While the Valentine was acceptable as a training vehicle, it was realized early on that it was past its prime as a combat tank. The most obvious choice as a follow-on vehicle was the ubiquitous Sherman then set to equip the majority of Commonwealth armoured regiments. The Duplex drive tanks in the British and by extension Canadian forces in Normandy were based on the M4A4 Sherman V variant. This version of the Sherman had a slightly longer hull and as a result the erect screen had slightly more volume giving a small increase in buoyancy. Later in the war the DD conversions were also made on the M4A2 Sherman III hull. The US used only the M4A1 for their conversions. British production of the Duplex Drive Shermans only began in March 1944 and the majority of the vehicles produced were required to equip the assault regiments. As a result, the five British and Canadian regiments had little more than one week apiece for the crews to cycle through the few vehicles available for training and familiarize themselves with their new equipment prior to launching on D-Day.
Fortunately, other than size the Sherman DDs were operationally very similar to the Valentines. The one major difference had to do with the method of propulsion. Where the Valentine had a single propeller connected directly to the engine, the Sherman, used sprockets bolted to the rear idler wheels and the rotation of the tracks to drive a system of gears providing power to twin propellers. In the water the Sherman DD had a top speed of about 4.5 miles per hour. As with the Valentine the canvas screen was raised by means of pumping compressed air into 36 rubber air pillars and was held in place by a metal frame and collapsible struts. Raising the screen took about 15 minutes. Collapsing the screen took only seconds.As mentioned previously the Sherman travelled with its gun facing forwards and was able to engage targets as soon as the screen was down.
As mentioned previously the Crew Commander’s were expected to judge when the water level had dropped sufficiently to allow the screens to be lowered while the main part of the hull of the tank was still under water. This was known to tankers as a “Hull down “position.Even in the down position the screen still extended above the engine deck and it was expected that this would prevent the incoming waves and tide from flooding the engines. Unfortunately, on D Day with the many competing priorities some crews deflated early while others failed to notice the rising tide and several tanks were disabled for just this reason
The videos below describe the principal components and operation operation of the Sherman DD the first video has no audio but clearly shows the various components of the vehicle and their operation. The second video with audio briefly describes the principles involved in creating a 34 ton floating tank and then covers its operation. from loading to landing. Of particular note is the fact that this video was taken in essentially dead calm conditions while the conditions on D-Day saw winds of up to 28 Kph and waves of up to 2 metres