Question

(a) State why fixed pitch propellers have a poor efficiency when going astern. (8)

(b) With reference to controllable pitch propellers state:

(i) Why is it preferable that the main servomotor be housed in the propeller hub rather than in the shafting forward of the propeller shaft? (4)

(ii) What regular maintenance and checks should be carried out to ensure maximum reliability of the gear at all times? (4)

Answer 1

Part (a)

Fixed pitch propellers are less efficient in the astern direction due to:

When moving astern, the angle of attack on the back of the blade is high, causing significant eddying and turbulence on the blade's surface, which reduces efficiency.
Thrust generation primarily depends on the high pressure exerted on the face of the blade. During astern movement, this thrust diminishes significantly, leading to a loss in propeller efficiency.
The thrust acting on the back of the blade during reverse motion is less effective because the blade curvature reduces the area available for thrust production.

Part (b)

Controllable Pitch Propellers (CPP)

(i) Servomotor Location

It is preferable to house the main servomotor for a controllable pitch propeller (CPP) in the propeller hub rather than in the shafting forward of the propeller shaft. This design, known as a hub servo system, is preferred over the external servo system, which uses a long push-pull rod extending from the engine room. The primary reason for this preference is that the long push-pull rod in an external servo system is prone to bending. This bending can occur due to the rod's length and the significant water pressure acting against the propeller blades, which can compromise the pitch control mechanism's reliability and precision. Housing the servomotor directly in the hub eliminates the need for this long rod, resulting in a more robust and reliable system.

(ii) Regular Maintenance and Checks

1. Hydraulic System Maintenance

Periodic cleaning of hydraulic filters and oil coolers.
Regular oil sampling (both onboard quick checks and shore analysis) to detect contamination or wear particles.
Monitoring oil consumption to detect leaks in the system.

2. Mechanical Components

Greasing all linkages to prevent corrosion and wear.
Checking and lubricating moving parts as per manufacturer’s recommendations.

3. Operational Checks

Ensuring familiarization of all relevant personnel with correct operating and maintenance procedures.
Regular testing of alarms and safety devices.
In dry dock, verify actual pitch position against remote indicators at the wheelhouse and engine room.
Test the fail-safe arrangement to ensure it operates correctly in emergencies.

✓ Verified Model Answer (Text Solution)

Fixed pitch propellers are less efficient in the astern direction due to:

Controllable Pitch Propellers (CPP)

(i) Servomotor Location

(ii) Regular Maintenance and Checks