Propeller
A Propeller (or screw) is the rotating, bladed device at a ship's stern that converts the rotational power of the engine into thrust, pushing water astern to drive the vessel forward (or, in reverse, astern). Turned by the propeller shaft from the main engine, it is the component that actually propels the ship.
Propellers come in types: the common fixed-pitch propeller (FPP), and the controllable-pitch propeller (CPP) whose blade angle can be adjusted for efficiency and manoeuvring without changing engine direction. Propeller design and condition strongly affect fuel efficiency: fouling or damage, or a rough surface, wastes fuel, so hull-and-propeller cleaning and polishing are common efficiency measures. Excessive load or speed can cause cavitation (vapour bubbles that erode the blades and cause vibration and noise). The propeller works together with the rudder (which steers by deflecting the propeller's flow) and is a focus of both propulsion efficiency and, increasingly, of measures to reduce fuel and emissions. It is fundamental to how a ship moves.
The propeller is where engine power becomes motion — and its design and cleanliness directly affect how much fuel that takes, which is why propeller polishing and CPP efficiency matter to voyage cost and emissions. Together with the rudder it also governs manoeuvring. Understanding the propeller is understanding the final link in how a ship is driven.
What is a controllable-pitch propeller?
A propeller whose blade angle can be adjusted to change thrust and direction without changing engine rotation, aiding efficiency and manoeuvring.
How does the propeller affect fuel use?
Its design and condition strongly affect efficiency; fouling, damage or roughness wastes fuel, so cleaning and polishing are common efficiency measures.