A jet propulsion system for a watercraft includes a duct defining an inlet, a venturi unit, an impeller housing disposed between the inlet and the venturi unit, and an impeller disposed within the impeller housing. The impeller is rotatable in a forward direction and a reverse direction. The venturi unit includes a venturi conduit and at least one door connected thereto. The venturi conduit has a peripheral wall defining at least one aperture. The at least one door is movable between a closed position when the impeller rotates in the forward direction, and an open position when the impeller rotates in the reverse direction. In the closed position, the at least one door closes the at least one aperture. In the open position, the at least one door opens the at least one aperture such that water flows into the venturi conduit via the at least one aperture.

A thrust enhancing device is disclosed. The thrust enhancing device enhances thrust of a thrust generation part in a state coupled to the thrust generation part configured to obtain a propulsive force by using a reaction force of a fluid. The thrust enhancing device includes a venturi part configured to receive a basic fluid allowed to flow by the thrust generation part and allow the basic fluid to pass through the inside thereof, and an ejection induction part disposed in an inner flow field of the venturi part and configured to linearize a flow of a fluid to be ejected to the outside of the venturi part.

A thrust enhancing device is disclosed. The thrust enhancing device enhances thrust of a thrust generation part in a state coupled to the thrust generation part configured to obtain a propulsive force by using a reaction force of a fluid. The thrust enhancing device includes a venturi part configured to receive a basic fluid allowed to flow by the thrust generation part and allow the basic fluid to pass through the inside thereof, and an ejection induction part disposed in an inner flow field of the venturi part and configured to linearize a flow of a fluid to be ejected to the outside of the venturi part.