Most of the propellers that are used in ships are fixed pitch
propellers (FPP). What does this propeller term means? Let’s understand this
from a beginner’s point of view.
If you look at a
propeller axially, you will notice a twist in the propeller blade along the
length of the propeller hub. It is this twist in the blade that results in the
forward motion of the propeller with each rotation.
It is pretty much what
you see on a screw. The angle of the thread is actually what is called the
pitch angle. The more the pitch angle, the more distance is travelled by the
screw in one rotation, that is, the more is the pitch. Similarly, propellers act
on the principle of screw. Hence the term screw propeller.
But what is a fixed
pitch propeller or the FPP?
The propeller is
designed in a way such that, if you cut the propeller radially at any random
radius from it’s centre, then the cut section of the propeller blades will have
a certain pitch (magnitude depends upon how twisted it is along the length of
the hub). Now if you rotate this cut section in water, the axial distance
travelled in one rotation is the pitch of this section. If you vary the radius at
which you cut a section, the twist of the blade also varies from the root to
the tip. This makes it evident that each section of the propeller has a
particular pitch.
The root of the
propeller will have a different pitch from the tip of the propeller. So the
pitch variation from the root to the tip is as shown in Figure 1. The question
now is, if each section has a different pitch, then what is the resultant pitch
of the propeller? Answer is the resultant pitch is the average of all the
pitches of each section of the propeller.
Note very carefully,
that the pitch varies along with the blade section (or increasing distance from
the propeller centre), but the net average or the net pitch of the propeller
remains constant. Such a propeller is a fixed pitch propeller.
Now, let’s go back to
the concept of a screw. What if you could somehow change the angle of
inclination of the threads along the screw?
If you could, what
would you actually be doing is changing the pitch angle and subsequently, the
pitch of the screw.This is exactly why the controllable pitch propeller was
developed by the industry. We will be dealing with the applications in detail a
little later. But for now, it is vital to know the fundamental working of a Controllable Pitch Propeller
(CPP) also commonly known as Variable Pitch Propeller.
The twist (to be
referred as pitch from now on) of the root section of the blade is always with
respect to the angle it makes with the propeller hub. What if we can change the
angle of orientation of the root? And since all the other blade sections had
their previous pitch angle already, they attain a new pitch angle. In other
words, you are able to control the pitch of the entire propeller, by changing
the orientation of the root section on the hub. This is done by the means of a
hydraulic cylinder.
When the required
orientation or pitch of the propeller is obtained, the blades are locked in
that position. So the blades are not an integral part of (or not casted along
with) the propeller hub. They are mounted on spindles that have axes
perpendicular to the shaft axis.
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.
·
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 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 faster response of speed change.
·
The efficiency of
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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