By 1849 the screw propeller had been invented and was slowly being introduced as iron increasingly was used in ship construction and the stress introduced by propellers could be compensated for. As the 1800s progressed the timber and lumber needed to make wooden ships got ever more expensive and the iron plate needed for iron ship construction got much cheaper as the massive iron works at Merthyr Tydfil, Wales, for example, got ever more efficient. The propeller put a lot of stress on the rear of the ships and would not see large spread use till the conversion from wood boats to iron boats was complete—well underway by 1860. By the 1840s the ocean-going steam ship industry was well established as the Cunard Line and others demonstrated.