Iveco Marine Diesel Engines, Installation Handbook

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MARCH 2004 ENGINE/BOAT CHOICE FACTORS
2.31
MARINE ENGINES INSTALLATION
2.5 MECHANICAL AND AUXILIARY COMPONENTS
For the evaluation of the engine installed it is necessary to consider the presence of each component
producing power absorption and to know exactly the absorbed powers, as the power taken by auxil-
iary components is no longer available for the flywheel. It is also necessary to check that these powers
comply with the absorption limits.
Winches, generators, pumps and other elements actuated by power takeoffs generate torque and
power absorption from the engine, according to their performance and the moment they are used.
The reducers-inverters, the thrust bearings and the supports for the axis line, between the engine and
the propeller, absorb power as a result of the friction during sailing.
The absorptions of the reducers-inverters vary according to the type and are included in the 3-5%
range. The supports, the axis line and the thrust bearing have an incidence amounting to approx. 1%
for each element.
2.6 SPEED AND POWER PERFORMANCE
The designed and/or wished speed cannot disregard the economic and critical speed values.
The forecast of the maximum speed and the speed required by the propeller axis for the different types
of hull can be based on the following mathematical methods.
Displacing hulls
The forecast of the maximum boat speed in relation to the engine installed can be obtained through
the following admiralship formula.
where:
• V : Hull speed (Kts)
• SHP :Total installed engine (HP)
• D : Displacement
• C : Admiralship coefficient
- L : Waterline length
- B : Waterline width
Summarising table of C values
L/B 1 2 3 3,5 4 4,5 5 5,5 6 6,5 7
C 20 30 40 47 60 80 90 110 125 145 165