Operation
0070−1/A1
Winterthur Gas & Diesel Ltd.
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The Relation between Engine and Propeller
1. General 1..........................................................
2. Fixed Pitch Propeller (FPP) 2........................................
2.1 Continuous Service Rating (CSR) 2............................
2.2 Engine Margin (EM) / Operational Margin (OM) 2................
2.3 Load Range Limits 3..........................................
3. Controllable Pitch Propeller (CPP) 4.................................
3.1 Load Ranges 4...............................................
3.2 Control System 5.............................................
1. General
There is a specified relation between the propeller speed and the absorbed power in
ships that have fixed pitch propellers. The relation is between the propeller and the
speed at which it turns.
The formula that follows (where P = power and n = speed) gives an approximate
result, which is sufficient for conventional vessels:
=
P
1
P
2
n
1
n
2
3
The graph from this formula is known as the propeller characteristic.
If the engine is in good condition, correctly supplied with air (i.e. turbocharger(s) are in
good condition and the resistance of the air and exhaust lines is in the specifications)
and the fuel injection quantity is correctly adjusted (see the shop test protocol), then
the mean effective pressure (mep) developed during service conditions (in
accordance with the specified load indication), is related to the approximate mep for
this position on the test bed.
In the diagram (see paragraph 2), the propeller property line through the CMCR point
(100% power at 100% engine speed) is known as the nominal propeller characteristic.
Engines which are to be used for the propulsion of vessels with fixed propellers have
a load applied on the test bed in accordance with this propeller characteristic.
However, during sea trial of a new ship with a smooth and clean hull, the power
requirement is lower and the operation point is below the nominal propeller
characteristic.
During service, a higher torque will be necessary for the propeller to keep its speed
than at the time of the sea trial (sea margin) because:
D There are changes in wake flow conditions because of marine growth on the hull.
D The cargo load has an effect on the depth of the vessel in the water.
D The propeller has a rough surface or has mechanical damage.
D The vessel operates in bad sea and weather conditions.
D The vessel operates in shallow water. The mep of the engine (and thus the fuel
injection quantity) will increase. In such a condition, the operating point will then
be at the left of the initial propeller curve which was calculated during sea trials.
A hull that was cleaned and painted will help to decrease the resistance as the vessel
moves through the water. It is not possible, to get the hull back to its initial condition.
Because the thermal load of the engine is related to the mep, the position of the
operating point is also important. The air supply to the engine and the operating
conditions will become unsatisfactory if the operation point is far above the propeller
curve.
To get the best conditions, the operation point of the engine for service range must be
on or below the nominal propeller characteristic.
General
2014
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