Why screw propulsion displaced paddlewheels

HMS Amphion illustrates the transitional screw frigate: ordered as a sailing frigate, then completed with propeller propulsion. Paddle and screw systems coexisted for decades, partly because officers worried about propellers lifting clear of the water and about cutting a shaft opening through the hull. The Royal Navy tested the alternatives rather than assuming the newer mechanism would be better at sea. Rattler first beat the similarly powered paddle vessel Alecto in a hundred-mile race and then pulled her astern in the famous tug-of-war trial. Those results gave the Admiralty practical evidence that screw propulsion offered both greater speed and greater usable power.