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A watercraft, equipped with a jet propulsion system with improved efficiency of operation is disclosed. Structures are disposed within a water passage of the jet propulsion system at a position upstream of the jet propulsion unit that modulate the amount of water that is allowed to pass through the

A watercraft, equipped with a jet propulsion system with improved efficiency of operation is disclosed. Structures are disposed within a water passage of the jet propulsion system at a position upstream of the jet propulsion unit that modulate the amount of water that is allowed to pass through the water passage. The structures can be a flexible fluid filled bag, an adjustable ride plate, and an additional water passage. Each of the structures allow a greater amount of water into the water passage during acceleration than during constant speed travel of the watercraft.

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1. A watercraft comprising:a hull having a bow and a stern; an engine supported by the hull; a water passage in the hull having an inlet at a position forward of the stern of the hull, an interior, and an outlet at a position proximate to the stern of the hull; a jet propulsion unit disposed in a po

1. A watercraft comprising:a hull having a bow and a stern; an engine supported by the hull; a water passage in the hull having an inlet at a position forward of the stern of the hull, an interior, and an outlet at a position proximate to the stern of the hull; a jet propulsion unit disposed in a portion of the water passage, the jet propulsion unit being operatively connected to the engine; a flexible barrier having a water contacting surface with a shape, the flexible barrier disposed in the water passage upstream of the jet propulsion unit; and a fluid disposed on one side of the barrier such that the barrier separates the interior of the water passage from the fluid, and wherein the barrier is movable with respect to the water passage such that water flowing in the passage changes the shape of the water contacting surface of the barrier. 2. The watercraft of claim 1, wherein the water passage is formed at least in part by the hull.3. The watercraft of claim 1, wherein the jet propulsion unit comprises a pump housing and an impeller disposed within the pump housing.4. The watercraft of claim 3, wherein the water passage upstream of the jet propulsion unit comprises a water intake ramp, the water intake ramp comprising at least an interior wall supported by the hull, the fluid being disposed between the interior of the water passage and the interior wall.5. The watercraft of claim 4, wherein the flexible barrier comprises a flexible bag containing fluid that is attached to the interior wall of the water intake ramp.6. The watercraft of claim 1, wherein the flexible barrier comprises at least one fluid containing elongate bag, the fluid containing elongate bag allowing fluid within to flow toward the water passage inlet and away from the water passage inlet.7. The watercraft of claim 6, further comprising a plurality of generally parallel fluid containing elongate bags.8. The watercraft of claim 7, wherein a first set of bags is placed on a starboard side of the water passage and a second set of bags is placed on a port side of the water passage.9. A watercraft comprising:a hull having a bow and a stern; an engine supported by the hull; a water passage having an inlet at a position forward of the stern of the hull and an outlet at a position proximate to the stern of the hull; a jet propulsion unit disposed in a portion of the water passage, the jet propulsion unit being operatively connected to the engine; and a ride plate having a water contacting surface, the ride plate supported by the hull proximate to the inlet, the entirety of the water contacting surface of the ride plate being operatively moveable through a predetermined range from at least a first position where the ride plate portion is disposed behind the inlet and a second position where the ride plate portion partially covers the inlet, such that the position of the ride plate portion determines the size of the inlet. 10. The watercraft of claim 9, wherein the ride plate portion is the entire adjustable ride plate.11. The watercraft of claim 9, wherein the jet propulsion unit comprises a pump housing and an impeller disposed within the pump housing.12. The watercraft of claim 9, wherein the ride plate portion is disposed in substantially the same plane as the portion of the hull immediately proximate to the ride plate portion, and the ride plate portion remains substantially disposed within the same plane throughout its movement.13. The watercraft of claim 9, further comprising an actuator operatively connected to the ride plate portion to actuate movement of the ride plate.14. The watercraft of claim 13, further comprising an ECU in communication with the actuator, the ECU being disposed to receive a condition of the watercraft and to transmit a signal to the actuator in response to the condition received by the ECU.15. The watercraft of claim 14, wherein:the ECU communicates with the actuator through an electrical connection; and the ECU is disposed to receive a signal representative of the acceleration of the watercraft. 16. The watercraft of claim 13, wherein the actuator mechanism is electro-mechanical.17. A watercraft comprising:a hull having a bow and a stern; an engine supported by the hull; a primary water passage having an inlet at a position forward of the stern of the hull and an outlet at a position proximate to the stern of the hull; a jet propulsion unit disposed in a portion of the primary water passage, the jet propulsion unit being operatively connected to the engine; a secondary water passage longitudinally aligned with the primary water passage, the secondary water passage including a secondary inlet and a secondary outlet through which water may flow from the secondary water passage into the primary water passage; a valve disposed adjacent to the secondary water passage that modulates the flow of water from the secondary water passage into the primary water passage, the valve being movable between at least a first position and a second position; and an actuator operatively connected to the valve to move the valve between the first and second positions based on sensed conditions of the watercraft, the actuator is operatively connected to a controller that controls movement of the valve to open toward the first position when the watercraft accelerates and to close toward the second position when the watercraft is operated at a high speed. 18. The watercraft of claim 17, wherein the jet propulsion unit comprises a pump housing and an impeller disposed within the pump housing.19. The watercraft of claim 17, wherein the secondary water passage outlet is disposed rearwardly toward the stern relative to the secondary water passage inlet.20. The watercraft of claim 17, wherein the secondary water passage is disposed forward of the primary water passage and extends rearwardly to connect with the primary water passage.21. The watercraft of claim 17, wherein the secondary water passage is disposed rearward of the primary water passage and extends rearwardly to connect with the primary water passage.22. The watercraft of claim 17, wherein the valve comprises a sliding gate.23. The watercraft of claim 17, wherein the valve comprises a pivoting plate.24. The watercraft of claim 23, wherein the valve comprises a pair of pivoting plates.25. The watercraft of claim 17, wherein the controller controls the actuator based at least in part on a pressure in the primary water passage.26. A watercraft comprising:a hull having a bow and a stern; an engine supported by the hull; a primary water passage having an inlet at a position forward of the stern of the hull and an outlet at a position proximate to the stern of the hull; a jet propulsion unit disposed in a portion of the primary water passage, the jet propulsion unit being operatively connected to the engine; a secondary water passage and a tertiary water passage each disposed laterally adjacent to the primary water passage, the secondary water passage including a secondary inlet and a secondary outlet through which water may flow from the secondary water passage into the primary water passage, and the tertiary water passage including a tertiary inlet and a tertiary outlet through which water may flow from the tertiary water passage into the primary water passage; a valve disposed adjacent to each of the secondary water passage and the tertiary water passage that modulates the flow of water from the secondary water passage and the tertiary water passage into the primary water passage, each valve being movable between at least an open position and a closed position; an actuator connected to the valves to move the valves between the open position and the closed position based on sensed conditions of the watercraft; and a controller operatively connected to the actuator that controls movement of the valves to progressively open when the watercraft accelerates and to progressively close when the watercraft is operated at a high speed. 27. The watercraft of claim 26, wherein the secondary water passage outlet and the tertiary water passage outlet are disposed at an upper portion of the primary water passage.28. The watercraft of claim 26, wherein the jet propulsion unit includes an impeller and the secondary water passage outlet and the tertiary water passage outlet are disposed upstream of the impeller.29. The watercraft of claim 26, wherein the secondary and tertiary water passages are connected.30. The watercraft of claim 26, wherein the secondary and tertiary water passages are in the form of an arch that straddles the primary water passage.31. The watercraft of claim 26, wherein the actuator comprises a first actuator connected to the valve adjacent to the secondary water passage and a second actuator connected to the valve adjacent to the tertiary water passage.