Alamarin Jet AJ 160, Operation And Maintenance Manual

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Operation and Maintenance Manual
1616
Version 1.5
16
Figure 2.4-2
The most common malfunction in water jet propulsion units manifests as cavitation.
Engine overdrive and stopping of thrust are signs of this.
Cavitation is a phenomenon in which the water pressure decreases locally in such an
extent that water vaporizes on the surface of the impeller blade, creating steam bubbles.
The bubbles move on the surface of the blade and when they reach a higher pressure
area they collapse. Cavitation incurs considerable reduction in capacity and damages the
impeller.
Usually the cause is the reduction of pressure in the whole intake duct, which is induced
by a blockage.
Cavitation can often be heard as a rumbling sound.
Any factor that hinders the flow of water in the jet increases the chance of cavitation.
The jet may be run by the engine even out of the water, because the bearings are greased
and oiled. This is a significant benefit in comparison to water-lubricated bearings,
which do not sustain dry running well. During dry runs, a temporary water inlet must be
arranged for the motor sea water cycle to cool the engine.
With these simple instructions it is
possible practically without exception
to clean the jet even after the boat has
stopped completely in reedy water.
Unlike with other propulsions, in boats
equipped with the jet the engine stops
very rarely as a result of a reed blockage.
If the measures mentioned above are
not of help, the propulsion is equipped
with an inspection hatch (figure 2.4-2),
through which the blockage can be seen
and removed.
Remember to close the hatch! The wing screw is tightened by hand!
2.6. Cavitation and Ventilation
2.5. Dry running
2.6.1. Cavitation