2.11 Waterjet propulsion

Waterjet propulsion is the most common propulsion system for high-speed vessels of nonplaning type. A comprehensive and practically oriented article on waterjet propulsion is presented by Allison (1993). Cavitation is easier to avoid for a waterjet than for a propeller at high speed.

If the engine power and the resistance curve of the vessel are known, we can easily make a preliminary selection of a waterjet propulsion system with diagrams provided by the manufacturer. There are different series with different outlet nozzle diameters and blade pitch angles. Let us illustrate this with an example. Figure 2.52 shows, for one particular waterjet system, different lines of constant brake power in kW (BKW) as a function of vessel speed. The brake power is the power delivered by the propulsion machinery. The vertical axis is the net thrust accounting for mechanical losses. In Figure 2.52, we have also plotted the resistance of the vessel together with the waterjet propulsion data. The intersection between the resistance and BKW-curves determines the needed power for a given vessel speed. The vessel should operate less than 10% of the time in zone 2, according to the diagram. This is associated with pump cavitation. Figure 2.52 also shows a zone 3, where a vessel must be only less than 1% of the operating time. However, Figure 2.52 shows that the studied vessel will never be in zone 3 with this particular waterjet propulsion system. Different loading conditions, that is, resistance curves, of the vessel must...