Propeller Diameter:

• A larger diameter generally increases efficiency by allowing the propeller to operate at a lower rotational speed (RPM). However, maximum diameter is limited by the need for sufficient clearance between the propeller, hull, and rudder. Excessively large diameters can also lead to increased wake variation, negatively impacting efficiency.

Number of Blades:

• Fewer blades typically result in higher propeller efficiency. However, a higher number of blades reduces the exciting force per blade, improving vibration characteristics and potentially increasing strength. The optimal number represents a balance between these competing factors.

Propeller Speed (RPM):

• Lower RPM, in conjunction with a larger diameter, generally leads to higher efficiency. However, higher RPMs can increase the likelihood of cavitation, which significantly reduces efficiency and can damage the propeller. The chosen speed must also avoid resonance with the natural frequencies of the hull and propulsion shafting system.

Propeller Pitch Ratio:

• A higher pitch ratio generally increases the power delivered at a constant advance coefficient. However, an excessively high pitch ratio can lead to negative effects on efficiency.

Blade Area Ratio:

• This ratio needs careful consideration. A large blade area ratio increases blade section drag, reducing efficiency. Conversely, a very low ratio makes it difficult to generate sufficient thrust.

Propeller Boss Diameter Ratio:

• This should be minimized to reduce drag, but practical limitations due to the propeller shaft diameter must be considered.

Propeller Blade Rake:

• Raking the blades aft increases clearance between the hull and propeller blade tips, permitting a larger propeller diameter and thus potentially improved efficiency.

Blade Skew:

• Skewing the blades aft reduces the magnitude of unsteady forces generated by the propeller operating in a circumferentially varying wake, leading to smoother operation and reduced vibration.

Pitch Angle:

• The pitch angle must be optimized to avoid both back cavitation (due to high angles of attack) and face cavitation (due to low angles of attack), both of which significantly reduce efficiency.

Blade Section:

• The efficiency of the propeller is heavily influenced by the blade section profile. Aerofoil sections, with their high lift-to-drag ratios, are preferred for improved efficiency.

1. Pitting: For pitting up to 1mm, grinding and polishing can restore surface smoothness, improving efficiency. Synthetic resin fillers can provide a temporary solution for minor roughness.
2. Blade Distortion: Distorted blades should be carefully and uniformly heated to a specific temperature and then straightened using weights and levers.
3. Cracks: Minor edge cracks can be addressed through flaring. Larger cracks require drilling, welding, and subsequent grinding and polishing to restore the blade's structural integrity and hydrodynamic performance.
4. Conduct periodic checks to detect early signs of pitting, distortion, or cracks.