Propeller

The propeller, sometimes called the screw, is the device that turns the power of a ship's engine into forward motion. Mounted on the tail shaft at the stern, it consists of a boss carrying two or more helical blades, and as it rotates it acts like a screw working in the water, accelerating a column of water astern; by Newton's principle of action and reaction, the water pushed backward pushes the ship forward, producing the thrust that drives the vessel. The propeller is the final link in the propulsion chain that runs from the engine through the shafting to the water.

Propellers come in two main types. A fixed-pitch propeller has blades set permanently at one angle, or pitch, and the ship's speed and direction are controlled by changing the engine's speed and, for going astern, reversing the engine or the shaft. A controllable-pitch propeller has blades that can be rotated on the boss to change their pitch while the propeller keeps turning, which allows fine control of thrust (including going astern) without changing engine direction, and suits ships that manoeuvre frequently such as ferries and offshore vessels. Large ships often turn a single large, slow propeller for efficiency, while some vessels have two or more.

Several factors govern how well a propeller works. Its diameter, the number and shape of its blades, and its pitch are designed to match the ship's hull, engine and intended speed for the best efficiency. A phenomenon called cavitation (the formation and collapse of vapour bubbles when local pressures fall too low) can erode the blades and cause vibration and noise, so propellers are designed to limit it. Fouling and damage reduce efficiency and raise fuel consumption, which is why propellers are polished and inspected, often during dry-docking. As the point where all the engine's power finally meets the sea, the propeller is central to a ship's speed, efficiency and manoeuvrability.