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
), 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
). 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
3
CMCR
CMCR
P
n
P
n
æ
ö÷
ç
÷
ç
=
÷
ç
÷÷
çè
ø
