Shafting and propellers | Marine Engineering Fundamentals

The transmission system on a ship transmits power from the engine to the propeller. It is made up of shafts, bearings, and finally the propeller itself. The thrust from the propeller is transferred to the ship through the transmission system. The total items in the system include the thrust shaft, one or more intermediate shafts and the tail shaft. These shafts are supported by the thrust block, intermediate bearings and the stern tube bearing. A sealing arrangement is provided at either end of the tail shaft with the propeller and cone completing the arrangement. These parts, their location and purpose are shown in Figure 1.

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Fig.1 Transmission system in Ship

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Thrust block

The thrust block transfers the thrust from the propeller to the hull of the ship. It must therefore be solidly constructed and mounted onto a stiff seating or framework to perform its task. It may be an independent unit or an integral part of the main propulsion engine. Both ahead and astern thrusts must be catered for and the construction must be strong enough to withstand normal and shock loads.

The casing of the independent thrust block is in two halves which are joined by fitted bolts (Figure 2). The thrust loading is carried by bearing pads which are arranged to pivot or tilt. The pads are mounted in holders or carriers and faced with white metal. In the arrangement shown the thrust pads widen three quarters of the distance around the collar and transmit all thrust to the lower half of the casing. Other designs employ a complete ring of pads. An oil scraper deflects the oil lifted by the thrust collar and directs it onto the pad stops. From here it cascades over the thrust pads and bearings. The thrust shaft is manufactured with integral flanges for bolting to the engine or gearbox shaft and the intermediate shafting, and a thrust collar for absorbing the thrust.

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Fig.2 Thrust block

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 Where the thrust shaft is an integral part of the engine, the casing is generally fabricated in a similar manner to the engine bedplate to which it is bolted. Pressurized lubrication from the engine lubricating oil system is provided and most other details of construction are similar to the independent type of thrust block.


Shaft bearings

Shaft bearings are of two types, the aftermost tunnel bearing and all others. The aftermost tunnel bearing has a top and bottom bearing shell because it must counteract the propeller mass and take a vertical upward thrust at the forward end of the tail shaft. The other shaft bearings only support the shaft weight and thus have only lower half bearing shells. An intermediate tunnel bearing is shown in Figure 3. The normal journal bush is here replaced by pivoting pads.

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Fig.3 Tunnel bearing/ Shaft bearing

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The tilting pad is better able to carry high overloads and preserve a thick oil lubrication film. Lubrication is from a bath in the lower half of the casing, and an oil thrower ring dips into the oil and carries it round the shaft as it rotates. Cooling of the bearing is by water circulating through a tube cooler in the bottom of the casing.


Stern tube bearing

The stern tube bearing serves two important purposes. It supports the tail shaft and a considerable proportion of the propeller weight. It also acts as a gland to prevent the entry of sea water to the machinery space. Early arrangements used bearing materials such as lignum vitae (a very dense form of timber) which were lubricated by sea water. Most modern designs use an oil lubrication arrangement for a white metal lined stern tube bearing. One arrangement is shown in Figure 4.

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Fig.4 Oil lubricated stern tube bearing

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Oil lubricated stern tube bearing Oil is pumped to the bush through external axial grooves and passes through holes on each side into internal axial passages. The oil leaves from the ends of the bush and circulates back to the pump and the cooler. One of two header tanks will provide a back pressure in the system and a period of oil supply in the event of pump failure. A low-level alarm will be fitted to each header tank. Oil pressure in the lubrication system is higher than the static sea water head to ensure that sea water cannot enter the stern tube in the event of seal failure.


Stern tube seals

Special seals are fitted at the outboard and inboard ends of the tail shaft. They are arranged to stop the entry of sea water and also the loss of lubricating oil from the stern bearing. Older designs, usually associated with sea water lubricated stern bearings, made use of a conventional stuffing box and gland at the after bulkhead. Oil-lubricated stern bearings use either lip or radial face seals or a combination of the two. Lip seals are shaped rings of material with a projecting lip or edge which is held in contact with a shaft to stop oil leakage or water entry. A number of lip seals are usually fitted depending upon the particular application.

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Face seals use a pair of mating radial faces to seal against leakage. One face is stationary and the other rotates. The rotating face of the after seal is usually secured to the propeller boss. The stationary face of the forward or inboard seal is the after bulkhead. A spring arrangement forces the stationary and rotating faces together. There may be one or more sections of intermediate shafting between the thrust shaft and the tail shaft, depending upon the machinery space location. All shafting is manufactured from solid forged ingot steel with integral flanged couplings. The shafting sections are joined by solid forged steel fitted bolts. The intermediate shafting has flanges at each end and may be increased in diameter where it is supported by bearings. The propeller shaft or tail shaft has a flanged face where it joins the intermediate shafting. The other end is tapered to suit a similar taper on the propeller boss. The tapered end will also be threaded to take a nut which holds the propeller in place.


Propeller

The propeller consists of a boss with several blades of helicoidal form attached to it. 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. A solid fixed-pitch propeller is shown in Figure 5. Although usually described as fixed, the pitch does vary with increasing radius from the boss. The pitch at any point is fixed, however, and for calculation purposes a mean or average value is used.

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Figure 5 Solid propeller

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A propeller which turns clockwise when viewed from aft is considered right-handed and 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.


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 has a series of axial and circumferential grooves machined into it. 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.

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The Pilgrim Nut is a patented device which 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 it is fitted. The Pilgrim Nut is, in effect, a threaded hydraulic jack which is screwed onto the tail shaft (Figure 6). A steel ring receives thrust from a hydraulically pressurized nitrile rubber tyre.

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Figure 6 Pilgrim Nut operation

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 pressurised the propeller is drawn off the taper. Assembly and withdrawal are shown in Figure 6.


Controllable-pitch propeller

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. A typical arrangement is shown in Figure 7. When a pitch demand signal is received a spool valve is operated which controls the supply of low-pressure oil to the auxiliary servo motor. The auxiliary servo motor moves the sliding thrust block assembly to position the valve rod which extends into the propeller hub.

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Figure 7 Controllable pitch propeller

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1 Piston rod

2 Piston

3 Blade seal

4 Blade bolt

5 Blade

6 Crank pin

7 Servo motor cylinder

8 Crank ring

9 Control valve

10 Valve rod

11 Mainshaft

-pitch propeller

12 Valve rod

13 Main pump

14 Pinion

15 Internally toothed gear ring

16 Non-return valve

17 Sliding ring

18 Sliding thrust block

19 Corner pin

20 Auxiliary servo motor

21 Pressure seal

22 Casing

The valve rod admits high-pressure oil into one side or the other of the main servo motor cylinder. The cylinder movement is transferred by a crank pin and ring to the propeller blades. The propeller blades all rotate mutually 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 usually from the bridge. Varying the pitch will vary the thrust provided, and since a zero pitch position exists the engine shaft may turn continuously. The blades may rotate to provide astern thrust and therefore the engine does not require to be reversed.


Cavitation

Cavitation, the forming and bursting of vapour-filled cavities or bubbles, can occur as a result of pressure variations on the back of a propeller blade. The results are a loss of thrust, erosion of the blade surface, vibrations in the after body of the ship and noise.

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It is generally limited to high-speed heavily loaded propellers and is not a problem under regular operating conditions with a well designed propeller.


Propeller maintenance

When a ship is in dry dock the opportunity should be taken to systematically examine the propeller, and any repairs necessary should be carried out by skilled dockyard staff. A careful examination should be made around the blade edges for signs of cracks. Even the smallest of cracks should not be ignored as they act to raise stresses locally and can result in the loss of a blade if the propeller receives a sharp blow. Edge cracks should be welded up with suitable electrodes. Bent blades, particularly at the tips, should receive attention as soon as possible.

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Except for slight deformation the application of heat will be required. This must be followed by more general heating in order to stress relieve the area in the region of the repair. Surface roughness caused by slight pitting can be lightly ground out and the area polished.  More severe damage should be made good by welding and following heat treatment. A temporary repair for deep pits or holes could be done with a suitable resin filler.


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