Abstract: The rotor and stator wheels of a hydrodynamic retarder, with confronting annular gorges together defining a toroidal channel of circular or slightly elliptical cross-section, have blades within their gorges which are of wedge-shaped cross-section and have slightly rounded edges that may or may not coincide with the radial direction. The blades have flat leading and trailing flanks, the former including an angle of about 45.degree. - 55.degree. with the plane of rotation so as to be inclined to the axial direction at an angle of about 35.degree. to 45.degree.; the vertex angle of the wedges lies between about 15.degree. and 20.degree.. Such a blade structure can be cast in a unitary mold from which it is extracted by a helicoidal twisting motion. A suitable radius of curvature for the edge rounding is in the range of about 1.5% to 4% of the channel diameter.