim. We were prepared and equipped for this swim but, under the existing circumstances, the decision to launch close in was well justified...This Regt is ready to try another landing if it is so desired at any time".



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The period reports reproduced above, provide figures regarding the number of DDs that were actually launched, along with the number that sank or landed. Note that the reports do not completely "agree" with each other. However, even modern sources vary, so while the documents may not have been entirely accurate, they were the basis for assessing the worth of the DDs, particularly in regard to their use in future operations. What they revealed was that, of the approximately 294 DDs embarked from England, 129 were stated to have launched. Thus, the "launch rate" was less than 50%. Of those launched, according to these reports, either 45 or 49 sank or "foundered". Thus the swimming success rate was about 65%. Despite these outcomes, the British continued production, and even development work on DDs, and both the British and US used stocks of existing DDs in future amphibious operations in Europe.


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The period reports failed to include the number of DDs that were successfully landed from their LCTs. In fact, the expected carnage to the LCTs on the approach, one of the reasons for the DD program in the first place, did not occur. Approximately 170 DDs were discharged on or very close to the beach. Numerous tanks were disabled by swamping in the rising tide, and this may the reason for the over count of DD sinkings in the Canadian sector in the original reports. Above is an "amended" version of the 79 Armoured Division Table. Figures are primarily based on Richard Anderson's "Cracking Hitler's Atlantic Wall."


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While wading tanks were to be "de-waterproofed" as soon as practical, orders were that "DD equipment on tanks...will not be removed or damaged". Given the fragility and the fire hazard of the canvas screens (to say nothing of combat), this hardly seems realistic. The above photos show the condition of a few DDs of C Company, 743rd Tank Battalion in St. Fromond on July 8 1944. Note that the rear photo shows that the propellers had been removed. The steel chevron tracks seen on these tanks were considered "absolutely necessary for launching from LCT," and were a standard element of US DD conversions.


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