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The keyway on the shaft taper is a source of defects leading to fatigue cracks developing because of the sudden change on the surface shape leading to stress concentrations around the keyway because of its sharp corners. If the key and keyway are eliminated about 75% of this problem is solved. Hence the need for keyless propeller shafts.
Method of fitting keyless propellers
The present method of fitting the propeller is called ‘ Wet fit method”. In this method the shaft taper is either 1;18 or 1;20 as per the individual classification rules. The propeller boss has a hole connecting the boss surface to the hollow core and the boss is pressurised by high pressure hydraulic fluid injected into the boss by a high pressure pump and at the same time pressurising the pilgrim nut for fitting and withdrawal. In all other respects the method is similar to the keyed propeller shaft fitting and withdrawal. This method uses force fit coupled with shrinkage to give a much higher grip than keyed propellers. The method of fitting is shown in sketch given as under.Keyless propeller fitting (Wet Method)
The classification societies recommend fitting keyless propellers for large ships to avoid fatigue cracking to occur on the taper fitting surface with the following conditions:
(1) the survey interval is enhanced to ten years , provided that at every dry docking the seals are subjected to a leak test similar to the keyed shaft assembly procedure.
(2) The condition of the lub oil is tested every six months at a laboratory approved by the classification society.
(I) Renewal of seals in afloat condition or in dry dock without withdrawal of shaft as well as chrome liner :-
At times occasionally the aft seal starts to leak and develop into a big leak within a few days making it necessary to either dry dock the ship or renew the seals in afloat condition. The reasons for the seals to leak are as follows:
(1) Some gritty matter like sand can get between the seals and the chrome liner and erode the seals and cause grooving of the liner surface leading to leakage.
(2) If the liner has been machined by 1mm during last dry dock, the spring pressure is reduced and can contribute to the leakage.
(3) The viscosity of the oil has reduced caused by using oil of lower viscosity as recommended by the shore office or the oil company.Controllable Pitch Propeller
In Controlled Pitch type propeller, it is possible to alter the pitch by rotating the blade about its vertical axis by means of mechanical and hydraulic arrangement. This helps in driving the propulsion machinery at constant load with no reversing mechanism required as the pitch can be altered to match the required operating condition. Thus the manoeuvrability improves and the engine efficiency also increases.
This drawback includes the possibility of oil pollution as the hydraulic oil in the boss which is used for controlling the pitch may leak out. It is a complex and expensive system from both installation and operational point. Moreover, the pitch can get stuck in one position, making it difficult to manoeuvre the engine.
Advantages and Disadvantages of CPP and FPP
The following points will discuss the applications and their relations with the advantages and disadvantages of CPP and FPP systems:
- The controllable pitch propellers can be used to run the ship in forward and astern direction both, without the requirement to change the direction of rotation of the engine. How? Well, the propeller works on the principle of lift generated by each aerofoil section of the blade. What if we can change the orientation of the blade, such that the direction of life force is opposite to what it was during forward running of the ship? That’s exactly what is achieved by changing the pitch angle of the blades, as shown in Figure 2.
Blade sections shown in various pitch orientations
- A non-reversible engine can be used for both forward and astern operation of the ship. Hence, the weight of the engine and propulsion machinery is reduced considerably when compared to what is required in propulsion systems equipped with fixed pitch propellers.
- Since the pitch of the propeller determines the amount of thrust generated by the propeller, a change in the pitch angle can bring about a change in the speed of the ship. So, a controllable pitch propeller is useful in changing the speed of the ship without changing the speed or rpm of the main engine. But why are we focusing on this advantage? What happens if we need to reduce the engine rpm to reduce the ship’s speed? Well, many ships have a shaft driven diesel alternator system. So the rpm of the main engine shaft controls the electric power production on board. In that case, if the speed was to be reduced using a fixed-pitch propeller, it would require a reduction in the shaft rpm, which would result in a reduction of power generated.
- Due to the above reason, the speed of the ship can be handled from the navigation bridge directly. In case of fixed pitch propellers, the marine engineer needs to be intimidated in order to change the ship’s speed. Therefore CPP technology is used for obtaining a faster response of speed change.
- The efficiency of an astern condition in case of fixed pitch propellers is less than that of controllable pitch propeller in astern condition.
However, there are many disadvantages of using a controllable pitch propeller to a fixed-pitch propeller. It is important to know them too.
- The pitch control mechanism installed in the hub is complicated enough to demand proper inspection at regular intervals. The installation process is also more complicated as compared to that of fixed-pitch propeller system.
- Controllable pitch propellers have a very high initial cost. And this cost increases rapidly with increasing diameter of the propeller.
- Due to pitch control mechanisms housed inside the hub, the length and the diameter of the hub is also high as compared to the fixed-pitch propeller systems.
To conclude, it is vital to know the applications of controllable pitch propellers. These are used in ships that are required to operate at variable speeds even when operating at constant power requirements. Such ships are tugs, trawlers, ferries, ice breakers, and small scale warships that are equipped with gas turbines.
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