"Great Britain"s main propeller shaft, built by the Mersey Iron Works, was the largest single piece of machinery. long and in diameter, the shaft was bored with a diameter hole, reducing its weight and allowing cold water to be pumped through to reduce heat. At each end of the main propeller shaft were two secondary coupling shafts: a , diameter shaft beneath the engine, and a screw shaft of in diameter at the stern. Total length of the three shafts was , and the total weight 38 tons. The shaft was geared upward at a ratio of 1 to 3, so that at the engines' normal operating speed of 18 rpm, the propeller turned at a speed of 54 rpm. The initial propeller was a six-bladed "windmill" model of Brunel's own design, in diameter and with pitch of .