The authors proposed the new propulsion system and the method of performance improvement by blowing moist air for a water jet ship in previous papers. The performance of the water jet ship by blowing moist-air into the nozzle was investigated and it is found that the thrust increased about 20% in comparison with the thrust by the conventional method in a former paper. In this paper, the flow visualization experiment by PIV method was performed to make clear the reason of the increase of the thrust by blowing moist-air. From the analysis of velocity vectors, it was found that the steam blown into the nozzle condensed just after the outlet and that the acceleration of water flow was remarkable. Accordingly, the average density of fluid increased by the increase of density. Therefore, it is clear that the thrust increased by using the moist-air.
