While the American-made M4 "Sherman" Medium Tank was not the finest combat tank of World War 2 (1939-1945), it was a war-winning instrument because of the sheer numbers available. This sort of availability led to it serving as the basis for other battlefield vehicles including dedicated tank-killer (the Sherman Vc "Firefly") and rocket projecting platform (as in the Sherman "Calliope"). Over 50,000 Shermans were completed by war's end in 1945. The British Army standardized to the Sherman, moving away from their early-war "Cruiser/Infantry Tank" doctrine, and eventually enacted their own developments to the line. One creation became the "Sherman DD", a component of the "Hobart's Funnies" collection of mission-specific tank developments that appeared through the work accomplished by British military engineers. The "Sherman DD" name stemmed from the words "Duplex Drive" which was generated as a front to protect the amphibious tank program during its development.
The "floating Sherman" was born in an initiative of 1941 by engineer Nicholas Straussler in an effort to produce an amphibious version of the classic combat tank that would become standardized within the Allied inventory soon enough. The challenge lay in how to balance and float a mammoth, heavy design in water while retaining a useful fighting capability once the tank had made it to shore. Early work used the British Tetrarch Light Tank as the starting point with development eventually moving to the larger Valentine Cruiser Tank before all was settled on the Sherman series. Valentine DD tanks also saw service in the war alongside Sherman DDs.
The Sherman hull was slightly modified with a new, boat-like hull structure added around the existing hull lines and to this was installed a collapsing wading screen and flotation system which featured some thirty-six, air-filled rubber tubes providing the needed buoyancy. When not in use, the floatation screen was worked down and around the edges of the tank's hull so the tank could operate in a normal combat role. When water-crossing was required, the wading screen was erected by a pair of onboard, air-filled tanks and solid struts were raised along with it to brace and lock the screen in place. The screen rose to pass a point just above the turret roof line so as to prevent seawater from passing over the top and sinking the vehicle or damaging its sensitive running components - the freeboard measured about 3 feet which provided some leeway though this restricted the tank to certain sea states (Sea State 5, about 8 to 13 foot waves, was the suggested absolute maximum). Propulsion was managed by a pair of small, steerable propeller units fitted to the lower rear hull of the tank, these lowered for water work and raised for ground travel. Drive power came from the tank's standard drivetrain. Crews were afforded inflatable rafts and breathing gear should the worst happen.
The entire preparation process took a well-trained crew around fifteen minutes to complete. One of the added benefits of the process was that it could be done anywhere and without an external equipment required. As such, many Sherman DD tanks were prepped when already in their landing transports prior to a beach assault. Once launched, the tank's crew could expect to make about 4 knots at full speed though this period was a particularly vulnerable time for the Sherman DD - endangered from both the current sea state as well as enemy fire directed upon it.
The standard Sherman armament of the 75mm main gun, coupled with the coaxial 0.30 caliber machine gun, was retained which gave the tank considerable firepower during amphibious actions - especially considering the light-armed nature of troops moving in ashore from their respective, lightly-armed transport craft. Sherman crews could clear enemy positions through use of High-Explosive (HE) shell fire and suppress enemy groups through machine gun fire. The bow-mounted 0.30 caliber machine gun hull position, standard to the basic Sherman combat tank, was eliminated in Sherman DD tanks.
Once the Sherman DD's tracks could interact with the ground, typically in around five feet of water or less, the wading screen was collapsed, the propellers raised for improved ground clearance, and the vehicle proceeded in a traditional fashion (the lowered wading screen did not impede the inherent traversal span of the turret at all). This allowed armor to engage enemy beach positions, much to the defender's surprise, as well as fight alongside allied infantry elements in clearing out a beachhead.
This sort of approach proved the norm in the successful, though ultimately costly, D-Day beach landings in northern France on June 6th, 1944. Omaha Beach, in particular, became a slog-fest after many of the expected Sherman DD tanks were lost on their approach in Channel waters - waves reaching up to six feet high that day. Sherman DD tanks were then used in the assault on southern France during "Operation Dragoon" which followed on August 15th, 1944. The vehicles remained in constant use until 1945 - one of their final actions being the crossing of the Elbe River in late April. Both Sherman DD and Valentine DD tanks were used in the campaign to retake Italy. Sherman DD tanks were used by both the British and American armies during the war.
Various Sherman models were modified for the amphibious tank role - this included M4A1, M4A2, and M4A4 types. Only a few are preserved today as showpieces and can be found in places around the world.