Most ships push water backward to move forward. A propeller is a rotating fan-like device, driven by the main engine or motor, that converts torque into thrust by accelerating water astern.

Fixed-pitch and controllable-pitch propellers serve different operational needs. Blade number, diameter, and pitch are matched to the hull and engine. Cavitation, vibration, and noise are design constraints. Shafts, bearings, and stern tubes support and seal the propeller system.

Propeller condition affects speed and fuel efficiency. Damage from grounding or debris requires drydock or underwater repair. Divers inspect propellers regularly. The propeller is one of the few parts of the ship whose performance the engineer can feel in every turn of the shaft.

• Rotating device that produces thrust for propulsion
• May be fixed or controllable pitch
• Matched to hull and engine characteristics
• Requires inspection and protection from damage