Marine info 58: Propeller &Shaft Survey

Propeller and the stern tube is an independent survey items from the docking survey.

But the classification rules in docking survey say as follows: “The propeller and the after end of the stern bush are to be examined. The clearance in the stern bush and the efficiency of the oil gland should be ascertained. In the case where a controllable pitch propeller is fitted, it isto be ascertained that the pitch control device is in good working order, and,if considered necessary, the device is to be opened up for further examination."

11-1 Propeller Damage The biggest damage of propeller is the broken blades; while sailing, when abnormal vibration suddenly happens in the stern and continues, it may be caused because of the broken propeller blade. In this case,the engine should be stopped, and then the extent of damageto be checked and depending on the damage, the ship shouldvisit to the nearest port for an Occasional Survey. If more than one-third of the blade is broken, there is apossibility that the damage also occurred to the stern tube.In the dry docked not only propeller but stern tube should becarefully examined. Sometimes propeller shaft should be withdrawn. The causes of this damage are contacting with a submerged or floating object or small material defect during manufacturing which develop to hair cracks, and or metal fatigue.

11.1.2 Bend of blade Bend occurs because of impact of the propeller with other objects; they can bedetected easily even before the propeller is cleaned.

11.1.3 Cracks Small cracks are overlooked in many cases. They cannot be detected unless staging are erected and after cleaning of each blade. Even the hair cracks they have a possibility of expanding and breaking the propeller blade. Thse hair cracks are discovered by dye penetration test.Generally this test need not be carried out for the entire surface of the blade; it is customary to check only the area from the root to 0.4R of the blade. In the previous dock if a stop-hole has been drilled at the crack end and the hole has been filled with a wooden plug, pay attention whether crack ia extended or not. In this case refer to previous Survey Record on this matter.

Photo.11-2 Blade broken Photo.11-3 Blade bent

11.1.4 Corrosion Similar to the rudder, the propeller blade may be subjected to spongiform corrosion (erosion) due to cavitation. There is no good repair method; the surface can be smoothed using a grinder, or depending on the position, the blade can be built up by welding, If the corrosion is severe, the corroded part may be cut out and using the approved material and repaired by welding. These welding repairs should be entrusted to the propeller manufacturers because welding of copper alloys is very difficult.

Photo.11-4 Hair crack on the blade

Photo.11-5 Blade errosion

11-2 Inspection of Blades According to data on cracks and broken blades, the blade is frequently cut at the location called the P point; therefore, this part should be inspected withparticular care. The P point lies on the pressure side of the blade, and it isthe point where the thickness of the blade is maximum and where the rounded radius of the boss terminates. The indication of the position on the propelleris similar to the concept of the frame space indicating the position of the frame and beam shown in the figure 11-1 below.

The blade is divided into parts formed by measuring arcs from the centre of the propeller at every 10% of the radius of the propeller, such as 0.4R, 0.7R. The names of the blades are generally assigned as A, B, C, D and E or (1), (2), (3), (4), and (5) in case of five-bladed propeller. The name of each blade is engraved at the root of the blade. It can be find easily after green algae or dirt on the propeller blade has been removed. The area shown in the figure should be inspected with special care. After polishing the surface using a disk sander along the length of the blade, the surface shouldbe examined using a hand magnifying glass or by performing the dye penetrant test. For details of the dye penetrant test,

Fig.11-1 Propeller Nomenclature (1)

Fig.11-2 Propeller Nomenclature (2)

11.3 Propeller Repair The surface of the propeller blade is divided into three regions: A, B and C. The table below shows the kinds of repairs that can or cannot be carried out in each region.

However, only typical damage and inspection procedures are covered here.

Fig.11-3 The area for dye penetration test

Fig.11-4 Propeller bladeR = Radius, Ct = Chode lenght in rudius r

Table 11-2-2 Kind of repair

11.4 Fall of Guard Ring A guard ring (also called a rope guard) is fitted between the propeller front face and the stern frame to prevent floating objects such as fishing nets frombeing fouled with the propeller shaft. The guard ring is a split ring welded

to the boss of the stern frame. It can come off easily if an object hits it. Aguard ring is not prescribed by the Rules, but if it has fallen off, it must be replaced by manufacturing a new ring and fitting it in place.

Fig.11-4 Gurd ring

12.5 Clearance Measurement The clearance between the propeller shaft and the bearing should be measured during the bottom inspection. There are two methods of measuring clearance, depending on the water—sealing method for the shaft. For the rudder, the clearances in the longitudinal (fore-aft) and transverse (p-s) directions of the rudder shaft are measured, but for the propeller shaft, because the lower surface of the shaft is in contact with thebearing, only the clearance of the upper surface of the shaft has to be measured in case of a water--lubricated system.

Fig.11-5 Clearance of stern tube bearingFor an oil lubrication system, also measure the clearance at the lower surface.However, for oil—lubricated systems, clearances of the upper and lower surfaces have to be measured, because the shaft sinkage is small. Clearances in the transverse direction need not be measured because the shaft is rotatingall the time.

11.5.1 Clearances of sea water—lubricated bearings Similar to measuring the clearance of the rudder pintle, remove the guard

ring, insert the feeler gauge or the measuring wedge from the stern tube side and measure the clearance. Before the lignumvitae dries out after the ship is drydocked, measure the clearance of the upper surface and record the results of the measure-ments in the Inspection Record Form M-1. For a ship with twin shafts, item 1. in the form is for the propeller on starboardside, and item 2. is for the port side. If a twin—shaft ship has shaft brackets, enter the measurements in the lower Dart of the form

Fig.11-6 Measurement of Clrarance

Table 11-2 Results of mesurement (1)The allowable maximum clearances according to shaft diameter are given below. If the values below are exceeded, the stern tube bearing material should be replaced or repaired.

Table 11-3 Allowable max. clarance12.5.2 Weardown of oil—lubricated bearings Unlike lignumvitae used in water—lubricated bearings, metal can be used in oil—lubricated bearings, and the clearance between shaft and bearing can be reduced. Wear is also small, and most modern ships use oil—lubricated bearings. Clearances should be measured at the same shaft positions as the previous measurement. To specify the shaft position, use the position of

the propeller blade or the position of the main engine piston. Generally, thisposition is specified according to the position of the propeller blade; for example, measure clearance with blade A at the top. If no hole for measuring sinkage is provided in the guard ring, the guard ring should be removed, the screwed cap of the measuring hole removed, and a measuring instrument such as Vernier calipers, so—called wear down gauge, inserted and distances from the bearing to the upper and lower surface of the shaft measured. The measued records is to be entered in the survey report showing the position of measured point sucha sa the Key Top or No. 6 Crank Top.; however, unless the bonnet is removed, the key position cannot be judged; therefore, it is convenient to take a specific blade (cylinder) as the reference for sinkage measurement. In large ships today, propellers are generally keyless. Sinkage is the difference in measurement at the time of inspection and measurement when the ship was built. The standard limit for sinkage is 0.3 mm irrespective of the shaft diameter. Sinkage should be determined by studying the properties of lubricating oil, and the history of temperatures of the lubricating oil and bearing material. An example of the Inspection Record is shown below.

Photo.11-6 Measuring clearance

Example of position propeller shaft

Fig.11-7 Position of propeller blade

Table 11-4 Results of measurement (2)

(1) Example of measuring clearance in a oil-lubricated bearing

(1) Original(2) Overhaul of guard ring and cap bolt

(3) Inserting the gauge(4) Measurement

Fig. 11-5 Measuring of propeller shaft clearance

(2) Example of measurement results for sinkage Measurement results for two ships and their graphs are provided below for reference. The part above the kinked line shows the top, and the part below the kinked line shows the bottom measurement values. The position of the blade does not conform to the key top position but the No. 8 piston top position.Ship (1) : Tanker, 72,368G/T,built in 1975

Month Measurement Month Measurement4/1977 114.30114.85 6/1978 115.40/114.75

9/1979 115.50/114.80 11/1980 115.50/114.80

5/1982 115.60/114.80 9/1983 115.30/114.70

5/1985 114.90/114.70 11/1986 114.90/114.60

10/1988 114.90/114.60 10/1990 115.10/114.90

9/1992 115.40/115.00

Ship (2) : Tanker, 43,444G/T, built in 1975

Month Measurement Month Measurement12/1981 72.7/73.1 4/1984 72.6/73.25/1986 72.9/73.1 6/1988 73.3/73.34/1990 73.4/73.3 5/1992 73.3/73.4

Fig.11-6 Results of measurement (3)

11.6 Inspection of Stern Tube Seal Oil leaks from the propeller boss in the stern frame of an oil—lubricated system can sometimes be detected during a bottominspection. This is probably due to a defective seal,therefore, a detailed examination of the seal is necessary.

Fig.11-7 Oil leakage from propeller boss

11.7 Bow Thruster and Side Thruster These items are ship owner’s options, therefore, they need not be inspected under class requirements. However, if these items are installed on the ship, they should be inspected at the docking survey. Frequently observed damage includes damage to guards at sea water ports due toimpact with submerged or floating objects, and bent propeller blades. If the shaft seal is defective, water entering to the ship; however, such incidents are not reported. Inspection results should be entered in the Survey Report.When the stainless plate is used in the surface of the nozzle, the corrosion of adjacent steel plate should be carefully inspected.

Fig.11-8 Damage of side thruster guard

Fig.11-9 Damage to side thruster

Read Also1. Propeller Shafts

1.1 Propeller Shaft Clearances - Measurement

Periodical docking surveys and periodical propeller shaft surveys are incomplete without propeller shaft clearances. These may be determined as bellows:

When the shaft is removed for survey or maintenance, by calibration of journals and bearing bores.

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.

Normal running condition, by poker gauge comparing readings with previous measurements.

In normal running condition, by lifting shaft and measuring the lift by dial gauge (Less accurate). Case needed to avoid undue force. Assessmentof poker gauge readings can only be madeif records of the previous readings areavailable and the relative shaftclearance is known. Poker gauge readingstaken in isolation produce no reliablevalue of bearing clearance.

NOTE: When stem tube bearings are renewed orre-metallic, clearances will be back tooriginal. On refit, it is essential that theseclearances are recorded and also thecorresponding new poker gauge readings for future comparison.

With most modem stem tube seal designs, the poker gauge measures from the sealbox “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 Propeller Shaft Clearances - Initial and Maximum Allowed

1.2.1 The following clearances are intendedfor the guidance of Surveyors.

Recommendations of designers and manufacturersmay differ and the Surveyor should take noticeof these in deciding whether departures fromthe values given here are justified.

In the case of initial clearances drawingsshould be checked and for special materialssuch as “Tufnol” the manufacturer’s technical information should be heeded to allow for expansion of the bearing material on submersion in water.

1.2.2 Propeller shafts running in white metal bearings - oil lubricated.

1.2.3 Propeller shafts running in lignum-vitae or "Tufnol" type lined bearingswater 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 Crack detection of propeller shafts

1.3.1 Area to be crack detected.

1.3.2 Crack detection may be by dye penetrant method or magnetic particle method. Magnetic particle inspection is preferred because it is more sensitivewhen 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 ofsealing rings etc.

Slight surface defects (corrosion or cracks) can be machined out provided thatrule 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-flangeitself have to be carefully inspected for electric-chemical corrosion.Seawater has to be avoided and adequate protection is to be foreseen.

1.4 Repairs to Propeller shaft cone

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 maximumamount 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 compressionand 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.

1.5 Protection of propeller shaft against corrosion

Arrangements are to be made to allow any air present in these spaces to escapeat 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.

1.6 Propeller shafts with bronze liners

1.6.1 General

Always check carefully the inboard part of the shaft where water from the stern gland may have caused corrosion cracking, characteristic X-shaped fissures particularity on the coupling Flange filled. This part of the shaft is best protected by a special coating.

Beware also of electro-chemical corrosion in the region of the end of the liner.The bronze liners protect those parts of the shaft which would otherwise be incontact with sea water. 

Continuous liners -either in one piece or in several sections cover the shaft from forward propeller boss to forward part of the stern gland. Parts of linermost liable to wear are those in way of stern gland packing, or the liner can be eroded by vibration or hammering of the shaft. Su objected to torque reaction with the shaft; the liner can crack (on its surface, or in other places). Cracks may also result from occasional over-heating. Sea water can seep through the cracks and cause rapid corrosion of the shaft.

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.

1.6.2 Checking the fit and condition of liners

The Surveyor shall hammer test the liner and joints between sections - bearingin mind that liners may be chamfered beyond the bearing surfaces and will, therefore, give a different sound, not to be mistaken with that of a slackening liner.Slackness is usually noted at extremities of liner. It may be caused by heating (of aft bearing, gland) or cracks in liner.

The Surveyor shall also examine carefully the surface of the liner for cracks or porosity, and if in doubt, he will carry out a non-destructive check.Any cracking or slackening of the liner (especially all, next to propeller) orloosening of copper joints will result in corrosion of the shaft.

s for such defects, loose copper joints will need to be withdrawn and the liner surface in way machined off and other corroded areas should also be machined so that the shaft surface can be inspected.The Surveyor will also check the bearing surface and wear in way of bush(es) and stern gland.

1.6.3 Renewal/Repairs of liners

Cracks, wear

Fully penetrating cracks always imply renewal of damaged part of liner.Fissures can be machined down provided that liner's thickness remains within the limits given below for wear, otherwise the damaged part shall be renewed.Major wear means that liner is unlit for use; the maximum wear allowed being as follows:

25 % of rule thickness in way of the bearing area. 50 % of rule thickness in way of out the stern-gland.

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 Society’s 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.

1.6.4 Ribbing in way of the stern-gland

Ribbing requires the complete - or part renewal of the liner only if thicknessinside of grooves is inferior to hall the regulation thickness. When ribbing is slight, however, i.e. when depth of grooves is 3 mm, liner shall be machined over the whole length in way of aft stuffing box.Slight ribbing needs only to be filed down or smoothed away with emery cloth.

What Is Propeller Drop and How to measure propeller drop?

What is propeller drop?

The propeller shaft is having inboard and outboard seals. It sealsagainst the bronze liner shrunk fit around the cast iron propeller shaft. After some years it creates groves on them and naturally looses sealing and sea water can come inside. These reduce the lubrication and create wear if thebronze liner have enough clearance the shaft will come down by certain amount because of the propeller weight. This drop in propeller shaft is termed as propeller drop.

How measure propeller drop?

Propeller drop' is actually amount the stern tube bearing wears down and is measured by a poker gauge which is placed between the last and second last stern tube seals (aft- near the propeller). The reading is taken every dry dock and recorded. A hole is provided on top of the seal whichis closed by a plug. This plug is removed the poker gauge is inserted and a reading taken. So since the propeller is connected to the shaft and the poker gauge measures how much the shaft has 'dropped' due to the wear down of the bearing on which the shaft runs.

Reference http://docking.ship-doctor.com

http://marinesite.blogspot.com/

http://marinesurveypractice.blogspot.com/2013/01/surveyor-guide-note-on-tailshaft-survey.htmlKeep in touch

Ahmed Samir Ghowel

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