Iveco Marine Diesel Engines, Installation Handbook, Page: 43 / 132

MARCH 2004 DRIVE
3.43
MARINE ENGINES INSTALLATION
Another important value is the blade area-disc area ratio (As/Ad), i.e. the ratio between the total area
developed by the blades and the area corresponding to the circle formed by the propeller rated diam-
eter. With the same absorbed power, rotation speed and diameter, this ratio increase contributes to
reduce the superficial pressure and therefore the potential cavitation risks.
The propeller dimensioning is extremely important to establish what engine power enables the boat
highest speed. It depends on the bottom shape (displacing, gliding, semi-gliding) and on the distribution
of the weights onboard. The precise propeller dimensioning in relation to the diameter, the pitch, the
number of blades and the speed shall be carried out at the shipyard.
On displacing work boats, such as tugs and trawlers, it is better to use propellers with big diameter and
low rotation speeds. On gliding boats, such as yachts and patrol boats, it is better to use propellers with
small diameter and high rotation speeds. It is suggested to size the propeller for 90% of the rated max-
imum power, so that the engine, on a new boat fully laden, can reach a speed above 3% that of the
maximum power.This way it will be possible for the engine to reach the maximum power speed when
the bottom is dirty.
Propellers with the wrong size do not allow the transfer of the maximum power supplied by the
engine.
1. Engine power curve - 2. Propeller absorption curve - 3. Gap curve - 4. Work point with new boat -
5. Point of maximum power to be reached when the boat is laden and the bottom not perfectly clean.
Propellers too small
As the power required is lower than the engine rated power, the work point is to be found on the gap
curve. In this case, the engine speed is higher than the rated speed and the power supplied is lower
than the maximum one.
Propellers too big
The propeller power absorption is higher than the engine rated power. The engine cannot reach its
rated speed and therefore it is overcharged.
As a result, black smoke could be emitted at the exhaust and this could lead to long-term engine dam-
ages and wear.
Figure 10
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