Waterjets are common place in vessels needing to achieve speeds of 30+ kts where conventional propeller solutions are unable to overcome the associated issues of cavitation, which in turn can lead to thrust breakdown and material failure. Any vessel designed for high speed requires a low resistance and corresponding slender hull. Of course this is not enough, the propulsor needs to also be capable of accepting a high level of power and hence for a given diameter a high power density.
Wärtsilä identified the need for a more compact, performant waterjet installation simply because the design of the vessel dictates the maximum size of the waterjet transom flange diameter. Due to this constraint the operational envelope of a waterjet can be limited at speeds well below the designed top speed. For fast patrol boats and fast attack craft flexibility and good acceleration at patrolling speeds is one of the key performance indicators, however.
The task for Wärtsilä was clear but by no means easy as the mechanism by which the jet delivers enough thrust at top speeds (small inlet diameter and high power) is opposite to that for low patrolling speeds (large inlet and low power). By changing the pump geometry, Wärtsilä discovered a break through. The new pump is characterized by its axial geometry, whilst keeping the typical top efficiency of the commonly applied mixed- flow pumps.