Abstract: A water jet propulsion system for moving a boat through the water includes apparatus for varying the thrust of the jet by varying the size of the stream of water being discharged from the nozzle of the jet propulsion unit. The apparatus for constricting the size of the stream of water also serves to direct the thrust of the stream upwardly or downwardly as desired as the stream discharges rearwardly of the boat so as to counteract any tendency of the boat to "porpoise". Interconnected closure members located respectively in the flow path leading directly to the nozzle and in a forwardly directed secondary flow path serve to close off the latter when the water is being passed under pressure via the nozzle and vice versa.