Propeller (Ship)

A ship’s propeller (also called a screw or marine propeller) is a rotating mechanical device that converts the rotational energy from the main engine or motor into thrust — the forward or backward force that moves the ship through water. It works on the Archimedes screw principle.

Answer in Brief

The propeller rotates in water, accelerating water astern (for ahead movement) or ahead (for astern). By Newton’s third law, the reaction to this acceleration is a thrust force on the ship. More RPM = more thrust = higher speed (up to a limit set by hull resistance and cavitation).

How a Propeller Works

Each propeller blade has an aerofoil cross-section — similar to an aircraft wing. As the blade rotates through water:

• The face (pressure side) pushes water aft
• The back (suction side) creates low pressure, drawing water forward

The combination generates thrust forward and induces a torque (rotational resistance) that the main engine must overcome.

Slip: The difference between the theoretical advance (pitch × RPM) and the actual advance of the ship. Typically 10–15% for commercial ships. High slip = inefficiency.

Types of Propellers

Fixed Pitch Propeller (FPP)

The blade angle (pitch) is permanently fixed. The only way to change thrust or speed is to change engine RPM or engine direction.

Advantages:

• Simple and robust — no hydraulic pitch control system
• Lower manufacturing cost
• Reliable — fewer mechanical failure points

Disadvantages:

• Engine must slow down and reverse for astern movement (time-consuming)
• Cannot optimise efficiency at different speeds
• Reversing the engine causes wear and is mechanically demanding

Used on: Most bulk carriers, tankers, and cargo ships with slow-speed 2-stroke main engines.

Controllable Pitch Propeller (CPP)

The blade pitch angle can be changed while the propeller is rotating. The propeller shaft continues to rotate in one direction; pitch change achieves ahead, zero, or astern thrust.

Advantages:

• Constant engine RPM — engine efficiency optimised
• Rapid pitch change = quick thrust change without engine reversal
• Smooth speed control
• Excellent for vessels needing frequent speed/direction changes (ferries, OSVs, dredgers)

Disadvantages:

• Complex hydraulic pitch control system (failure potential)
• Higher cost and maintenance
• Blade pitch control mechanism in hub is a vulnerable component

Used on: RoPax ferries, OSVs, cruise ships, dredgers, some container ships.

Azimuth Thruster (Azipod)

A propeller mounted in a pod that can rotate 360°. Replaces both the propeller and rudder — steering is achieved by rotating the pod.

Advantages:

• Excellent maneuverability
• No need for conventional rudder
• Can push or pull the ship

Used on: Cruise ships, ice-breakers, some OSVs.

Cavitation

Cavitation occurs when the pressure on the suction side of a propeller blade drops below the vapour pressure of water. Water vaporises, forming bubbles. When these bubbles collapse (implode), they generate intense local pressure waves that erode the blade surface.

Signs of cavitation:

• Vibration from propeller area
• Blade erosion visible during drydock inspection
• Reduced thrust efficiency despite increasing RPM

Causes:

• Too high RPM for the draft condition (light ship, propeller near surface)
• Design operating outside intended parameters

Prevention: Operate within designed RPM range. Maintain adequate draft. During propeller design, appropriate blade area ratio is selected to avoid cavitation.

Propeller Inspection and Maintenance

Propellers are inspected during drydocking:

• Blade surface for erosion, cracking, or impact damage
• Blade edges for fouling or deformation
• Shaft clearance (stern tube bearing wear)
• Zinc anodes on shaft/hub (cathodic protection)

Underwater inspection by divers is also done between drydockings for blade condition check and hull cleaning.

Frequently Asked Questions

Q: Why does a ship’s propeller cause the stern to swing when going astern? This is called the transverse thrust effect or paddle wheel effect. The propeller blades at different depths operate in water of slightly different densities and encounter different resistance, causing a sideways force. For a right-handed propeller going astern, the stern kicks to port.

Q: What is a right-handed vs left-handed propeller? Viewed from astern, a right-handed propeller rotates clockwise when going ahead; left-handed rotates anti-clockwise. Twin-screw ships often have one of each (outward-turning) for better handling.

Q: Can a propeller be repaired underwater? Minor repairs (polishing, straightening minor bends) can be done by divers. Significant damage requires drydocking.

Questions about ship machinery, propulsion systems, or engine room equipment? Chat with SailorGPT