|Propulsion machinery of ship|
While describing in terms of marine ships, the propulsion machineries can be described as the machineries that provides the power to drive the propeller of the ship which in turn provides the force (thrust) to move the ship through the water.
There are three principle types of propulsion machineries found on the ship. They are
1)Direct drive slow speed diesel engines
2)Geared medium speed diesel engine
Amongst these the diesel engines can be classified on the basis of their speed as:
1) Slow speed engines- Engines having speed up to 150 rpm.
2) Medium speed engines-Engines having speed between 300 to 1000 rpm
3) High speed engines-Engines having speed above 1000 rpm.
This figure shows the arrangement of the propulsion machineries. These propulsion machineries along with the propeller help in the movement of the ship in both forward and astern direction. The arrangement of these propulsion machineries is such that they normally propel in the forward direction. To make the ship move in reverse direction, there are following three methods. They are
1)Direct reversal of engine so that the propeller turns in the opposite direction
2)A controllable pitch propeller (CPP) which changes the angle of the blade as the engine rotates in the same direction
3)There are reverse gear and clutches which causes the propeller to turn in the opposite direction
For slow speed diesel engines mostly method 1 is used, but sometimes method 2 can also be used. For medium speed diesel engines all the three methods can be used, but method 2 is most common amongst all.
But in case of steam turbine, a separate stern turbine is required to drive the propeller in reverse direction. This astern turbine is mounted on the same shaft as the astern turbine or it can be separately geared to the main shaft. The ahead and astern turbines have their own separate steam stop valves.
Coming back to the propeller machineries, there are mainly four things which contributes to the propulsion unit. These are as following:
1) TRANSMISSION SYSTEM
This system helps in transferring the power developed in the engine to the propeller. This contributes main engine, thrust shaft, thrust block, tunnel bearings, stern tube, and propeller. All these constitute together and help in transmitting power from main engine to propeller.
2) THRUST BLOCK
The function of the thrust block is to absorb the thrust forces. The power developed by the propeller is so large that at once it can de root the main engine from its position. To absorb this power and uniformly distribute it all over the ship there is one arrangement called thrust block, the work of this thrust block is to absorb the thrust forces developed by the rotating propeller and transmitting this force to the ship's structures. Thrust collar is forged as the part of the shaft and tilting pad bearings absorb the transmitted force and delivers it to thrust bearing housing and hence the ship's structure.
3) PLUMMER BLOCK- the Plummer block (also known as tunnel bearing or intermediate bearing) is there to support the weight of the intermediate shaft. They only have lower shaft bearing shells with exception of the aftermost bearing which has a top and bottom bearing shells because of the tilting effect of the overhanging propeller.
4) STERN BEARING- Stern bearing or stern tube gearing is there to serve two important purposes i.e. to support the tail shaft (propeller shaft) and considerable propulsion of the propeller weight, which is its main work. Apart from this it can also act as a gland to prevent the entry of seawater to the machinery spaces.