Marine Installation Manual 2021-08 2-3
2 Engine Power and Speed
2.4 Power range
X72DF
Maximum propeller
diameter
The maximum propeller diameter is often determined by operational require-
ments, such as:
• Design draught and ballast draught limitations
• Class recommendations concerning propeller/hull clearance (pressure im-
pulse induced on the hull by the propeller)
The selection of a main engine in combination with the optimum propeller (effi-
ciency) is an iterative process where also commercial considerations (engine and
propeller prices) play an important role.
According to the above approximation, when a required power/speed combina-
tion is known (for example, point Rx1 in Figure 2-1,  2-1), a contracted max-
imum continuous rating line can be drawn which fulfils the ship’s power
requirement for a constant speed. The slope of this line depends on the ship’s
characteristics (see coefficient α in Formula 2-1,  2-2). Any other point on this
line represents a new power/speed combination, for example Rx2, and requires
a specific propeller adaptation.
2.4 Power range
Propeller curves and operational points
To establish the proper propeller curves, it is necessary to know the ship’s speed
to power response.
Determining power/pro-
peller speed relationships
Normally, the curves can be determined by using full-scale trial results from sim-
ilar ships, algorithms developed by maritime research institutes, or model tank
results. With this information and by applying the propeller series, the
power/speed relationships can be established and characteristics developed.
The relation between absorbed power and propeller speed for a Fixed Pitch Pro-
peller (FPP) can be approximated by the following cubic relation:
Formula 2-2
where:
P .............. = propeller power
n .............. = propeller speed
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