Auxiliary 1 Shafts and propellors Flashcards
(25 cards)
How is a change of alignment in propulsion shaft intermediate bearings due to vessel condition allowed for?
Many intermediate bearings are designed to be self-aligning, which allows them to adjust automatically to changes in shaft alignment caused by vessel movement. This is essential to compensate for hull deflections in rough seas or uneven loading.
Bearings may also be mounted on floating or semi-floating supports, enabling them to shift slightly in response to dynamic changes in vessel structure and conditions.
Why must the propulsion shaft be able to move longitudinally?
- To accommodate thermal expansion and contraction of the shaft during operation.
- To adjust for hull movement due to ship flexing or deformation in varying sea conditions.
- To distribute load evenly and prevent jamming or misalignment, ensuring smooth operation.
Why does the aft most bearing require a complete bush, while other bearings may have only a lower half?
- The aftmost bearing supports the propeller shaft, which experiences higher thrust and radial forces.
- A full bush provides maximum support and absorbs both axial and radial forces.
- Intermediate bearings need only lower halves for support and easier maintenance, as they face lighter loads.
What are three advantages of hydraulically fitted shaft coupling bolts compared to parallel interference fit bolts?
- Ease of Installation and Removal: Hydraulic bolts require minimal force and are easy to install and remove without damaging the components.
- Uniform Force Distribution: Hydraulic fittings evenly distribute force, reducing stress points and enhancing durability.
- Reduced Risk of Damage: No hammering or excessive force is needed, minimizing the chance of deformation or material damage.
With reference to propellers, what are:
(a) Two advantages of high skew?
(b) Two advantages of aft rake?
Advantages of High Skew:
- Reduces vibrations for smoother operation and less wear on the propulsion system.
- Minimizes stress concentrations, extending the propeller’s lifespan.
Advantages of Aft Rake:
- Improves thrust efficiency, reducing fuel consumption.
- Reduces cavitation, protecting the propeller from erosion.
With reference to propellers, explain
SKEW
Skew in a propeller blade refers to the curvature or backward sweep of the blade when viewed in the plane of rotation. Instead of being straight, a skewed blade is twisted or curved relative to the hub.
With reference to propellers, explain; RAKE
- Rake is the inclination of the propeller blades in the axial direction (forward or aft relative to the hub).
- Aft rake improves thrust efficiency and reduces cavitation.
- Forward rake enhances flow dynamics and reduces drag.
- Aft rake is commonly used to improve propulsion performance and minimize wear.
With reference to propellers, explain; Pitch
Pitch is the calculated distance the ship would move forward with one full rotation of the propeller under ideal conditions with no losses.
With reference to propellers, explain SLIP
Slip is the difference between the theoretical and actual movement of a propeller through water.
* Water resistance prevents the propeller from achieving perfect forward movement. * Higher slip means lower efficiency, while optimized slip improves propulsion effectiveness.
With Reference to Propellers
Two Reasons for Having High Skew
1. Reduction of Vibrations and Noise
A high-skew propeller reduces pressure fluctuations as each blade enters the water, minimizing vibrations.
This results in quieter operation, improving comfort on board while reducing structural stress on the vessel.
2. Enhanced Durability and Load Distribution
High skew helps distribute hydrodynamic forces more smoothly across the propeller blade.
This minimizes localized stress concentrations, reducing the risk of fatigue cracks and extending the propeller’s lifespan.
With Reference to Propellers
Two Reasons for Having Aft Rake
1. Improved Thrust Efficiency
Aft rake optimizes water flow around the propeller, increasing propulsion efficiency and reducing fuel consumption.
This is particularly beneficial for high-speed vessels where energy efficiency is critical.
2. Reduced Cavitation and Erosion Risk
Aft rake helps maintain a cleaner flow of water around the blades, reducing the occurrence of cavitation (vapor bubble formation).
Less cavitation means lower erosion risk, helping to maintain propeller performance over time.
Explain the emergency operation of a pneumatically operated friction clutch
A pneumatically operated friction clutch relies on compressed air to engage and disengage the clutch mechanism. In an emergency situation, the operation of the clutch must ensure rapid disengagement to prevent damage . Here’s how it typically works:
- Loss of Air Pressure – If the system experiences a sudden drop in air pressure, the clutch may automatically disengage due to spring-loaded mechanisms that push the friction plates apart.
- Emergency Release Valve – Many systems incorporate a manual or automatic emergency release valve that quickly vents air from the actuator, ensuring immediate disengagement.
- Fail-Safe Spring Mechanism – Some designs use a fail-safe spring that forces the clutch to disengage when air pressure is lost, preventing unintended engagement.
- Backup Air Supply – In critical applications, a secondary compressed air source or accumulator may be used to maintain clutch operation temporarily until the main system is restored.
- Manual Override – Certain systems allow for manual disengagement using a mechanical lever or override mechanism in case of pneumatic failure.
with reference to comparing modern water lubricated stern tube bearings with those that are oil lubricated explain advantages and dis-advantages
Here’s a comparison of modern water-lubricated stern tube bearings and oil-lubricated stern tube bearings:
Advantages of Water-Lubricated Bearings:
1. Environmental Friendliness: Water-lubricated bearings eliminate the risk of oil spills, making them more eco-friendly and compliant with stricter environmental regulations.
2. Lower Maintenance: They require less maintenance since there’s no need for oil changes or filter replacements.
3. Simpler Design: These bearings often have fewer components, reducing weight and manufacturing costs.
Disadvantages of Water-Lubricated Bearings:
1. Lower Load Capacity: Water has a lower lubricating capacity compared to oil, which can limit the load these bearings can handle.
2. Stricter Tolerance Requirements: They demand tighter clearances between the shaft and bearing material to maintain proper lubrication.
3. Potential Corrosion: Exposure to seawater can lead to corrosion, though modern materials and coatings help mitigate this.
Advantages of Oil-Lubricated Bearings:
1. Higher Load Capacity: Oil provides superior lubrication, allowing these bearings to handle higher loads.
2. Better Durability: Oil-lubricated systems are generally more robust under heavy-duty operations.
3. Established Technology: These systems have been widely used and refined over decades.
Disadvantages of Oil-Lubricated Bearings:
1. Environmental Risks: Oil spills can harm marine ecosystems, and even minor leaks can lead to regulatory penalties.
2. Higher Maintenance: Regular oil changes and seal replacements are necessary to prevent leaks and maintain performance.
3. Complexity: These systems often involve more components, increasing initial costs and potential points of failure.
Describe the checks carried out during the inspection of a main thrust bearing
Inspecting a main thrust bearing involves several critical checks to ensure its proper function and longevity. including
- Visual Inspection: Checking for signs of wear, scoring, or pitting on the bearing surface.
- Lubrication Assessment: Ensuring the oil supply is adequate and free from contamination.
- Temperature Monitoring: Measuring operating temperatures to detect overheating.
- Axial Clearance Measurement: Verifying that the bearing clearance is within acceptable limits.
- Vibration Analysis: Identifying any unusual vibrations that could indicate misalignment or damage.
- Wear Measurement: Evaluating the extent of wear using precision instruments.
- Structural Integrity Check: Inspecting the housing and support structures for cracks or deformation.
Regular inspections help prevent failures and ensure smooth operation.
Describe three types of wastage that may occur on the internal surfaces of a ships side valve that is made of nodular cast iron
Three types of wastage that can occur on the internal surfaces of a ship’s side valve made of nodular cast iron include:
- Corrosion – Exposure to seawater and oxygen leads to electrochemical reactions, causing material degradation over time. Nodular cast iron is more resistant than grey cast iron, but prolonged exposure can still result in pitting and surface weakening.
- Erosion – High-velocity fluid flow, especially in ballast or cooling systems, can wear down the valve’s internal surfaces. This is exacerbated by suspended particles in seawater, leading to gradual thinning of the material.
- Cavitation Damage – Rapid pressure changes within the valve can cause vapor bubbles to form and collapse violently, creating localized shockwaves that erode the metal surface. This is common in valves handling high-pressure fluid flow
Describe 2 reasons other than imbalance for propellers induced vibration
Two causes of propeller-induced vibration, aside from imbalance, are:
- Cavitation – When pressure drops around the propeller blades, vapor bubbles form and collapse violently, creating shockwaves that lead to vibration and structural damage.
- Wake Flow Disturbance – Uneven water flow from the hull or nearby structures disrupts the propeller’s movement, causing irregular forces that result in vibration.
State why centrifugal pumps aren’t self-priming.
Centrifugal pumps aren’t self-priming because they rely on liquid at the impeller to generate suction. If the pump casing is empty, air prevents proper fluid movement, making external priming necessary. Unlike positive displacement pumps, they cannot create enough vacuum to draw liquid into the system on their own.
State the purpose of the hunting gear in a hydraulic-hydraulic steering system
The hunting gear in a hydraulic-hydraulic steering system acts as a feedback mechanism that ensures precise rudder positioning. Here’s how it works:
- Rudder Position Feedback – It transmits the rudder’s actual position to the pump control lever, preventing overcorrection.
- Automatic Correction – When the rudder reaches the desired angle, the hunting gear adjusts the pump stroke to stop further movement.
- Smooth Steering – It prevents oscillations and ensures stable rudder control, improving maneuverability.
whilst a single screw vessel is on passage it is noticed that an intermediate shaft bearing is running hot
1. state FIVE possible causes
2. explain the procedure that should be followed in order to reach port for further investigation if there were no obvious causes
1. Five Possible Causes of a Hot Intermediate Shaft Bearing
1. Insufficient Lubrication – Low or degraded oil can increase friction and heat buildup.
2. Misalignment – Incorrect shaft alignment causes uneven loading and excessive wear.
3. Overloading – Excessive torque or load beyond design limits can lead to overheating.
4. Bearing Wear or Damage – Worn-out or defective bearings generate excess heat due to increased friction.
5. Cooling System Issues – Blocked or ineffective cooling pathways prevent proper heat dissipation.
2. Procedure to Reach Port Safely
If no obvious cause is found, follow these steps to minimize damage and ensure safe arrival:
- Monitor Temperature Trends – Continuously check bearing temperature and look for abnormal increases.
- Reduce Load & Speed – Lower engine power to reduce stress on the bearing.
- Increase Lubrication – Verify oil levels and circulation, adding lubricant if necessary.
- Check for Vibration & Noise – Listen for unusual sounds that may indicate worsening conditions.
- Prepare for Emergency Measures – Keep cooling systems operational and be ready for shutdown procedures if overheating becomes critical.
With reference to controllable pitch propellers
- describe the mechanism that changes the pitch of the blades
- explain how the pitch of the blades is indicated
1. Mechanism for Changing Blade Pitch in Controllable Pitch Propellers
Controllable Pitch Propellers (CPP) use a hydraulic system to adjust the blade angle dynamically. The key components involved in pitch adjustment include:
- Hydraulic Actuator – Located inside the propeller hub, it moves the blades by applying hydraulic pressure.
- Oil Distribution Unit – Directs hydraulic fluid to the actuator, controlling blade movement.
- Linkage & Pin-Slot Mechanism – Converts hydraulic force into rotational movement, adjusting the blade pitch.
- Control System – Uses sensors and feedback loops to regulate pitch based on operational needs.
This system allows the propeller to optimize efficiency under varying load conditions, improving maneuverability and fuel economy.
2. Indication of Blade Pitch
The pitch of the blades is indicated using:
- Mechanical Indicators – A physical scale or pointer linked to the pitch control mechanism.
- Electronic Sensors – Provide real-time feedback to the control panel, displaying pitch angle digitally.
- Hydraulic Pressure Readings – Changes in hydraulic pressure correspond to pitch adjustments.
- Bridge Control Display – Shows pitch settings for monitoring and adjustment by the operator.
With reference to intermediate shaft bearings of the roller type describe with the aid of a sketch each of the following
a) how some angular misalignment of the shaft is accommodated.
b) how longitudinal movement of the shaft is accommodated.
a) The spherical seating of the housing allows some angular movement therefore accommodating some misalignment.
b) The inner race and roller assembly is clamped to the shaft so as the shaft moves they move with it. The outer race is fixed into the housing and is wider than the rollers, allowing the rollers to move fore and aft.
Describe with the aid of sketches the fitting of a hydraulically tensioned bolt suitable for main propulsion shaft flanges
Fitting a Hydraulically Tensioned Bolt for Main Propulsion Shaft Flanges
Hydraulically tensioned bolts are used in propulsion shaft flanges to ensure a secure and high-strength connection while allowing for easy removal when necessary. The fitting process involves several key steps:
1. Preparation
- Ensure the flange surfaces and bolt threads are clean and free from debris.
- Apply a suitable lubricant to the bolt threads to reduce friction during tensioning.
2. Bolt Installation
- Insert the bolt into the flange hole and ensure proper alignment.
- Hand-tighten the bolt to engage the threads correctly.
3. Hydraulic Tensioning Process
- Attach a hydraulic tensioning device to the bolt.
- Gradually apply hydraulic pressure to stretch the bolt, increasing the clamping force between the flanges.
- Monitor the pressure gauge to ensure the correct tension is achieved.
4. Locking and Verification
- Once the desired tension is reached, secure the bolt using a locking mechanism if required.
- Verify the tension using a calibrated torque wrench or other measurement tools.
5. Completion
- Repeat the process for all bolts in the flange, following the recommended tightening sequence.
- Inspect the assembly to ensure uniform load distribution.
a) Sketch an arrangement for a transverse water jet thruster.
b) Explain how thrust is created in a water jet thruster
- Water Intake – Seawater is drawn into the thruster through an opening in the hull.
- Pump Acceleration – A powerful pump, typically centrifugal, increases the velocity and pressure of the water.
- Nozzle Direction – The high-speed water is directed towards the the vanes that act as a nozzle. the actuators open and close directing thrust in the required direction.
Describe with the aid of a sketch the operation of a transverse thruster that is hydraulically driven
In this design the prime mover drives a
variable displacement bi-directional hydraulic
pump whose speed and direction of delivery has
infinite variation. The fluid is delivered to a
hydraulic motor in the propeller hub so that the
only connection between the pump and the motor
are two pipes (flow and return).
