Abstract: An accessory device that produces a variable-aperture orifice in the outlet pipe of conventional water-jet nozzles. In one embodiment, the attachment is mounted on the exterior of the outlet pipe; in another, it is installed as an insert inside the outlet pipe. The device comprises two opposite, substantially semi-cylindrical shells slidably coupled along their longitudinal edges and hingedly connected at one end to form a cylindrical structure which is capable of deformation at the other, free end to produce a variable cross-section. The aperture of the structure at the hinged end is sized to fit tightly with the size of the nozzle's outlet pipe, while the cross-section at the free end may be progressively reduced by clamping the two shells together. A clamping mechanism, such as a hydraulic bladder or a cable-driven clamp, is mounted around the free end and is used to reduce its cross-sectional size during operation of the watercraft.