Iveco Marine Diesel Engines, Installation Handbook, Page: 28 / 132

In addition, it is important to make the right choice to prevent:
■
Requesting a power above the rated one
■
Requesting a power needed for gliding incompatible with the power which can be supplied by
the engine
The wrong design of the propeller substantially reduces the boat performance: in the first case the max-
imum speed reached by the engine will be below the rated value and equal to the balance between
the engine torque and the propeller resistance.
In the second case the time need to reach gliding will be prolonged or it will not be possible to reach glid-
ing and the engine maximum speed will be noticeably lower than the rated value (see also Section 3).
In both cases we are persuaded that the engine setting is wrong, whereas these conditions are due to
a propeller which, with the same rotation speed, has actually been designed for an engine greater than
the one installed.
It is possible to avoid the above mentioned cases by following this rule: foresee a maximum speed
reached by the engines when sailing with a new boat fully laden 3% above the engine rated speed.
This rule is based on the fact that the power required to the engine increases when the boat is used,
because of the weight growth and the presence of incrustation and vegetation under the bottom and
on the propeller.
Displacing boats
1.Torque limit curve/real average pressure/input, for the engine - 2. Absorption curve of a propeller
too big for the application - 3. Absorption curve of a propeller with the right size (pattern closer to
the cubic one) - 4. Absorption curve of a propeller too small for the application - 5. Gap curve.
MARCH 2004 ENGINE/BOAT CHOICE FACTORS
2.28
MARINE ENGINES INSTALLATION
Figure 2
M
rpm
1
2
3
4
5