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.