HMS Rattler and the screw-propeller turn

The Siamese double-cylinder engine and HMS Rattler show why the form of propulsion dictated machinery layout. A screw propeller required power to travel aft along a shaft, so the engine had to deliver rotation in a useful direction or add complicated gearing and pivots. Paddle wheels demanded a different arrangement and left bulky, vulnerable structures along the ship's sides. Selecting a propulsor also meant choosing an engine-room layout, a power-transmission system, and a fighting arrangement for the hull.