The screw propeller was introduced in 1835 by Francis Pettit Smith who discovered a new way of building propellers by accident. Up to that time, propellers were literally screws, of considerable length. But during the testing of a boat propelled by one, the screw snapped off, leaving a fragment shaped much like a modern boat propeller. The boat moved faster with the broken propeller. The superiority of screw against paddles was taken up by navies. Trials with Smith's SS "Archimedes", the first steam driven screw, led to the famous tug-of-war competition in 1845 between the screw-driven and the paddle steamer ; the former pulling the latter backward at 2.5 knots (4.6 km/h).