Side-lever and beam engines in early warships

The side-lever engine was a beam-engine adaptation rearranged to save height inside a hull. It was large, heavy, and less powerful than designers might have wanted, but it could sit low below the waterline where gunfire was less likely to damage it. At this point, a torpedo meant a mine rather than the later self-propelled Whitehead weapon, placing the protection argument in its contemporary underwater-threat context. Its low centre of gravity also helped a ship remain stable in heavy seas. From roughly 1820 to 1840, about 50 of the Royal Navy's approximately 70 new steam vessels used side-lever machinery, which shows how strongly those advantages outweighed the drawbacks. Their boilers still ran at very low pressure because designers feared explosions and the stresses imposed by a ship moving at sea.