A boat drive has a housing, a tube shaft rotatable in the housing about a shaft axis and having axially opposite front and rear ends, a hub carried on the shaft rear end, and a plurality of radially projecting vanes pivotal on the hub. An axially shiftable core shaft extending coaxially in the tube shaft has a rear end rearward of the tube-shaft rear end and a front end forward of the tube-shaft front end. Mechanism connecting the core-shaft rear end with the vanes pivots the vanes on the hub on axial displacement of the core shaft. A pair of oppositely directed axial-thrust bearings on the core-shaft front end are contained in and surrounded by a hollow piston having front and rear faces axially flanking the thrust bearings. A cylinder fixed in the housing and surrounding the piston forms with the respective faces thereof front and rear compartments so that pressurization of the front compartment shifts the core shaft rearward and pressurization of the rear compartment shifts the core shaft forward.