The Mark VII tank was a development of the Mark V that used a Williams-Janney hydraulic steering system, but only a handful were built in 1918 and it never entered service.
The Mark VII was designed to use a Williams-Janney hydraulic steering system. This system used the main engine to power two hydraulic pumps, which in turn powered hydraulic motors in the track frames that were then connected to the drive sprockets. Steering and speed were both controlled by altering the speed of the pumps, in theory giving very fine control of both. This was an adaptation of the company’s successful gear for rotating naval gun turrets. One Mark II tank was given the Williams-Janney drive and took part in trials at Oldbury in March 1917. At the time the gear had underperformed, mainly because it had produced too much heat, so hadn’t been selected for the Mark V. However the overall concept was still seen as very promising, so in October 1917 Brown Brothers of Edinburgh were given a contract to develop the Mark VII, which would use the basic layout of the Mark V but with the Williams-Janney steering system.
The resulting tank was slightly longer than a standard Mk V, but was still very crowded. It was powered by a 150hp Ricardo engine, located just behind the driver. This powered a par of Variable Speed Gear pumps which were coupled directly to the two hydraulic motors. These drove pinions in the track frames that were linked to the drive sprockets by loops of chain. It was one of the first tanks to be given an electric starter motor.
In order to cope with the heat the tank had armoured louvers in the roof, extraction fans in the tank and two radiators in the track frames, one for the engine coolant and one for the hydraulic fluid.
The first Mark VII was completed in July 1918 and an order for 74 more tanks was then placed, split between Brown Brothers and Kitsons of Leeds. However it does not appear to have reached the test unit for tanks, probably because the heat was still causing problems. Only three were built, all by Brown Brothers.
Hull Length: 29.4ft
Hull Width: 12.84ft (male)
Weight: 33 tons
Engine: 150hp Ricardo gasoline engine
Max Speed: 4mph
Armament (Male): Two 6-pounders, four machine guns