Tidal power generation by means of marine current farms is potentially a large renewable energy resource which could be harnessed in many coastal waters. Its availability is highly predictable in time, and the technology promises high energy conversion efficiency along with a relatively low impact on sea life due to its relatively small disturbance of natural tidal flows. A series of devices have so far been proposed and developed for the extraction and conversion of kinetic energy present in tidal flows into useful electrical power [1]. Designs include horizontal axis turbines, vertical axis turbines, and devices with oscillating lift surfaces. Up to date no technology has firmly established itself. This paper describes a novel hydraulic control mechanism designed for vertical-axis marine current turbines of the straight-bladed Darrieus type. It has been found to significantly improve turbine efficiency over conventional Darrieus turbines when operated at low blade tip-speed to tidal-flow-velocity ratios (TSR) and to give the turbine the ability to self-start reliably. The control mechanism enforces a cyclic pivoting motion on the turbine blades as they move around their circular flight-path. The movement of the pitch control is of sinusoidal shape and is continuously variable in amplitude. The blade actuation is powered by the turbine's own rotation and is implemented using a swash-plate mechanism in conjunction with a hydraulic circuit for every blade. For surface piercing turbines, this control mechanism may be remotely positioned in a dry nacelle above sea level. If the appropriate design is applied, this can offer access to the cyclic pitch control mechanism, gearbox and generator, even when the turbine is operational, promising lower maintenance and operating costs compared with submerged systems. (c) 2006 Elsevier Ltd. All rights reserved.