|Prerequisite||Steam Ship Propulsion|
|Research Points Needed||500|
|Building Needed||Steam-Powered Factory|
A screw propeller drives a ship through the water, powered by steam and independent of the wind.
Unlike a paddlewheel, a screw propeller is entirely underwater, and is largely invulnerable to enemy fire as a result. However, it does add to the draft of a ship, so a pilot must have his wits about him in shallow water. The mechanism is, however, far more efficient than any combination of paddlewheels (either stern or side), and can drive a ship at much higher speed for a given engine size.
Historically, the screw propeller had its origins in Archimedes’ Screw, an ancient method of lifting water to a higher level. The screw’s potential for propulsion was recognised early on, but not actually implemented. It required steam power, coupled with further refinements to the shape of the screw propeller, to finally see it adopted instead of paddle wheels. British engineer Francis Pettit Smith (1808-1874) accidentally discovered that smaller propellers work better than big ones. During one of his many tests, a piece of his screw propeller snapped off, and he noticed that, rather than grinding to a halt, the ship actually accelerated.
While researching Screw Propeller always guarantees access to 38-gun Steam Ships and 80-bun Steam Ships, Ironclads additionally require Iron Plating, an end-game military technology, to be researched before they may be built.