Propellors and Thrusters Flashcards
Explain the precautions that should be take prior to starting the thruster sketched above?
We need to ensure that there is zero thrust before starting otherwise during start up there will be
thrust generated each way. The prime mover is operated at a single speed and in a single direction
so the blade will always spin the same way. Thrust generated is proportional to the pitch. Varied
between maximum to port and maximum to STBD
Before starting the thruster it is important to check that the area around the tunnel is clear. The
damage caused from sucking in rope can be dangerous.
Describe, with the aid of a sketch, an electrically driven tunnel thruster?
The electrical motor (if directly connected to the propeller) will need to have frequency drive
controlling its rotational speed and direction. If the prop is not CPP then the load on the motor will
be significant.
With reference to a vessel with a single electro/hydraulic controllable pitch propeller?
How maneuvering may be maintain if the system fails?
The action to be taken should the hydraulic system irreparably fail whilst on route and the
blades assume zero pitch?
b)
blades assume zero pitch?
We can make the assumption that the hydraulic system is intact then we can take local control
of the system to manually change the pitch from the solenoid block. If the hydraulic pumps
have failed then the use of a manual hand pump which is installed can be used to adjust the
pitch accordingly until the system can be repaired. A clear communication with the bridge will
need to be established. This will likely be an open loop system, one where there is no feedback
so a close eye on the system will be required.
Depending on the manufacturer the failure of the system can force the pitch into the
maximum ahead position, enabling the vessel to get to a port where the system can be fixed.
b)There are a few options, this will depend on where the hydraulic failure is occurring.
Option 1. Local hand pump if the hydraulics downstream of the solenoid block are functional.
Option 2. Failure to full ahead where the push pull rod is moved against a large spring putting
the pitch into the full ahead position. Relief of the pressure in the piston enables the spring to
press this forward.
Option 3. Providing the rods come all the way through to the engine room a bolted plate can
be used to lock the blades by locking the control rods within the shaft.
Option 4. Generally the blades will tend to rotate to the direction of maximum pitch,
depending on their direction as a result of the hydrodynamic forces acting on the blades.
Therefore the blade will move to a position of increased pitch in the event of failure providing
there is no restraint on the pushrod.
with CPP - Why should there be a small amount of pitch when in the neutral position?
Generally due to the shape of the blades there is a tendency when run up in a zero pitch
position provide some thrust astern. Therefore the zero thrust position is slightly offset in the
fwd thrust to account for this.
Due to the blade pitch reducing with an increasing radius there may be some astern pitch at
the blade tips.
Sketch how a CPP is secured to the tail shaft?
The tail shaft of a CPP propeller is flanged to allow attachment of the CPP propeller hub. There must
be an approved seal arrangement (depending on the manufacturer) The flange is stepped to ensure
correct alignment of the hub.
Fitted bolts and dowels are used to ensure that we get a full and correct installation, the bolts
ensure that the faces are clamped together tightly so the bolts and the friction between the flanges
can handle the forces and thrust developed by the power train.
Explain how this is used to provide a correct fitting of a keyless propeller for a pilgrim nut
This is the most common way of securing a propeller onto a shaft, its by pressing the propeller onto
the taper using hydraulic jack, this jack is incorporated into the nut.
This jack is the form of an hydraulic nut with a loading ring expanded by a nitrile Tyre. (pilgrim nut)
The Tyre is pressurized and the ring moves our pushing the propeller onto the taper by a pre-
determined distance (to a shop mark). The pressure is then released and the nut is tightened up fully
and then locked into place. A Fairing cone is then placed around the fitting.
Describe two reasons, other than imbalance, for propeller induced vibration?
Surface finish of the propeller can cause an increased cavitation where the implosion of bubbles can
cause vibrations
Ventilation Air is sucked down into the blade causing and increase in the amount of slip and varied
loading onto the propeller.
Incorrect number of blades for the operation (too many)
Blade Tip Clearance, if the propeller tip and underside of the stern of the vessel is too tight we can
see pulses being formed that will induce vibrations into the hull
Speed of the propeller coupled with the main engine can align with the nodes of natural vibration of
the vessels hull causing significant vibration.
List 3 defects that can be found during an inspection of a fixed bladed propeller stating the possible
cause of each?
State the likely consequences if these are not rectified?
Cavitation - imploding of vapor bubbles penning the blade tip edge
Roughening - through service life, can be ground or polished out
Deformation - Bending of the blade
Edge Cracks - Can lead to large cracks if not treated correctly
Serious Edge Damage - May require a new blade section to be made to class approval
Cracks at the blade root - Could lead to the loss of the blade
Increased vibration which can lead to premature failure of shaft bearings and effect other ships
structure and components
Loss of efficiency
Loss of Power
Propeller imbalance
Destruction of bearings
6 advantages and 4 Disadvantages of controllable pitch propellers?
Faster Response
Greater Bollard Pull
Unidirectional Motor
Can absorb full engine power when stopping
The pitch can match the state of the hull and sea conditions to allow better fuel economy and higher
service speeds
Simple bridge controls
Increased maneuverability which on occasions can eliminate the need for a tug in port.
Expensive
Maintenance
More weight
Shaft needs to be removed externally
Increased vigilance in monitoring oil quality
More to go wrong with increased complexity
