held at Studland Bay with the Valentine DDs. The trial run of the tanks ran into difficulty when a change in the weather adversely affected the sea conditions. Six tanks sank with the loss of six crew members. The sunken wrecks of at least 10 tanks, lost during training, are known to lie off the British coast. Another sunken DD tank remains at the bottom of Fritton Lake. Sherman DD Modifications to the Sherman included the sealing of the lower hull, the addition of the propeller drive and the addition of Straussler's flotation screen around the hull, together with its inflation system. The base of the canvas flotation screen was attached to a horizontal mild steel boat-shaped platform welded to the tank's hull. The screen was supported by horizontal metal hoops and by 36 vertical rubber tubes. A system of compressed air bottles and pipes inflated the rubber tubes to give the curtain rigidity. The screen could be erected in 15 minutes and quickly collapsed once the tank reached the shore. In practice there was about of freeboard. In combat, the flotation system was considered expendable and it was assumed the tank crew would remove and discard it as soon as conditions allowed. In practice, some units kept the flotation equipment and their tanks were used in several amphibious operations. A pair of propellers at the rear provided propulsion. One problem presented by the Sherman was that the configuration of the transmission (gearbox at the front) made it impossible to take a drive-shaft directly from the gearbox to the propellers. The solution to this was to have sprocket wheels at the rear of the tank so power was delivered to the propellers by the tank's tracks. DD Tanks could swim at up to . Both the commander and the driver could steer in the water, although with different methods. A hydraulic system under the control of the driver could swivel the propellers; the commander from a platform at the rear of the turret, where he could see over the skirt, could contribute by operating a large tiller. The first DD Shermans produced by the British were used by both British and US units. Later production was by both the US and the UK. British Shermans were Sherman III (M4A2) and Sherman V (M4A4) conversions. The US used the M4A1 only for their conversions. Experience from D-Day led to an improved,