Résumé : A tidal turbine and generator assembly having a turbine with rotor blades 16 disposed around a rotor axis C and mounted on a tower 14, wherein a pair of aerofoils 22 is mounted upstream of the blades either side of the rotor 13, each aerofoil mounted on a rotatable support 26 and able to turn around a vertical axis Y while its chord line remains orthogonal thereto. Simultaneous and symmetrical swivel of the aerofoils can concentrate or diffuse the water flow incident on the turbine and thereby control rotor speed and power and also protect the turbine from overload in high tidal flows. The aerofoils effectively provide an inlet funnel and are preferably arc-shaped, curved around the rotor axis. They are preferably mounted in line with the yaw axis Z of the turbine, which is preferably weight balanced on axis Z and yaws with respect to the tower on a pintle 12. The turbine blades are preferably retractable, so that collision with large, heavy objects coming their way and detected by a sonar 34 may be avoided. Finally the rotor may have a buoyant part to counteract the weight of generator 15 and turbine blades 16.