This paper argues that although widely-spaced horizontal-axis rotors with axial flow are universal for wind turbines in an open flow field, the transfer of the design to tidal streams may be wrong for the installation of large numbers of units in channels like the Pentland Firth where walls and the sea bed form a restricted flow passage. Vertical-axis rotors can be placed very close to one another so as to give a high blockage and performance well above the Betz limit. The vertical axis configuration also allows better mountings for blades: if a beam is supported at both ends the bending moments are one quarter of those of a cantilever. Blade element analysis shows that with proper control of blade pitch, a cross-flow rotor can give a constant pressure drop across 95% of the width of a rotor rather than the uneven drop across the disk of an axial flow one.
This can make the down-stream wake less turbulent than the input flow and so improve the performance of successive rows. Rigid, neutral-density struts with spherical end-bearings can provide an efficient transfer path for very large forces to the sea bed.
