Procedure For Ship Propeller Renewal
Procedure For Ship Propeller Renewal
Procedure For Ship Propeller Renewal
Propeller is the most significant machinery system of the ship. No matter what type of
propeller it is, ship owners never make compromises on such important equipment as it
directly affects the efficiency and safety of the vessel. Here we discuss about fixed pitch
keyless propeller renewal.
Why is propeller renewed?
There are various reasons for which a ships propeller could be renewed. Some of the
significant reasons are:
1) Damage to the propeller
2) Dynamically unbalanced
3) Renewing the existing propeller with a new propeller that is of better material, larger
blade area etc.
Once the intermediate shaft is removed and secured in a safe location in the ship, check
the condition of the plumber block bearings. Look for any white metal debris or contact
marks.
4) Dry fit: Actuate the high-pressure pump connecting the Pilgrim nut, allow the propeller
to slide in to certain distance and set the dial indicator to zero. The maker always
recommends initial load
5)Wet fit: Then actuate the high pressure pump connecting the propeller boss expansion
oil port and simultaneously actuate the nut pressure pump. Raise the pressure gradually
until the predetermined push up length is achieved (determination of push up length is
explained below)
6) Once the required push up length is achieved gradually release the pressure of boss
expansion port and then release the nut pressure
7) Remove all the connections and plug both propeller boss port and pilgrim nut port
8) Remember to take shaft jack up test post the installation
9) Verify the coupling alignment using sag and gap method
Alternatively, there are graphs provided by manufacturer, which can be used in lieu of the
above-mentioned formulae recommended by the class.
Permissible push up length can be captured for the corresponding temperature.
Conclusion
Propeller renewal is not about removing the existing propeller and just installing the new
one, there are many other significant factors to overcome. Following are important points
to be taken care of:
1) Good interference fit to transmit the torque: Damaging factor here is the varying
temperature and differential thermal expansion of the propeller hub (bronze) and
propeller shaft (steel).
Thermal expansion coefficient of bronze is about 10 and steel is about 7 (in 10 -6
in/Fahrenheit). This will affect the fitting, hence keeping all this in mind required push up
length is calculated such that the sufficient friction is achieved to transmit the torque.
2) Reduced stress on propeller hub and propeller shaft: Von misses stress should be less
than 0.7 times yield stress of propeller material. Von misses criterion is one of the most
commonly used failure theory.
To assess the von misses stress:
1) Derive the stress elements
2) Find the principal stresses (Mohrs circle)
3) From the principal stresses von misses stress can be derived (for 2d). If it is greater
than the yield strength, then the material fails. It is just a simple explanation about von
misses stress. Taking all these into account, maker represent the minimum push up
length graphically.
Note: The above description of propeller renewal may slightly vary with ships but
underlying concept is similar in all the cases.
Over to you..
Do you know about more points that can be added for the propeller renewal procedure?
Lets know in the comments below.
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When bearing is exposed with shaft in place for partial survey or seal
maintenance, by leveler gauges, or soft wood wedge driven between shaft and
bearing then measured with calipers.
In normal running condition, by lifting shaft and measuring the lift by dial gauge
(Less accurate). Case needed to avoid undue force. Assessment of poker gauge
readings can only be made if records of the previous readings are available and
the relative shaft clearance is known. Poker gauge readings taken in isolation
produce no reliable value of bearing clearance.
NOTE: When stem tube bearings are renewed or re-metallic, clearances will be back to
original. On refit, it is essential that these clearances are recorded and also the
corresponding new poker gauge readings for future comparison.
With most modem stem tube seal designs, the poker gauge measures from the seal box
OD to the seal sleeve OD. Hence, whenever the seal is fitted with a new sleeve or the
sleeve is machined, the poker gauge readings as fitted should be recorded in
conjunction with the measured clearance, by feeler gauges, calibration or both, since the
bearing will be accessible during seal overhaul.
1.2.3 Propeller shafts running in lignum-vitae or "Tufnol" type lined bearings water
lubricated:
NOTE: The stern gland should be repacked or at least partially repacked at each
periodical bottom survey.
1.2.4 Propeller shafts lubricated by grease.
1.3.2 Crack detection may be by dye penetrant method or magnetic particle method.
Magnetic particle inspection is preferred because it is more sensitive when properly
carried out.
In both cases the area to be checked must be thoroughly cleaned. Where possible a
qualified technician is preferred working to recognized standards.
The sensitivity of the magnetic particle inspection is easily checked using a field strength
indicator (burmah castrol strip).
If cracks are detected and not easily removed by light polishing it is advisable to check
the crack depth by ultrasonic before proceeding.
1.3.3 It is allowed to reduce the rule diameter by 3 % (corresponding to a decrease in
torsion strength of 10 %) by machining or grinding. Therefore if crack depth deeper than
1.5 % or shaft diameter (shaft being rule size) the shaft shall be rejected. Repairs of
corroded or cracked shafts within limits given above shall be smoothly ground out to
reduce stress concentrations to a minimum. The hollows should be filled with propriety
metal filler if in way of sealing rings etc.
Slight surface defects (corrosion or cracks) can be machined out provided that rule
diameter is not reduced by more than 3 %.
1.3.4 In the case of a propeller-shaft in normal steel and the propeller hub in stainless
steel, the holes of the propeller-flange and the propeller-flange itself have to be carefully
inspected for electric-chemical corrosion.
Seawater has to be avoided and adequate protection is to be foreseen.
Corrosion on the conical part of the propeller shaft may be repaired by machining the
taper.
This will result in the propeller moving forward which must be contracted by fitting a
spacer between the shaft couplings. The maximum thickness allowed for this spacer is
25 % of the intermediate shaft flange thickness. It is therefore the intermediate shaft
flange thickness which determines the maximum amount which can be machined off the
cone.
e.g. Intermediate shaft coupling flange thickness 100 mm, then maximum spacer which
may be employed = 25 mm, if propeller shaft taper = 1 in 12 the radial amount which may
be machined = 25/12= approx. 2 mm.
Surface contact of propeller bore to shaft cone should be checked using Prussian blue.
There should be a minimum of 70 % contact equally distributed.
NOTE: As well as rectification of the damage, the cause must also be determined and
repairs and preventative action taken to avoid a recurrence. The usual source of
leakages are from a badly jointed and sealed lairing cone on the ack of the propeller or
leakage past the sealing ring and/or gasket on the forward face - ensure the O ring is
the correct size so that compression and sealing is achieved.
All free spaces between propeller shaft cone, propeller boss, nut and propeller cap are to
be filled with a material insoluble in sea water and non-corrosive.
Arrangements are to be made to allow any air present in these spaces to escape at the
moment of filling. It is recommended to test these spaces under a pressure at least equal
to that corresponding to the immersion of the propeller in order to check, after filling, the
tightness obtained.
Other types of corrosion are the result of liner assembly. Liners are generally held to tail
shaft by shrinkage. Shafts are submitted to helical stress and it is unlikely that the same
distortions are equally applied over the whole length of liner.
Such fretting is often the cause of corrosion noted under aft extremity of liners where sea
water filters in. What has been said about shaft and liner relative motions is also true for
liner sections connected by (hammered or shrunk) red copper joints. Connections of two
adjacent liner sections are submitted to torque reactions and if copper joints no longer
ensure the necessary water tightness: corrosion is then frequent under joints and
adjacent area.
These measurements shall be taken alter machining of corroded or ribbed areas of the
liner. When damage (fissures or wear) is important and requires the renewal of the liner,
the Surveyor shall proceed as for the liner of a new shaft.
The new liner or liner sections ordered shall be submitted to the Societys factory
inspection test (quality of material, hydraulic test), and liner lit and good condition (no
defects) shall be checked alter machining.
When renewal of the liner is not necessary and it is difficult or impossible to change only
a section of it without removing the part of liner in good condition, it is often easier to
have the damaged part re-metallic.
Some yards can replace the damaged liner portion by two hail shells fitted on to the shaft
and welded together longitudinally. Any such repair shall be submitted to Technical Office
for approval.