In August 1942 Canadian and British Troops experienced disaster when landing craft came under significant attack whilst unloading
tanks at the beaches of Dieppe. Infantry and craft were easy targets for the German guns, and losses were high. It was
considered by Military Commanders that a future direct assault on a fortified coast held by the enemy would be catastrophic,
yet without such an offensive, it would be impossible to win the war.
However, there was simply no time to develop new vehicles
and equipment to meet this challenge.
So a specialised unit, the 79th Armoured Division was established. They were charged, under the leadership of Major-General Percy Hobart, with the adaptation, evolution, development, training and operation of a series of tanks specifically modified to overcome the problems associated with an invasion. The collective name given to these specialised tanks was known as "Hobart's Funnies", owing to the often un-conventional, yet practical, appearance of the tanks. Over the following year several tanks were developed to overcome German defences and beach invasions. These included:
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| The SHERMAN CRAB had a rotating flail attached to the front of the tank to tackle the problem of Landmines.© IWM (H 38079) | The CHURCHILL FASCINE CARRIER mounted a chepsale bundle which could be dropped into anti tank ditches © IWM (H 37472) | The CHURCHILL LAKEMAN ARK had its turret removed, and replaced with 30ft of ramp, over which other tanks could drive © IWM (H 36593) |
Tanks weighing anything up to 30 tonnes are not the easiest things to float on water. The biggest problem of designing amphibious tanks was trying to create a tank light enough to float, but protected sufficiently to keep out armour piercing rounds. During the interwar years experiments had been undertaken by adding floats to the side of heavy tanks, but this made them cumbersome and too wide to launch from landing craft.
The problem was solved by the Hungarian born engineer, Nicholas Straussler (1891-1966). He devised a collapsible screen, made of waterproof canvas. The screen was fitted to the tank's hull, just above the running gear, and a series of tubular frames were mounted horizontally giving shape to the screen. The screen was held taut by metal struts and vertical rubber tubes filled with compressed air. When collapsed, it would fold like a concertina, and enable the tank to continue in its combat role.
The water tight canvas walls provided extra displacement to discount the total weight of the tank, thus enabling it to float. The tanks in the water looked similar to innocent canvas boats, with only about three feet of the screen showing above the water line (the majority of the vehicle itself being below the waterline). Thus concealing the lethal weapon beneath.
Archimede's principle states that if the mass of an object is greater than its weight, then an object will float.
Straussler
proved that by increasing the volume of the tank, it was possible to make it positively buoyant. However the height of the screen
was limited by the width of the tank.
Initial trials were undertaken with the Tetrarch Light Tank. Weighing in at around 7 tons, it required a shallow screen to facilitate the necessary displacement. It was put through its paces initially at Hendon Reservior in June 1941, prior to saltwater trials at Langstone Harbour later the same year. Satisfied with results of these, Straussler was given the opportunity to develop the flotation gear with the current British Infantry Tank, the Valentine.
Strausller and his team set about converting the 17 ton tank to its DD role. They developed a number of modifications including the cutting of a hole in the rear of the tank to enable drive to be taken directly off the gearbox. One slight problem with the tank was that its turret gun would foul the screens, so necessitated it to be traversed to the rear, prior to screen inflation. Trials were undertaken at Brentwood Reservoir, followed by Saltwater trials at Rye. It was during these in November 1942 that the versatility of the tanks was assessed during firing at trials, resutling in the fragile air pillars being punctured, and collapsing the screen. Thus modifications were made to attach struts to support the screen after inflation.
Further Saltwater Trials continued in Scotland at Castle Toward, before moving down to Linney Head. The small Cadre of men involved were developing the tactics and skills required to pass onto the crews who would shortly be trained.
In 1943 the War Office were concerned about the versatility of the DDs, and naturally involved the USA. This proved to be a good move, becasue the US were able to recommend their Shermans for conversion. The 32 ton tanks had many advantages over the Valentine, including a larger 75mm gun, thicker armour, electronic turret and space for a crew of five.
Trials proceeded at Fritton in November 1943 with one of the three protype Sherman DDs. The demonstration was going well until the DD Sherman struck an underwater object resulting in the sinking of the tank. This accident caused minor delays, particularly as the 79th Armoured Division were keen to pursue with Saltwater trials. Once these trials had been completed, production could continue of converting sufficient numbers of Sherman to DD. However UK production was slow, and so the US contributed to the cause by converting a significant number in the States, and shipping them across to England just in time for DDay.
These issues relating to shortages of both Valentine and Sherman DD tanks would frequently affect training schedules and the planning for DDay. However the continued pressure exerted on the Ministry of Supply and Home Forces by Percy Hobart ensured that this disruption was minimised through the reworking and overhaul of overswum tanks for training.
Through the course of 1943-4 625 Duplex Drive Valentines were created. These were all utilised for training purposes. The majority were obselete after DDay, whereupon Sherman DDs were the main vehicle used.