The term steamboat is used to refer to smaller, insular, steam-powered boats working on lakes and rivers, particularly riverboats. As using steam became more reliable, steam power became applied to larger, ocean-going vessels.
Early steamboat designs used Newcomen steam engines. These engines were very large and heavy and produced little power (unfavorable power to weight ratio). Also, the Newcomen engine produced a reciprocating or rocking motion because it was designed for pumping. The piston stroke was caused by a water jet in the steam-filled cylinder, which condensed the steam, creating a vacuum, which in turn caused atmospheric pressure to drive the piston downward. The piston relied on the weight of the rod connecting to the underground pump to return the piston to the top of the cylinder. The heavy weight of the Newcomen engine required a structurally strong boat and the reciprocating motion of the engine beam required a complicated mechanism to produce propulsion.
James Watt's design improvements increased the efficiency of the steam engine, improving the power to weight ratio, and created an engine capable of rotary motion by using a double-acting cylinder which injected steam at each end of the piston stroke to move the piston back and forth. The rotary steam engine simplified the mechanism required to turn a paddle wheel to propel a boat. Despite the improved efficiency and rotary motion, the power to weight ratio of Boulton and Watt steam engine was still low.
The high-pressure steam engine was the development that made the steamboat practical. It had a high power to weight ratio and was fuel efficient. High pressure engines were made possible by improvements in the design of boilers and engine components so that they could withstand internal pressure, although boiler explosions were common due to lack of instrumentation like pressure gauges. Attempts at making high-pressure engines had to wait until the expiration of the Boulton and Watt patent in 1800. Shortly thereafter high-pressure engines by Richard Trevithick and Oliver Evans were introduced.
Early attempts at powering a boat by steam were made by the French inventor Denis Papin and the English inventor Thomas Newcomen. Papin invented the steam digester (a type of pressure cooker) and experimented with closed cylinders and pistons pushed in by atmospheric pressure, analogous to the pump built by Thomas Savery in England during the same period. Papin proposed applying this steam pump to the operation of a paddlewheel boat and tried to market his idea in Britain. He was unable to successfully convert the piston motion into rotary motion and the steam could not produce enough pressure. Newcomen's was able to produce mechanical power, but produced reciprocating motion and was very large and heavy.