Propellers and Stern Tubes

8/19/2019 Propellers and Stern Tubes

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S t e r n

t u b e s


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Stern tubes


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Stern tubes (Sea Water lubricated)


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Stern tubesThe stern tube is normally constructed of cast iron slightly larger at

the forward end to ease removal. The forward end is flanged and

bolted to a doubler-plate stiffened aft peak bulkhead. The forwardend is supplied with a stuffing box and gland, the after end with a

bearing comprising lignum vitae staves or similar, and is dove tailed

into a brass bush. The wood is machined and cut on end grain. ie.

The staves in the lower bearing area are cut and fitted in such away

that the end grain is vertical to facilitate better life and staves at thetop part are cut with grain in the axial direction for economy. The

bearing can be lined with Lignum Vitae , rubber composition (cutlass

rubber or an approved plastic material (!ertain plastics possess

good bearing properties being inert and very tolerant of slow speed

boundary lubrication conditions. "or water lubricated bearing length

shall not be less than # x the diameter of the steel shaft. $f the

bearing is over %&'mm diameter forced water lubrication must be

used, a circulating pump or other source with a water flow indicator.

The shaft is withdrawn for examination every 3 years.


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Stern tubes


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Oil lubricated bearingsnlike for the water lubricated stern tube a shaft liner is unnecessary.

)enerally a small one is fitted in way of the aft seal bolted on to the

propeller boss. $n this way it excludes sea water contact with the main

shaft and provides an easily replaceable rubbing surface for the seal.Lined with white metal are to have a bearing length so as not to exceed a

bearing pressure from the weight of the shaft and propeller of * kg+cm.

The limitations of a bearing are the load it can withstand without metal

cracking or sueeing out and the temperature it can withstand without

melting. Length of bearing not less than / 0 in any case. !ast iron andbrone bearings must have a bearing length not less than #0. Lubrication

system must be capable of maintaining oil tightness despite varying

temperature. )ravity tanks fitted with low level alarms, sual for aft peak

to be filled with water to provide cooling low suction valve to be fitted to be

locked shut.1ear down for the white metal should not exceed mm to avoid

hammering out and the period for inspection is 2 years. 3 highly resilient

reinforced plastic may be used in place of the white metal. $t is claimed to

have greater load carrying capacity, high resistance to fatigue and shock

loading, with good lubrication properties. !eramic liners can also be used.


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Oil lubricated bearings


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Oil lubricated bearings1hen large propellers are fitted the heavy overhanging weight

greatly increases the load at the after end of the stern tube

breaking down the hydrostatic lubrication causing metal tometal contact and seiure towards the aft end of bearing. To

obviate this it is usual to angle the shaft downwards for about

&mm over 4''m length thus attempting to ensure than the

weight of the bearing is taken on the full length of the bearing.$t is good practice to leave the oil tank open to the stern tube

when in port with machinery stopped, this prevents sea water

leaking into the system. 5owever, water has been known to

contaminate lubricating oil systems causing rusting of tail shaft

particularly when shaft is stopped for periods long enough for

water to settle in bottom of bearing.

In ships with large changes in draught it is usual to fit two

gravity tanks. The upper tank is used when fully loaded or

there is water leaking in.


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Oil lubricated bearings

If outboard seal leaks the following ste!s are to be

taken• Fresh water in gravity tank to emulsify and coagulate it, oil

pumped around system to seal and lubricated .• Recharge with high viscosity oil

Disconnect oil supply line and reconnect to 45 gallon drum

which is supported by block and tackle in order to give avariable head. By raising and lowering the drum the oil

pressure in the system can be made to match the water

pressure from outside taking into account the difference in

gravities


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Sim!lex shaft seal


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simplex seal 3 very common arrangement for oil lubricated stern tube bearings. 3

simplex seal arrangement is fitted to both inner and outer ends.The replaceable chrome liner prevents damage to the prop shaft which

would be expensive to repair.

6ot shown is a rope guard bolted to the hull which prevents material from

being 7wound7 into the gap and damaging the seal. 8ope cutters may be

fitted with a fixed blade attached to the hull and a moving blade to thepropeller.

9il pressure is fed to the area between the two opposite facing seals.

This pressure is governed by the draught of the vessel and is often

supplied via tanks situated at set heights. This pressure balances the sea

water pressure on the seal and prevents sea water ingress, by opening

the correct tank the pressure exerted by the oil is insufficient to cause oil

to leakage out.

:tern tube seals with oil lubrication have tended to use rubber rings

increasingly. Viton with additives has been shown to be more effective

than nitrile butadiene rubber for seal rings


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"stablishing Shaft #entre line

3 telescope with cross wire is set up at the shaft centre line on the foreword end

of the engine room. 3 plain cross wire target is established at the aft end on the

same axis. 1ith both in use the centre of the engine room and the centre of the

shaft on the aft peak bulkhead can be marked prior to hole cutting for the stern

tube. The reuired centre of the aperture in the stern frame boss can then be

found by the line of sight using a crosswire in an ad;ustable spider.

8eplacement of the crosswire with a plug with a centre gives a location for a

divider to be used for marking off the boss for boring.

Telescope and the cross wire system can be used to check up the accuracy of

the boring operation, installation of the stern tube and siting of the shaft

bearings.


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Shaft alignment

3t the for7d end of the

engine room a light boxemitting light through a

pin hole is fixed from the

design height of the

crankshaft. sing the

sighting gear in sternframe boss with solid

piece fitted. The stern

frame boss is marked off

for boring . The solid

piece is then exchanged

for a sighting piece


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3 second sighting gear with sighting piece is fitted to the bore hole in the aft peak

bulkhead. This is ad;usted until the light source can be seen through the boss and

aft peak bulkhead sighting pieces. The sighting piece is replaced by the fixedpiece and the bulkhead may be machined. The stern tube is scribed out and the

p.c.d. of the bolts which will support the stern tube flange marked off. 3 similar

procedure is repeated for other bulkheads. 1hen boring out is completed the

stern tube is hauled into position, wood packing being fitted under the flange

before bolting up at the aft peak bulkhead, the external stern tube nut is screwed

up hard making a rigid connection at the after end. The tail end shaft is now fittedinto the stern tube, subseuent to the installation of bearing. The flange of the tail

end shaft is now the standard by which the remaining line shafting will be aligned.


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Shaft alignment

Taut wire method ($ilgrims wire)

!onsists of steel wire anchored above shaft at one end of system and

led over a pulley with suspended weight at the other end.

The height of the pulley and fixed anchorage are ad;usted so that they

are the same distance above the shaft and are positioned vertically

over the shaft centre line. 3 microstaff is employed to measure the

differences in height at bearing support points between shafts and wire,

an allowance being made for wire sag.

3 master inclinometer is employed to monitor ships movement during

the aligning process.


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Tailshaft keys and keyways

3brupt changes of shape of section cause stress concentrations to build up

due to interruption of the stress flow lines.

This build up in stress causes cracks to develop and supports crack

propagation. 1ith this in mind it can be seen that shapes or sections which

may be sub;ect to great stresses< should be well rounded or gradually tapered

off to give smooth stress flow.

St i t il h ft


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Stresses in tail shafts

0ue to the considerable weight of the propeller, the tail shaft is sub;ect

to a bending stress. There are however other stresses which are likely

to be encountered. There is a torsional stress due to the propellerresistance and the engine turning moment, and a compressive stress

due to the prop thrust. 3ll these stresses coupled with the fact that the

shaft may be in contact with highly corrosive sea water makes the

likelihood of corrosion attack highly probable.

"xamining a tail shaft and stern tube%efore the !eriodic ins!ection the bearing wear down should

be measured&

'fter shaft removed given thorough examination&

On water lubricated shafts the integrity of the fit of the brone

liner should be checked by ta!!ing with a hammer along itslength listening for hollow noise indicating a se!aration&

easure wear of shaft&

"xamine key way for cracks es!ecially the nut thread area&

re!lace rubber rings

Sh ft % i


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Shaft %earingsThe intermediate shafting if supported in plain or tilting pad bearings, has an

after most bearing which is lined top and bottom. 8oller bearings are installed in

some vessels.

$lain and tilting !ad bearingsThe shaft supported in a plain ;ournal bearing, will as it rotates, carry oil to its

underside and develop a film of pressure. The pressure build up is related to

speed of rotation. Thus oil delivered as the shaft turns at normal speed, will

separate shaft and bearing, so preventing metal to metal contact. =ressure

generated in the oil film, is effective over about one third of the bearing area

because of oil loss at the bearing ends and peripherally. Load is supported andtransmitted to the ;ournal, by the area where the film is generated. The

remaining two thirds area does not carry load

8eplacement of the ineffective side portions of the ;ournal by pads capable of

carrying load will considerably increase its capacity. Tilting pads based on those

developed by >itchell for thrust blocks are used for the purpose. ?ach pad tilts

as oil is delivered to it so that a wedge or oil is formed. The three pressurewedges give a larger total support area than that obtained with a plain bearing.

The tilt of the pads automatically ad;usts to suit load, speed and oil viscosity.

The wedge of oil gives a greater separation between shaft and bearing than

does the oil film in a plain ;ournal. The enhanced load capacity of a tilting pad

design permits the use of shorter length or less bearings.


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Shaft %earings


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S*+ (uff) cou!ling

9utside dia7s at end of

outer muff measured

before fitting

3fter fitted, dia7s should

be approx. '.*mm

greater 8estraining

devices must be fitted

to prevent the muffs

separating too uickly


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#ou!ling bolts

?longation of a bar produces a related reduction in cross sectional area.

3 bar with the same elastic properties in all directions will have a constant

relationship between axial strain and lateral strain. This is termed the =oissons

8atio and given by the symbol n.

3 bolt when tightened similarly causes a loss in area and diameter. $n a

clearance hole this is not a problem. 1ith a fitted bolt however, the positivecontact or 7fit7 between the accurately machined bolt and reamed hole would be

affected.

:haft coupling bolts are tightened to force the faces of the flange together so the

friction between the faces will provide some proportion of the drive. 5owever,

fitted bolt shanks are also designed to take a proportion of the drive. 3 clearance

bolt could provide the first reuirement but not the second. 3 fitted bolt whentightened and sub;ect to reduction in cross section would also fail on the second

count and probably be damaged by fretting. 3 tapered bolt may be used instead

of a conventional coupling bolt to obtain a good fit and reuired tightening.


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#ou!ling bolts$arallel shank fitted bolts

have $nterference fit in holes so that in the event of loss of frictional grip between

flanges then each bolt will take on eual share of the shear stress due to toruetransmission.

=arallel bolts become slack after one or two refits. Therefore taper shank bolts

have been used. 3n alternative is the sleeved coupling bolts.

The fit of the bolt is achieved by the tensioning of the taper shank bolt. :hould

wear occur in the sleeve then this can be renewed, reusing the rest of the

assembly


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,ydraulically fitted bolts&The pilgrim or margrip hydraulic bolt

uses the principal embodied by =oissonratio to provide a calculated and definite

fitting force between bolt and hole

!enter load rod fitted into hollow

coupling bolt and hydraulic head fitted.

5igh pressure oil pumped into head

pushing down, seal, piston and rod.This action stretches the bolt ( within its

elastic limit and reduces its diameter

sufficiently for a sliding fit into the hole.

"luid pressure is released allowing bolt

to expand and tightly grip within the holewith a radial grip of about .%2

@g+mm . :imultaneous longitudinal

contraction of the bolt having already

fitted the nut hand tight, exerts

considerable compressive force whichis about 4+ x greater than that which

$I-./I 01T


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$I-./I 01T


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$I-./I 01T'ssembly

=ropeller bedded to tail shaft and ;acked up to usual shop mark. The =ilgrim nut

is then screwed on the shaft with the loading ring against the prop boss. 1iththe lever operated, high pressure grease gun, grease is pumped into the inner

tube inside the nut at around 2'' bar, ( w.p. stamped on nut, not to be

exceeded, the prop will be pushed sufficiently up the taper to give the reuired

frictional grip. The pressure is then released and the nut is rotated until it is hard

up against the aft face of the prop hub and locked, fair water cone then fitted

/emoval 3fter removal of fair water cone and the locking plate, the pilgrim nut is

removed, reversed and together with a loose shock ring is screwed back onto

the shaft. 3 strong back is fitted and secured with studs to the prop boss.

)rease is now inserted to the system expanding the inner tube forcing the

loading ring, strong back, withdrawal studs and prop aft.


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#ontrollable $itch $ro!ellers


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#ontrollable $itch $ro!ellers

The !== consists of a flange mounted hub inside which a piston arrangement

is moved fore and aft to rotate the blades by a crank arrangement. The piston

is moved by hydraulic oil applied at high pressure (typically 4#' bar via an 9il

transfer tube (9T tube This tube has and inner and outer pipe through which

3head and astern oil passes. The tube is ported at either end to allow oil flowand segregated by seals.

9il is transferred to the tube via ports on the shaft circumference over which is

mounted the 9T box. This sits on the shaft on bearings and is prevented from

rotation my a peg. The inner bore of the box is separated into three sections.

The ahead and astern and also an oil drain which is also attached to the

hydraulic oil header to ensure that positive pressure exists in the hub andprevents oil or air ingress

The 9T tube is rigidly attached to the piston, as the piston moves fore and aft

so the entire length of the tube is moved in the same way. 3 feedback

mechanism is attached to the tube, this also allows for checking of blade pitch

position from within the engineroom.


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#ontrollable $itch $ro!ellers 3dvantages

'llow greater maneuverability'llow engines to o!erate at o!timum revs

'llow use of $TO alternators

/emoves need for reversing engines

/educed sie of 'ir Start #om!ressors and receivers

Im!roves !ro!ulsion efficiency at lower loads

0isadvantages

.reater initial cost

Increased com!lexity and maintenance re2uirements

Increase stern tube loading due to increase weight of assembly

the stern tube bearing diameter is larger to acce!t the largerdiameter shaft re2uired to allow room for OT tube

-ower !ro!ulsive efficiency at maximum continuous rating

$ro! shaft must be removed outboard re2uiring rudder to be

removed for all !ro! maintenance&

Increased risk of !ollution due to leak seals

Propellers and Stern Tubes

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