Propeller and shafting
Sealing arrangement for oil lubricated stern tube.
Shaft Coupling
SKF or oil injection fitting
Propulsion efficiency enhancement Technologies
Reduction gear
Questions and Answers
Fixed pitch: - The pitch at any particular point on a blade is however fixed and an average value of the complete propeller is used for calculation.
Vibration: -The propeller when turning in the ship’s wake, is a potential source of vibration excitation. To some extent, this can be minimized by having the leading edge skewed back. Skew back is an advantage when the propeller is working in a varying wake as not all the blade is affected at the same time, vibrations in the thrust and torque are therefore smoothed out. Since the vibration is blade excited, then the number of blades is significant and determines the vibration frequency.
The surface of each blade when viewed from aft called the face is its driving surface when producing an ahead thrust. The other surface of the blade is called the back. The leading edge of the blade is the edge which when the ship is driven ahead, first cut the water; the other edge is termed as the trailing edge. In twin-screw ships, the starboard propeller is normally Right-handed and the port propeller is left-handed. Thus the propellers are outward turning. In this way, the cavitation is reduced.
Pitch (P): - If the propeller is assumed to work in an unyielding fluid then in one revolution of the shaft the propeller will move forward a distance which is known as the pitch.
Diameter(D): - the diameter of the propeller is the diameter of the circle or disc cut out by the blade tips. The theory of a Screw is best followed out by assuming it to be an inclined plane that would round to form a spiral on a cylinder. The baseline representing the circumference of the cylinder and the perpendicular representing the advancement of the pitch which we can see the hypotenuse becomes the thread of the screw the angle the screw line makes with the base then becomes the pitch angle. To find the pitch of the propeller, turn one of the blades to the horizontal, mark off a suitable radius at about the broadest part of the blade, and hang a pair of plumb bobs over the blade at this radius. Next place a straight edge parallel to the shaft by using a spirit level and just touching the leading edge at the plumb line. The distance between the plumb line BC represents the part pitch PP and the distance at right angles from the straight edge to the following edge AB is the part circumference PC, next to work out the whole circumference WC which is equal to 2πR. Part circumference(BC)/ part pitch(AB) = whole circumference 2πR/ whole pitch therefore whole pitch is equal to 2πR x BC/AB, The pitch angle is the acute angle which the driving face of the blade makes, with the plane of the propeller.
Propeller material:-
NIKALIUM or "Nickel aluminum bronze" or "Aluminum copper Nickel carbon"
Typical composition of NIKALIUM
Copper: - 80.2%
Aluminum: - 9.3%
Nickel: - 4.3%
Iron: - 5.0%
Manganese: - 1.2%
Mechanical properties: -
Tensile properties: -
2% proof stress- 27.4 kg/ mm square.
Tensile strength- 70 kg/ mm square.
Elongation- 27%
The direction of rotation:-
Which viewed from the stern, If the proper revolve in clock-wise direction when going ahead is known as a right-handed propeller.
Propeller Boss:-
To obtain the smooth flow, cone cover (fairwater cone) or the cone nuts are arranged on most propellers. The dimension of the boss is in relation to the shafting, keeping the rope guard in consideration.
The propeller can be divided into three categories:-
(a) Fixed pitch propeller.
(b) Controlled pitch propeller.
(c) Directional pitch Propeller.
Fixed pitch propeller
(a) Solid propellers: - Propeller cast the blades integral with the boss, machine and tapered bore and faces of the boss grooves cut in the surface.
(b) Build up propeller: - blades cast separately and secured to boss by studs and nuts.
Advantage of build-up propeller: - Easily replaceable damaged blades, ability to adjust the pitch, less repairing cost. To adjust the pitch slotted holes are provided.
Disadvantages of build-up propeller: - loss of efficiency, resulting from the restricted width at the blade root, the greater thickness required to maintain strength and large hub diameter. Blades may slacken, varying pitch leads to vibrations.
This tube also conveys the operating oil from the oil distributing box inboard to the control valve. The forward end of the shaft tube connects with a key which is moved forward and aft by a sliding ring within the oil distributing box. An auxiliary servomotor mounted externally to the box is used to move the sliding sleeve through a fork mechanism. Oil pressure is supplied to the system by means of electrically driven or shaft driven screw or gear pumps.
Operation: - Movement of 32 on the bridge actuates piston 'B' opening valve 'A'. H.P oil is now admitted to 25 which operates lever 24. This operates pin 38 which slides rod 17 along to open the oil port to piston 7. At the same time the movement of 25 lifts piston C opening valve 22 admitting h.p oil to the passage in 17 through which it reaches the main piston7; the movement this operates pin 9 which being eccentric to the centre line of the blade, turns the blade so changing the pitch.
Advantages:- Ability to change pitch in the event of a bent shaft. A very high force involved in pitch changing operation is constrained within the boss. In the unlikely event of electric or hydraulic failure, a spring/ a series of springs (large propeller) moves the blades into a full ahead pitch.
A shaft generator can be driven at constant ship speed while allowing at the same time a change in the ship's speed. Since possible to reverse the pitch completely, a Unidirectional engine, required to give full ahead or astern thrust, when manoeuvring, best for ferries, tugs etc.
The old method of attaching the propeller to the tail shaft was by taper and Key. Keyways are made in the shaft taper and in the bore of the hub. A taper 1 in 12 is used sometimes 1 in 15. The propeller hub and Shaft taper are accurately matched so that a good "interference fit" is obtained. A contact of 70-80% is considered necessary. In addition to this bedding, a key is fitted on a milled keyway and secured in place with the help of some Screw The key must be thoroughly bedded on the side of the keyway and must be clear on top from the propeller boss. A slack-key will result in hammering action which may result in the rapture of the shaft. A key having no clearance on top will have the tendency to keep the boss off the taper and as such the friction of the taper is lost and the entire torque transmission will come on the key which will share off in no time The propeller hub would be stretched by being forced past the point of fit on the shaft taper, by propeller nut. The push up of a few millimetres is calculated to give a good interference fit. Torque in ideal condition is transmitted totally by the interference fit with the Key being merely a backup. If conditions are not as intended, fatigue crack can occur at the forward end of the Key and more serious fatigue crack may result from fitting damage articles in high powered single screw ships. The keyway due to its form act as a stress riser. It is important that as far as possible there will be no sharp edges in the keyway to prevent stress concentration, which may ultimately result in cracking of the shaft. The development of the key design has to be done in many stages. i. Key with Square edge rectangular shape. ii. Expansion grooves are cut on the corner which prevents stress concentration in the corner but the bottom is still square the cracks would develop at the bottom. iii. Rounded kiwi which avoids stress concentration in the upper corner but the inside corner was still vulnerable. iv. The present-day design of the key has circular edges at the top and boat ended inside. This key was found to be most suitable.
[B] Keyless propeller.
The success of a keyless propeller depends on the accuracy of the hub and shaft tapers (85%) and the correct grip from the stretching of the propeller hub on the shaft. It must ensure adequate grip despite any temperature changes and consequent differential expansion of the bronze hub and steel shaft. It must also avoid overstressing of the hub and in particular any permanent deformation.
Attachment of propeller to the tail shaft: -
1. (a) assemble- steam heating of boss so metal to expand.
(b) withdraw- to take the weight of the propeller by means of Sling passed over the quarter of the ship, shaft coupling bolt removed. Two shores are placed between the back of the tail and coupling and the face of the stern tube. To slacken back the propeller nut on the tail shaft about 12mm. (note:- left-hand Threads on right-hand propeller). Now inserting long Steel wedges between the popular boss and sternpost and hammering it. Boss may be carefully heated. 2. (a) assemble- steam heating of Boss so metal to expand.
(b) withdraw- the boss of the propeller has two holes tapped in the after face so that a plate may be placed across the thimble point of the shaft and a good pull then be exerted to draw the propeller aft and trust the tail end shaft forward.
A. Sketch a section through a key-less sleeved propeller.
Shaft Coupling
SKF or oil injection fitting
Propulsion efficiency enhancement Technologies
Reduction gear
Questions and Answers
Theory of propeller
The propeller consists of a boss which has several helicoidally form blades, which rotated in the screws or thrusts their way through the water by giving Momentum to the column of water passing through it. The thrust is transmitted along the shafting to the thrust block and finally to the ship structure.Fixed pitch: - The pitch at any particular point on a blade is however fixed and an average value of the complete propeller is used for calculation.
Vibration: -The propeller when turning in the ship’s wake, is a potential source of vibration excitation. To some extent, this can be minimized by having the leading edge skewed back. Skew back is an advantage when the propeller is working in a varying wake as not all the blade is affected at the same time, vibrations in the thrust and torque are therefore smoothed out. Since the vibration is blade excited, then the number of blades is significant and determines the vibration frequency.
The surface of each blade when viewed from aft called the face is its driving surface when producing an ahead thrust. The other surface of the blade is called the back. The leading edge of the blade is the edge which when the ship is driven ahead, first cut the water; the other edge is termed as the trailing edge. In twin-screw ships, the starboard propeller is normally Right-handed and the port propeller is left-handed. Thus the propellers are outward turning. In this way, the cavitation is reduced.
Pitch (P): - If the propeller is assumed to work in an unyielding fluid then in one revolution of the shaft the propeller will move forward a distance which is known as the pitch.
Diameter(D): - the diameter of the propeller is the diameter of the circle or disc cut out by the blade tips. The theory of a Screw is best followed out by assuming it to be an inclined plane that would round to form a spiral on a cylinder. The baseline representing the circumference of the cylinder and the perpendicular representing the advancement of the pitch which we can see the hypotenuse becomes the thread of the screw the angle the screw line makes with the base then becomes the pitch angle. To find the pitch of the propeller, turn one of the blades to the horizontal, mark off a suitable radius at about the broadest part of the blade, and hang a pair of plumb bobs over the blade at this radius. Next place a straight edge parallel to the shaft by using a spirit level and just touching the leading edge at the plumb line. The distance between the plumb line BC represents the part pitch PP and the distance at right angles from the straight edge to the following edge AB is the part circumference PC, next to work out the whole circumference WC which is equal to 2πR. Part circumference(BC)/ part pitch(AB) = whole circumference 2πR/ whole pitch therefore whole pitch is equal to 2πR x BC/AB, The pitch angle is the acute angle which the driving face of the blade makes, with the plane of the propeller.
Propeller material:-
NIKALIUM or "Nickel aluminum bronze" or "Aluminum copper Nickel carbon"
Typical composition of NIKALIUM
Copper: - 80.2%
Aluminum: - 9.3%
Nickel: - 4.3%
Iron: - 5.0%
Manganese: - 1.2%
Mechanical properties: -
Tensile properties: -
2% proof stress- 27.4 kg/ mm square.
Tensile strength- 70 kg/ mm square.
Elongation- 27%
The direction of rotation:-
Which viewed from the stern, If the proper revolve in clock-wise direction when going ahead is known as a right-handed propeller.
Propeller Boss:-
To obtain the smooth flow, cone cover (fairwater cone) or the cone nuts are arranged on most propellers. The dimension of the boss is in relation to the shafting, keeping the rope guard in consideration.
The propeller can be divided into three categories:-
(a) Fixed pitch propeller.
(b) Controlled pitch propeller.
(c) Directional pitch Propeller.
Fixed pitch propeller
(a) Solid propellers: - Propeller cast the blades integral with the boss, machine and tapered bore and faces of the boss grooves cut in the surface.
(b) Build up propeller: - blades cast separately and secured to boss by studs and nuts.
Advantage of build-up propeller: - Easily replaceable damaged blades, ability to adjust the pitch, less repairing cost. To adjust the pitch slotted holes are provided.
Disadvantages of build-up propeller: - loss of efficiency, resulting from the restricted width at the blade root, the greater thickness required to maintain strength and large hub diameter. Blades may slacken, varying pitch leads to vibrations.
CPP
It has the operating servo motor positioned outboard in the hub body. the servomotor control valve is also the hub body and regulated by a tube down the hallow propeller shafting.This tube also conveys the operating oil from the oil distributing box inboard to the control valve. The forward end of the shaft tube connects with a key which is moved forward and aft by a sliding ring within the oil distributing box. An auxiliary servomotor mounted externally to the box is used to move the sliding sleeve through a fork mechanism. Oil pressure is supplied to the system by means of electrically driven or shaft driven screw or gear pumps.
Operation: - Movement of 32 on the bridge actuates piston 'B' opening valve 'A'. H.P oil is now admitted to 25 which operates lever 24. This operates pin 38 which slides rod 17 along to open the oil port to piston 7. At the same time the movement of 25 lifts piston C opening valve 22 admitting h.p oil to the passage in 17 through which it reaches the main piston7; the movement this operates pin 9 which being eccentric to the centre line of the blade, turns the blade so changing the pitch.
Advantages:- Ability to change pitch in the event of a bent shaft. A very high force involved in pitch changing operation is constrained within the boss. In the unlikely event of electric or hydraulic failure, a spring/ a series of springs (large propeller) moves the blades into a full ahead pitch.
A shaft generator can be driven at constant ship speed while allowing at the same time a change in the ship's speed. Since possible to reverse the pitch completely, a Unidirectional engine, required to give full ahead or astern thrust, when manoeuvring, best for ferries, tugs etc.
Propeller attachment
[A] keyed propeller:-The old method of attaching the propeller to the tail shaft was by taper and Key. Keyways are made in the shaft taper and in the bore of the hub. A taper 1 in 12 is used sometimes 1 in 15. The propeller hub and Shaft taper are accurately matched so that a good "interference fit" is obtained. A contact of 70-80% is considered necessary. In addition to this bedding, a key is fitted on a milled keyway and secured in place with the help of some Screw The key must be thoroughly bedded on the side of the keyway and must be clear on top from the propeller boss. A slack-key will result in hammering action which may result in the rapture of the shaft. A key having no clearance on top will have the tendency to keep the boss off the taper and as such the friction of the taper is lost and the entire torque transmission will come on the key which will share off in no time The propeller hub would be stretched by being forced past the point of fit on the shaft taper, by propeller nut. The push up of a few millimetres is calculated to give a good interference fit. Torque in ideal condition is transmitted totally by the interference fit with the Key being merely a backup. If conditions are not as intended, fatigue crack can occur at the forward end of the Key and more serious fatigue crack may result from fitting damage articles in high powered single screw ships. The keyway due to its form act as a stress riser. It is important that as far as possible there will be no sharp edges in the keyway to prevent stress concentration, which may ultimately result in cracking of the shaft. The development of the key design has to be done in many stages. i. Key with Square edge rectangular shape. ii. Expansion grooves are cut on the corner which prevents stress concentration in the corner but the bottom is still square the cracks would develop at the bottom. iii. Rounded kiwi which avoids stress concentration in the upper corner but the inside corner was still vulnerable. iv. The present-day design of the key has circular edges at the top and boat ended inside. This key was found to be most suitable.
[B] Keyless propeller.
The success of a keyless propeller depends on the accuracy of the hub and shaft tapers (85%) and the correct grip from the stretching of the propeller hub on the shaft. It must ensure adequate grip despite any temperature changes and consequent differential expansion of the bronze hub and steel shaft. It must also avoid overstressing of the hub and in particular any permanent deformation.
Attachment of propeller to the tail shaft: -
1. (a) assemble- steam heating of boss so metal to expand.
(b) withdraw- to take the weight of the propeller by means of Sling passed over the quarter of the ship, shaft coupling bolt removed. Two shores are placed between the back of the tail and coupling and the face of the stern tube. To slacken back the propeller nut on the tail shaft about 12mm. (note:- left-hand Threads on right-hand propeller). Now inserting long Steel wedges between the popular boss and sternpost and hammering it. Boss may be carefully heated. 2. (a) assemble- steam heating of Boss so metal to expand.
(b) withdraw- the boss of the propeller has two holes tapped in the after face so that a plate may be placed across the thimble point of the shaft and a good pull then be exerted to draw the propeller aft and trust the tail end shaft forward.
A. Sketch a section through a key-less sleeved propeller.
The propeller torque is taken only by friction between the propeller and propeller shaft. It has the following advantages surpassing the keyed propeller:
-simple construction, no key slot and consequently no concentration of stresses at the edge of key slot unlike the ordinary keyed propeller, and uniform distribution of stresses over the internal surface of the propeller boss. -The keyless propeller, therefore, aroused great interest among the maritime communities and that demand for it is on the steady increase.
C. State with reason, Which metal sleeve should be made for contact with the forged mild steel tail shaft? A cast iron sleeve is fitted between the Bronze propeller and mild steel tail shaft. The coefficient of friction between mild steel and Bronze is less. which is required to be high, the cast iron sleeve is providing the same.
Also bedding and gripping were enhanced.
Chances of slippage are reduced.
D. State the material uses to bond the sleeve to the propeller and the general thickness of the bonding material. see the first figure: material is Araldite. maximum thickness is 1mm (commonly ranges from 0.3 to 0.7mm)
Pilgrim fitting of propeller
Propeller hub with sleeve:-cast iron sleeve bonded into the propeller boss by using a special form of Araldite injected with pressure. C.I sleeve easier to handle to be bedded with taper shaft, machining and bedding than with a complete propeller. Cast iron has a coefficient of friction nearer to the shaft.
Pilgrim Nut has an internal nitrile rubber tube, which when inflated hydraulically forces the steel loading ring against the propeller hub. Loading ring travel should not exceed 1/3rd of loading ring width, to avoid rapture of rubber tube.
Preparation before assembly:-
(a) propeller or sleeve must be bedded to its tail shaft and checked with blue marking. if sleeve, to be bonded into propeller boss.
(b) propeller and tail-shaft the wipe clean and dry.
if cast steel propeller bosses, then light smear with clean oil.
(c) measure temperature of shaft and propeller hub. Determine from the pushup diagram, the push-up need depending on temperature.
(d) pilgrim nut and shaft thread should be liberally coated with "MolyGrease".
(e) Put the propeller on the tail shaft. Never jack the propeller by Loading ring.
Assembly:
(a) Mount pilgrim nut with loading ring on the tail shaft against propeller hub. The loading ring must flush with the face of the nut. Rotate the nut nearest position, so blanking plugs on top and bottom respectively.
(b) Connect the oil pipe at the bottom and keep the top plug open. Start the pump with clean oil, when bubble-free oil comes out from the top plug, then tighten it.
(c) Pump up until pressure gauge showing 1000psi;(67bar).
Make a reference mark on the shaft or liner 25mm forward end of the hub(initial mark). Also, fit a dial gauge to record the continuous travel as per rising pressure.
(d) vent loading ring, turn the nut so that the loading ring is flush with the force of the nut.
(e) pump up to the working pressure corresponding to boss and shaft temperature.
(f) Make a second 25mm mark (final mark), compare with a dial gauge.
(g) vent and remove the oil pipe and adaptor from the nut body.
(h) rotate the nut to force the loading ring "HOME", Fit the plug.
(i) Test with a feeler gauge between nut and propeller boss [no clearances]
(j) fit the locking plate.
(k) fill up with tallow, fit the fairwater cone and lock it in place.
Withdrawal:- (a) Remove fair water cone and slacken off the flange ring to secure the rubber ring at the FWD end of the propeller boss or sleeve.
(b) clean the weight of the propeller and remove the locking plate.
(c) Take the weight of the propeller and secure it.
(d) Unscrew the pilgrim nut completely, reverse and screw back again, so that the loading ring will now be facing aft.
(e) assemble the withdrawal plate and secure it with studs and nuts.
(f) Insert a pair of the wooden blocks between propeller boss aft end and pilgrim nut.
(g) The free gap between the faces of wooden blocks and propeller boss aft end should not exceed 1.6mm more than the "push up" distance.
(h) Connect oil pipe at the bottom of pilgrim nut and keep top plug open. Start the pump with clean oil, when bubble-free oil comes out from the top plug, then tighten.
(i) Pump up the nut up to the required pressure, which can be obtained from the ship's record with respect to the temperature of the dry dock.
(j) Once the propeller jumps off, remove the oil pipes and adaptor.
(k) Screw down the hexagonal nuts securing the withdrawal plate. This will drive the loading ring back to zero or flush position.
(l) Remove the hexagonal nuts and withdrawal plate and studs.
(m) Tighten the blanking plug after ensuring the Dowty seal is correctly seated.
(n) The pilgrim nut and then propeller may be removed.
Sealing arrangement for oil lubricated stern tube.
(A) Sealing arrangementThe SEAL is composed of an AFT SEAL, which prevents stern tube lubricant oil leakage outside the ship as well as seawater from entering the stern tube, and a FORWARD SEAL which prevents stern tube lubricant oil leakage into the engine room.
Aft Seal: The Aft Seal can be broadly divided into the casing, which is fixed to the hull, and the chrome steel liner, which is fixed to the propeller boss and rotates with the propeller shaft.
The casing is composed of three kinds of metal rings; flange ring, intermediate ring and Cover ring, which are tightened to each other with bolts. Three or four sealing rings are assembled between the metal rings with their pointed ends (lip section) touching the chrome steel liner. The lips are pressed hard against the rotating liner and maintain a sealing effect through water pressure, oil pressure, the elasticity of the sealing ring and the tightening force of the springs.
The sealing rings are numbered 1, 2 and 3 in order from the seawater side. The No.1 and No.2 sealing rings close out seawater, while the No.1 sealing ring also has the function of protecting the inside of the stern tube from foreign matter in the seawater. Lubricant oil in the stern tube is sealed with the No.3 sealing ring.
Forward Seal:
The Forward Seal is of similar construction to the Aft Seal except that it is composed of two sealing rings. The sealing rings are numbered 4 and 5 in order from the stern tube. The No.4 sealing ring seals the lubricant oil in the stern tube. The forward chrome steel liner is tightened with bolts to the split clamp ring mounted to the propeller shaft.
(B) seal failure, loss of oil and temporary actions.
The common form of seal failure is the garter spring compression on the seal reduces due to grooved wear on the chrome liner causing leakage of oil through the seal. Hence at every propeller shaft survey, the seals must be renewed to avoid stoppage of the ship and extra urgent docking of the ship.
In the event of oil loss from the seal which can be noticed by the watch-keeper during the rounds by comparing the oil level in the gravity tank.
if the oil is leaking from the forward seal into the engine room through the cover ring, then the oil can be collected in a clean tank and can be reused as a temporary action. inspect and repair the forward seal as soon as the situation permits.
if the oil is leaking from the aft seals into the seawater. adjust the oil pressure equal to water pressure. In case the aft seal is fitted with four seals in which two seals 1 & 2 are for seawater sealing and 3 & 3S are for sealing lube oil, close the lubricating oil supply into the oil chamber between rings 2 and 3.
other methods include changing of oil to a higher viscosity oil such as main engine cylinder oil leakage quantity can be reduced. But this is only a temporary correction and renewal of the seals is inevitable and has to be done at the earliest.
(C). Liner Material
The material of the liner is chromium steel since it is non-corrosive and the liner is in contact with the seawater. The material of the seal is VITON (flororic rubber) or NBR (Nitrile-butadiene rubber) which is resistant to wear.
Shaft Coupling
A. Sketch a hollow type coupling bolt and the hydraulic head/nut and loading rod which are used to fit it;
B. Describe how the bolt is fitted;
The Pilgrim hydraulic bolt uses the principle embodied in Poisson’s ratio to provide a calculated and definite fitting force between bolt and hole.
The bolt is hollow and before being fitted is stretched with hydraulic pressure applied to an inserted rod from a pressure cylinder screwed to the bolt head. Stretching makes the bolt diameter small enough for insertion into the hole, after which the nut is nipped up.
The release of hydraulic pressure allows the bolt to shorten so that
(1) Predetermined bolt load is produced and
(2) diametrical re-expansion gives a good fit of the shank in the hole.
C. State the advantage of the hollow coupling bolt as compared to the traditional type of coupling bolt.
These bolts prevent any misalignment between flanges.
These bolts when fitted in flange couplings and flange mounted propellers, have the advantage that they are easily removed for inspection and maintenance. Since the strain is calculated excessive stress is avoided.
Couplings become more rigid and thus less prone to fretting corrosion and fracture.
SKF or oil injection fitting
Preparation before assembly:-(a) the propeller must be bedded to its tail shaft and checked with blue marking.
(b) clean inside propeller boss and shaft taper surface with acetone.
(c) Measure temperature of shaft and propeller boss.
(d) Determine from the push-up diagram the push up needed depending on temperature.
(e) Put the propeller on the shaft, adjust the circumferential position of the propeller boss with moly grease.
Assembly:-
(a) Fit oil jacks (if non-hydraulic nut), screw the nut on the shaft to the aft face of the boss up to specified torque. Rotate the nut/jack nearest position, so blanking plugs on top and bottom.
(b) Mount pump holder and pump. connect the oil pipe at the bottom and keep the top plug open.
Start the pump with clean oil, when bubble-free oil comes out from the top plug, then tighten it.
(c) Fit a dial gauge to record continuous travel as per rising pressure.
(d) Mount pump holder, increase oil pressure of nut/jacks only. Measure pushup load when push up distance is 0.5, 1.0, 1.5 &2.0mm.
(e) Decrease oil pressure of nut/jack gradually.
(f) Determine "zero point" by measurement.
(g) Reset dial gauge to record continuous travel as per rising pressure. Make also reference mark on shaft (depends on push up distance).
(h) operate all pumps. The pressure of the pump(nut/jack) should not exceed 500atm. The final pressure of the other pump should be of the order of 500 to 700 atm depends on temperature.
(i) Push up boss to designed push up distance corresponding to temperature.
(j) Make a second "reference mark" ( final marking) compare with dial gauge should be the same as marking.
(k) After having reached the pushup (control by dial gauge) and confirmation from reference marking, release the interface pressure of the pump but maintain the pressure of the hydraulic nut/ jack for a further 15minutes to enable the oil film between shaft and propeller to disperse completely. At low temperature or when using higher viscosity oil the time should be longer. Fit back the plug on propeller boss.
(l) Remove oil pipes.
(m) Tighten up the nut against the boss. if jack, then it is to be removed and tighten the nut.
(n) Test with filler gauge between nut and propeller boss.
(o) No. clearance, Fit the locking plate
(p) Fill up with tallow, fit the fairwater cone and lock in place.
Withdrawl:-
(a) Remove fair water cone and slacken off the flange of simplex seal(chrome steel liner flange).
(b) Clean the area around the nut and remove the locking plate.
(c) Take the weight of the propeller and secure it.
(d) Loosen propeller nut and put a wooden piece between boss and nut. Keep the gap a little more than push up the distance.
(e) Fit oil pump and pressure gauge on propeller boss. Connect oil pipe at bottom of the boss and keep top plug open. Start the pump with clean specified hydraulic oil. When bubble free oil comes out from the top plug, then tighten it.
(f) Increase oil pressure of propeller boss interface gradually, wait until propeller comes free and slip back against the nut.
(g) The nut and then propeller may be removed.
Propulsion efficiency enhancement Technologies
(a) Optimization of propeller hull interface flow devices and improvement of propeller efficiency:-Adoption of EEDI (energy efficiency design index) was mandated by IMO for all vessels contracted since 2013. IMO also resolved to reduce the noise generated by ships to 3dB within 10years and 10dB within 30year to the conversion of marine mammals and fish.
The goal is to improve propulsion efficiency and protect the marine environment.
Propulsion efficiency of ship and cavitation:-
Only 70% of the power of the ship propeller is used for ship propulsion and the rest disappears due to friction, heat loss and vortex etc.
In order to improve the propulsion efficiency of the ship, it is necessary to minimize the loss by improving the design technique of the propeller and developing highly efficient propeller appendages.
Cavitation: - as the velocity of the fluid increases the pressure near the surface of the object touching the fluid is lowered, causing the fluid to vaporize, which then results in physical change such as the creation of empty space in the water. Various types of strong cavitation occurs in the propeller. This is the main cause of noise and vibration of the ship loss of propulsion efficiency and propeller & rudder erosion.
Technologies for enhancement of propulsion efficiency and noise reduction:-
The existing ship propeller has a round cup shape cover, this shape contrary to expectations create strong vortex flow and forms a strong hub vortex cavitation. In an attempt to inhibit the formation of the vortex, a new cap design called K-CAP is developed. It can improve the propulsion efficiency while efficiently inhibits the formation of hub vortex cavitation.
Technologies for enhancement of propulsion efficiency and noise reduction.
K-CAP & FIN:- it uses the combination of K-CAP and a simple shaped plate, the fin attached helps to absorb the rotational energy behind the propeller and removes the propeller hub vortex cavitation, while improving the propulsion efficiency, it reduces propeller broadband noise and prevent water erosion by completely eliminating hub vortex cavitation with increased efficiency of the propeller.
K-CAP & FIN:- it uses the combination of K-CAP and a simple shaped plate, the fin attached helps to absorb the rotational energy behind the propeller and removes the propeller hub vortex cavitation, while improving the propulsion efficiency, it reduces propeller broadband noise and prevent water erosion by completely eliminating hub vortex cavitation with increased efficiency of the propeller.
Vortex generator:- it is a structure built in front of the propeller which controls the flow of fluid into the propeller to improve the propulsion efficiency and reduces the noise and vibration of the ship.
Conventional energy saving devices (ESD): - they are installed in the form of the fin or duct close to the propeller. With such a structure the propulsion efficiency can be improved by the pre-swirl effect. But the possibilities of increased cavitation and erosion in there with such arrangements. They also form a large structure and are difficult in maintenance also they show structural problems.
Conventional energy saving devices (ESD): - they are installed in the form of the fin or duct close to the propeller. With such a structure the propulsion efficiency can be improved by the pre-swirl effect. But the possibilities of increased cavitation and erosion in there with such arrangements. They also form a large structure and are difficult in maintenance also they show structural problems.
(b) sketch and explain the optimization of Auxiliary machinery using VFDs.
production and distribution of high voltage is been proven economical over a long period of services. the high voltage also facilitates to instal of an electric propulsion system on the ship. electric propulsion systems have advantages over diesel propulsion.
The system generates a significant amount of power, excess power utilized by supplying it to cargo pumps, fire pumps and other important auxiliary machinery. The space required for the installation of electrical propulsion machinery is very less and compact as compared to a conventional system. There is no direct connection of the propeller shaft and prime mover, and hence transmission of severe stresses such as torsional and the vibration is restricted. There is more flexibility in the installation of machinery. It provides improved manoeuvrability and high redundancy. Increased payload through the flexible location of machinery components. Environmental benefits from lower fuel consumption and emissions. High performance in harsh ice conditions due to maximum torque at zero speed. Reduces life cycle cost by less fuel consumption and maintenance cost. Minimal standstill time for maintenance and service. Vessels with potential trim problems, such as stern Wheeler, where machinery needs to be located forward to avoid trim problems. Better comfort due to reduced vibration and noise. Much better dynamic response from zero to maximum propelling speed compared to other propulsion systems. Less reversing time compared to other propulsion systems. Availability of maximum torque across the entire speed range at the propeller. Reduced space requirements in the shaft system. The design and engineering of the propeller are independent of the drive. Flexibility in the choice of diesel engine speed.
The system consists of an AC generator that produces a fixed voltage at a fixed frequency. But for the purpose of slow streaming and manoeuvring the fixed output of the generator needed to be lowered frequency and adjusted voltage. The speed of the synchronous motor used for the propulsion shall be controlled with this adjustment. There are many methods to achieve this. Some methods are required to convert the AC to DC then DC to AC with a new controlled frequency.
One method is known as cyclo converter which does not have an intermediate DC form. This method of controlling frequency relies on the ability of the converter to accept current from the switchboard at constant frequency and voltage but to pass this current to the AC motor at a reduced frequency and with voltage adjusted. The fixed-frequency supply from the a.c. generators are applied simultaneously to the three pairs of Graetz thyristor bridges of the cyclo-converter. The upper and lower bridges of each pair are arranged to operate alternately so that a number of triggering occur in the top set of thyristors-followed by an equal number from the bottom set, to deliver an output with a lower frequency. The two bridges for each phase are required to supply both the positive and negative half-cycles.
The triggering of the thyristors is continually changed relative to the three-phase supply so that output can be tailored to provide the exact frequency and amplitude of voltage required. Frequency is variable from 0 to 60 Hz.
Reduction gear
Reverse and reduction Gear:-Input in the gearbox is from a four-stroke medium-speed diesel engine. Thus there is a need for the reduction of RPM before transmitting the torque to the propeller.
Output shaft rotation RPM depends on the gear ratio and it is generally between 2:1 & 6:1.
Reduction gear in marine engine for reversing
The reduction gear design shown in the figure above has a primary stage, a reduction stage and a reversing stage. The input and output shafts are co-axial.
It consists of helical reverse gears incorporating multi-plate clutches.
Housing is made of grey cast iron and provided with the reinforced ribs at the points to prevent distortion. At the bearing points and especially in the thrust bearing section the casings are extra stiffened to absorb thrust forces. All gears and pinion shafts are of a single helical design; tooth flank is case hardened (by gas carburization) and finish machined.
The shafts are made of high grade quenched and tempered steel and carried in rolling bearings. The propeller thrust is absorbed by Michel-type thrust bearing.
The integral multi-plate clutches with steel/sinter plating are pressure oil operated.
The two-stage pressure regulating valve on the gearbox results in smooth engagement. Multi-plate clutch engagement and disengagement is carried out by a pneumatically actuated gear controller with a manual emergency control. The bottom of the lower housing serves as an oil sump.
Stern tubes
The propeller shaft enters the ship through the stern tube which acts as the final bearing and a watertight seal to the sea. Traditional practice saw the use of lignum vitae and certain synthetic materials as bearing surfaces within the stern tube and these were lubricated by seawater. The increased loadings, as a result of slow speed shafts and heavier propellers on more modern ships, has 100 to the widespread use of oil-lubricated white metal bearings. With this arrangement wear down in service is much reduced but there is a need for more accurate alignment and for seals at each end of the stern tube.
Propellers
A propeller consists of a boss which has several helicoidal form blades. When rotated it 'screws' or thrusts its way through the water by giving momentum to the column of water passing through it. The thrust is transmitted along the shafting to the thrust block and finally to the ship's structure. The thrust block must therefore have a rigid seating or framework which is integrated into the ship's structure to absorb the thrust. The propeller will usually be either of the fixed pitch or controllable pitch type. ln addition some special designs and arrangements are in use which offers particular advantages.
Fixed pitch propeller
Although described as fix pitch, a solid single-piece cast propeller has a pitch that varies with increasing radius from the boss. The pitch at any particular point on a blade is however fixed and an average value for the complete propeller is used in aU calculations. A fixed-pitch propeller, where most of the terms used in describing the geometrical features are also given. It should be noted that the face is the surface farthest from the stern and is the 'working' surface. A cone is fitted to the boss to provide a smooth flow of water away from the propeller. A propeller that rotates clockwise, when viewed from aft, is considered to be right-handed. Most single-screw ships have right-handed propellers. A twin-screw ship will usually have a right-handed starboard propeller and a Left-handed port propeller Cavitation is the forming and bursting of vapour filled cavities or bubbles and occurs as a result of certain pressure variations on the back of a propeller blade. The results of this phenomenon are a loss of thrust. erosion of the blade surface, vibrations in the afterbody of the ship and noise. It is usually limited to high-speed, heavily loaded propellers and is not a problem under normal operating conditions with a well-designed propeller.
The propeller. when turning in the ship's wake, is a potential source of vibration excitation. To some extent, this can be minimised by having the leading edges skewed back. Skew back is an advantage when the propeller is working in a varying wake as not aIl the blade is affected at the same time. Variations in the thrust and torque are therefore smoothed out. Since the vibrations are blade excited. then the number of blades is significant and determines the vibration frequency. Where severe vibration problems exist it may ultimately be necessary to change the propeller for one with a different number of blades.
Propeller mounting
The propeller is fitted onto a taper on the tail shaft and a key may be inserted between the two; alternatively, a keyless arrangement may be used. A large nut is fastened and locked in place on the end of the tail shaft. A cone is then bolted over the end of the tail shaft to provide a smooth flow of water from the propeller.
One method of keyless propeller fitting is the oil injection system. The propeller bore is machined with a series of axial and circumferential grooves. High-pressure oil is injected between the tapered section of the tail shaft and the propeller. This reduces the friction between the two parts and the propeller is pushed up the shaft taper by a hydraulic jacking ring. Once the propeller is positioned. the oil pressure is released and the oil runs back leaving the shaft and propeller securely fastened together.
The pilgrim nut is a patented device that provides a predetermined frictional grip between the propeller and its shaft. With this arrangement, the engine torque may be transmitted without loading the key (where fitted). The pilgrim nut is, in the effect, a threaded hydraulic jack that is screwed onto the tail shaft. A steel ring receives thrust from a hydraulically pressurised nitrile rubber tyre. This thrust is applied to the propeller to force it onto the tapered tail shaft. Propeller removal is achieved by reversing the Pilgrim Nut and using a withdrawal plate which is fastened to the propeller boss by studs. When the tyre is pressured the propeller is drawn off the taper.
Controllable-pitch propellers
A controllable-pitch propeller is made up of a boss with separate blades mounted into it. An internal mechanism enables the blades to be moved simultaneously through an arc to change the pitch angle and therefore the pitch. When a pitch demand signal is received, a spool valve is operated which controls the supply of low-pressure ail ta the auxiliary servo-motor. This moves the sliding thrust block assembly to position the valve rad which extends into the propeller hub.
The valve rod admits high-pressure oil into one side or the other of the main servomotor cylinder. The cylinder movement is transferred by a crankpin and ring to the propeller blades. The propeller blades rotate together until the feedback signal balances the demand signal and the low-pressure oil to the auxiliary servo-motor is cut off. To enable emergency control of propeller pitch in the event of loss of power, the spool valves can be operated by hand. The oil pumps are shaft driven.
The control mechanism, which is usually hydraulic, passes through the tail shaft and operation is from the bridge. Varying the pitch will vary the thrust provided and since a zero pitch position exists the engine shaft may tum continuously. The blades may rotate to provide astern thrust and therefore the engine does not require to be reversed.
Special types
A number of specialised arrangements or types of propellers exist and have particular advantages or applications. The Voith-Schneider propeller is a vertically-rotating device. The blades are vertically positioned around a disc and can be rotated by cams in order to change the blade angle at a particular point in each revolution. This results in a thrust whose magnitude and direction is determined by the cams. It is, therefore, in some respects similar to a controllable-pitch propeller in that the disc is driven and the blades can be positioned independently of the main drive. This unit can effectively thrust in any direction and will respond rapidly to the pitch control mechanism. The complete assembly is unfortunately complex, noisy in operation and considerable maintenance is necessary. It is often used for main propulsion in ferries and vessels requiring considerable manoeuvrability. It may also be used as a thrust creator or propulsion device for drillships or floating cranes which require accurate positioning. The use of a duct or nozzle around the propeller can result in an improvement of the propeller performance. Furthermore, the aerofoil shape of the duct can produce a forward thrust which will offset any drag it creates. The duct also protects the propeller from damage and reduces noise. It is usually fitted on ships with heavily loaded propellers, e.g. tugs, and has been used on larger vessels. One particular patented design of duct is known as the Kort Nozzle.
The CLT (Formerly TVF) propeller is a recent special design that results in much-improved propeller efficiency. The blade tips are fitted with pieces at right angles to the plane of rotation. The initial impression is that the blade edges have been bent over towards the face, Le. away from the ship. The attachments at the blade tips serve to generate thrust across the whole propeller blade and thus improve the propeller efficiency. A nozzle surrounds the propeller and a tunnel structure under the stem on either side is used to direct the incoming flow of water. The Grim Wheel or vane wheel is mounted aft of the main propeller and is larger in diameter. It is a freely rotating propeller with high aspect ratio blades that vary from a coarse pitch at the boss to a very fine pitch at the tip. The wheel is rotated by, and extracts energy from, the propeller slipstream and produces an additional thrust from the tip region of its blades.
The contra-rotating propeller uses two driven propellers that rotate in opposite directions. A special gearbox and shafting arrangement enables a single engine to drive the two propellers. Significant efficiency gains have been achieved by the first unit which was fitted to a bulk carrier. Propeller boss cap fins convert the propeller hub vortex energy into additional torque and thrust which is transmitted back to the propeller shaft. The boss cap with its short blades is fixed to the main propeller boss.
Rudders The rudder is used to steer the ship. The turning action is largely dependent on the area of the rudder, which is usually of the order of one-sixtieth to one-seventieth of the length x depth of the ship. The ratio of the depth to width of a rudder is known as the aspect ratio and is usually in the region of 2. Streamlined rudders of a double-plate construction are fitted to all modem ships and are further described by the arrangement about their axis. A rudder with aH of its area aft of the turning axis is known as 'unbalanced'. A rudder with a small part of its area forward of the turning axis is known as 'semi-balanced. When more than 25% of the rudder area is forward of the turning axis there is no torque on the rudder stock at certain angles and such an arrangement is therefore known as a 'balanced rudder'.
Modem rudders are constructed with steel plate sides welded to an internal webbed framework. Integral with the internal framework may be heavy forgings that form the gudgeons or bearing housings of the rudder. The upper face of the rudder is formed into a, usually, horizontal flat palm which acts as the coupling point for the rudder stock. A lifting hole is provided in the rudder to enable a vertical in-line lift of the rudder when it is being fitted or removed. A special lifting bar with eye plates is used to lift the rudder.
This is welded in place after the rudder is fitted to provide streamlined water flow into the rudder. After manufacture, every rudder is air tested to a pressure equivalent to a head of 2.45 m above the top of the rudder in order to ensure its watertight integrity. The internal surfaces are usually coated with bitumen, or some similar coating, to protect the metal should the plating leak. A drain hole is provided at the bottom of the rudder to check for water entry when the ship is examined in drydock.
Rudder pintles and bearings
The rudder, depending on its type and arrangement, will tum on either pintles or bearings. The balanced rudder has a rudder axle fitted at its turning axis. Upper and lower bearings are fitted in the rudder. The bearing consists of a stainless steel bush in the rudder and a stainless steel liner on the axle.
The stainless steel bush is spirally grooved ta permit lubrication. Other materials are in use, such as gunmetal for the liner and lignum vitae or tufnol for the bush. The upper and lower pair of tapered bearing rings are fitted between the rudder and the stern frame. These are fitted with a small clearance but may support the weight of the rudder should the carrier fail. The semi-balanced rudder turns on pintles. Arrangements vary but the pintle consists of a bearing length of constant diameter and a tapered length which is drawn into a similarly tapered hole on the rudder or stern frame gudgeon. The pintle is drawn in by a large nut pulling on the threaded portion of the pintle. The pintle nut is securely locked in place after tightening. A locking pintle has a shoulder of increased diameter at its lower end which prevents excessive lift of the rudder. A bearing or heel pintle has a bearing surface at its lower edge which rests on a hard steel disco This bearing pintle is only required to support the weight of the rudder in the event of the rudder carrier failing.
Liners of brass or sometimes stainless steel are fitted to the pintle bearing surface. The bearing material is held in a cage in the gudgeon and is usually tufnol or some hard-wearing synthetic material. Lubrication is provided by seawater which is free to circulate around the bearing surfaces of both pintles.
Rudder stock and carrier
The stock passes through a gland and a rudder carrier before entering the steering compartment. The gland and carrier may be combined or separate items of equipment. The rudder carrier consists of two halves which provide an upper and lower bearing surface. The upper part of the rudder carrier is keyed to the stock so that they tum together. The major part of the rudder's weight is transferred to the rudder carrier by either a shoulder, as part of the stock forging, or a collar fitted between the tiller and the carrier. The rudder weight is thus transferred to the lower bearing surface of the carrier which is grease lubricated. A flat or conical bearing surface may be used depending on the particular design. The lower half of the carrier is bolted into a heavy insert plate in the deck of the steering flat and is chocked against fore and aft and athwartships movement.
A separate watertight gland is often fitted where the stock enters the rudder trunk. This arrangement provides access 10 a greater length of the rudder stock, removes the need for a watertight construction of the carrier bearing and reduces the unsupported length of the stock. A combined type of watertight gland and rudder carrier. It is essential for ease of operation of the rudder that the pintles and rudder stock turning axes are in the same vertical line. Great care must be taken during installation to ensure this correct alignment.
Questions and Answers
Q.What do you mean by Wake?
Ans:When a propeller rotates it sucks water into itself and discharges it in a well defined slip stream. Due to the rotation of the blades the fluid pressure immediately behind the propeller is increased and the effect of this is to increase the velocity of the mass in the slip stream. The change if momentum in this mass provides the propeller thrust. As the slip moves through the water, the friction of the water in the surface of the hull causes a surrounding layer of water on the surface of the hull causes a surrounfing layer of water to follow in the direction of motion. This belt of water is called the wake and as a result of its effect the speed of the propeller through the wake is generally less than the speed of the ship.
The wake velocity varies in magnitude and direction but is assumed as a matter of convenience, to have a constant forward velocity given by (V-Va), where V= speed of the ship and Va = Speed of advance of propeller. The wake speed can be expressed as a percentage of either the speed of advanve (Va) of the propeller or the speed (V) of the ship.
Froude selected the former whilst Taylor chose the latter as mentioned below.
Froude Wake fraction (Wf)= (V-Va)/Va
Taylor Wake fraction (Wt) = (V- Va)/V
Expressions frequently used in the estimating stages of a design to assess the wake speed are:
For single screw: Vw = (0.5Cb -0.05)V
For Twin screw: Vw = (0.5Cb -0.2)V.
Q. What is a Propeller Cavitation?
Ans: -The thrust of a propeller varies approximately as the square of the revolutions. The net pressure at any point on the back of the blade is the algebraic sum of the atmospheric pressure, water pressure and negative pressure or suction caused by the thrust. With increase of propeller revolutions the peak of the pressure reduction curve increases and if at any point the local pressure on the blades falls below the vapour pressure a cavity will be formed filled with water vapour and air which disassociates from the sea water. As the blade turns, the buble moves across the blade to a point where the net pressure is higher, causing the cavity to collapse. The forming and collapsing of these cavities is known as ‘Cavitation. It can produce holes in blade material due to severe erosion, and also can causes reduction in thrust and efficiency, vibration and noise also. It may be reduced or avoided by reducing RPM and by increasing the blade area for constant trust and thereby reducing the negative pressure. Since cavitation thrust and thereby reducing the negative pressure. Since cavitation is affected by pressure and temperature, it is more likely to occur in propellers operating near the surface than in those deeply submerged, and will occur more readily in the tropics than in cold regions.
Q. Explain Singing of Propeller?
Ans: Before the beginning of cavitation, the blades of the propeller give out a high pitched note due to the elastic vibration of material exited by resonant shedding of non -cavitating eddies from the trailing edge of the blade. This fault may be eliminated by a change in the shape of the trailing edge or increased damping of the propeller blade.
Q. Explain the scew propeller.
Ans: The screw propeller is the usual method by which a thrust developed by the propelling machinery to overcome the resistance of the ship and produce motion. Fundamentally, the marine screw propeller may be regarded as a helicoidal surface which, on rotation, screws its way through the water.
A screw propeller has two or more fixed blades projecting from boss. The surface of each blade when viewed from aft is called the FACE, which is the driving surface when producing an ahead thrust and the other surface of the blade is called the BACK. The Leading Edge of the blade is the edge which when the ship is driven ahead, first cuts the water; and the other edge is termed the TRAILING EDGE.
When viewed from aft, in producing the ahead thrust, if the propeller is rotating in clockwise direction is said to be right handed Propeller and if anti clockwise is said to be left handed Propeller. In twin screw ships, the starboard propeller is normally right handed and the port propeller left handed in order to reduce cavitation. DIAMETER (D) - It is the diameter of the circle swept out by the tips of the blades.
PITCH (P) - It is the distance any specified point on the face of the blade would move forward in one revolution. Pitch Ratio.- Pitch / Diameter = P /D.
The Developed blade area is the sum of face area of all the blades; the boss is excluded. The Projected Area is the projection of the blades on to a plane normal to the shaft exis. The dise area is the area enclosed within the tip circle, and is equal to $\mathrm{\frac{\pi D^{2}}{4}}$ where D is the diameter of the propeller. The Blade Area Ratio ( BAR) is the ratio of the developed blade area (Ad) to the disc area.
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