A system and method of starting a water jet propulsion system for a ship in which the propulsion system includes a stator shell adapted to be mounted to the hull and having a nozzle ending in an outlet with a cross-sectional outlet area, an impeller housing attached to the stator shell and having an upstream inlet, and an impeller rotatably mounted in the impeller housing for receiving water from the inlet and discharging it through the nozzle of the stator shell so as to create a water jet upon rotation of the impeller. The method includes reducing the outlet area by partly closing the nozzle during a start up phase of the water jet propulsion system, wherein there is provided a back flow hindering arrangement arranged to hinder air to enter into the impeller housing via the nozzle.