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Cross-section diagram of a proposed boat from the 1803 patent filed by Charles Dallery (1754-1835) in France. It was to be propelled by two 1.8m diameter helical screw propellers, one mounted in the stern and the other in the bows. The two screws were linked by a chain transmission and powered by a vertical steam tubular boiler. Whilst the stern screw provided the main propulsion, the forward screw was articulated so that it could be directed either to the right or left to act as a rudder. Dallery, an organ builder and jeweller, was one of a number of engineers and inventors at this time endeavouring to build a successful steam-powered marine propulsion system. Due to lack of funds his boat was never built. (KEYSTONE/SCIENCE PHOTO LIBRARY/SHEILA TERRY/SCIENCE PHOTO LIBRARY)