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
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.
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".
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
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.
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
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).
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
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
"with no operational need at this time".
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.
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.
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.
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.
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.
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
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
to have been based on the Sherman V (M4A4), pictured above. The 4/7
Dragoon Guards and the Nottinghamshire (Sherwood Rangers) Yeomanry,
which were both part of the 8th Armoured Brigade, are thought to have
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".
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.
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
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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