elements 84 the expansion elements 84 are cut or otherwise disposed through the strip 80 to allow for movement of the strip 80 in the plane thereof in response to twisting the strip 80 the expansion elements 84 shown in fig2 are in the form of shaped slots extending through the strip 80 once the strip as being twisted into position as described above the expansion elements 84 can be filled in fig3 illustrates a single sail 8 in its final position with the expansion elements 84 filled in it follows from the above that the curved surface of each sail 8 is not a simple geometric shape such as a cylindrical surface as best seen in fig4 each of the sails 8 extends between a lower disc 6 and an upper disc 6 but does not meet with the central core 7 thus there is a vertically extending gap or opening 9 between each of the sails 8 and the central core 7 the existence of the opening 9 between the sails 8 and the central core 7 constitutes a substantial difference between the arrangements of the present invention and the abovementioned prior art specification in particular the opening 9 permits air impinging on one sail 8 to be directed onto another so as to increase the rotational force created by wind flowing onto the stages 2 3 the discs 6 are not essential and can be dispensed with if desired however the discs 6 do provide additional structural rigidity for the turbine wind tunnel testing of a prototype indicates good power output and in particular as relatively high efficiency.