Amphibious tanks (called Duplex Drive, or DD) were designed by British General Percy Hobart to manoeuvre in water as well as on land. The chassis of the conventional Sherman tank was modified to make it watertight, and to improve its buoyancy it was fitted with a canvas flotation screen held in place with steel hoops, as well as two swivelling propellers for propulsion and steering. It could reach a speed of four knots on water. Its armaments were similar to land models: two 7.62 mm Browning machine guns and a 75 mm gun. Its crew of five included a commander, driver, gunner, loader, and co-driver/bow gunner. Once on land, the screen, or skirt, could be lowered in just a few minutes making the gun operational.