A scoop-trimmed fluid coupling wherein the transmitted torque decreases continuously with increasing ratio of runner speed to impeller speed at each degree of filling of the working circuit with liquid. During acceleration of the runner, the scoop tube is moved by its actuator from a "circuit empty" to a "circuit full" position in several stages and at different speeds so that the torque which is transmitted by the coupling during acceleration of the runner remains substantially constant. If the load upon the runner varies from acceleration to acceleration of the runner, the operation of the actuator during acceleration of the runner is modified by a controller system which monitors the energy requirements of the prime mover for the impeller or the acceleration of the runner. The actuator is a single-acting hydraulic cylinder having one or more auxiliary ports which are permitted to admit additional pressurized fluid into the cylinder chamber in predetermined intermediate position or positions of the scoop tube during movement from the "circuit empty" to the "circuit full" position. The speed of the scoop tube can increase from stage to stage; alternatively, and especially if the load upon the runner varies from acceleration to acceleration of the runner, the stages or intervals during which the speed of the scoop tube increases alternate with stages or intervals during which the scoop tube is idle or moves backwards toward the "circuit empty" position.