Alamarin Jet AJ 340, Operation And Maintenance Manual

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Problem situations
Operation and maintenance manual
KHO/340/EN/1.0.0. 35
5. Problem situations
5.1. Cavitation
The most common malfunction in water jet propulsion units manifests as
cavitation. Cavitation is a phenomenon in which the water pressure decreases
locally to such an extent that water vaporises 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.
Signs of cavitation are engine overdrive and stopping of thrust, and it can
often be heard as a thundering sound.
Cavitation reduces performance considerably and damages the impeller.
Any factor that hinders the flow of water in the jet increases the chance of
cavitation. Usually the cause is the reduction of pressure in the whole intake
duct caused by a blockage.
If cavitation is apparent or the boat runs slowly even though the engine runs
on high revolutions, investigate the cause.
Checking the causes for cavitation:
1. Open the inspection hatch (figure 31).
Figure 31. Opened inspection hatch
2. Check the grass rake (figure 32).
Make sure that grass, reed, plastic, stone or some other extraneous object
is not clogging the grass rake. Remove any possible blockages.