improve upon the design of the Tetrarch in a number of areas. It had thicker armour than the Tetrarch, with the front hull and turret armour increased to a thickness of and the side armour to , and the turret and hull given more sloped surfaces to help deflect shells fired at the tank. The dimensions of the Tetrarch were also changed; the Mark VIII was longer by , wider by and heavier. The new tank was no longer air-portable, as it was too heavy to be carried by a Hamilcar. The 12-cylinder engine of the Tetrarch was fitted to the Mark VIII, although the increased weight meant that its maximum speed decreased to ; its armament also remained the same as that of the Tetrarch. The War Office authorised the construction of three prototype models in April 1941. The new design was considered a success, and the Tank Board of the War Office ordered 1,000 to be constructed in September. However, problems were encountered with further tests of the prototypes, and a report issued in December 1942 stated that production of the Mark VIII had been delayed due to developmental problems. These problems continued to persist into 1943, when the War Office decided against using the tank in active service; approximately 100 Mark VIIIs were produced by 1945, when production ended. A second variant on the Tetrarch design was the Tetrarch Duplex Drive ("Tetrarch DD"). The Duplex Drive system was invented by Nicholas Straussler, and was designed to allow a tank to 'swim' through water and participate in amphibious operations. The system functioned by erecting a large waterproof canvas screen around the tank above its tracks, which was supported by thirty-six inflatable tubes and steel struts; this gave the tank sufficient buoyancy to float, and was then propelled along by a small propeller powered by the tank's engine. The screen could be collapsed by using a small explosive charge once the tank reached land. The system was fitted during June 1941, as the Tetrarch was the lightest light tank available at the time; the converted tank was successfully tested on a number of lakes and reservoirs, allowing the Duplex Drive system to be tested on heavier tanks, such as the Valentine. The system would be used during Operation Overlord, when M4 Sherman medium tanks would land on the invasion beaches. Survivors Two surviving Tetrarchs are on display, one each