Arnulf, good to hear from you... cavitation will always limit the tipspeed and as you say will be more of a limitation near the surface where the pressure is less than deeper down. It is possible we could run the lower turbine rotors faster, however the tidal flows are less at greater depth, so that might not be so efficient. We would try to operate the top rotors as near as possible to the surface but below the wave dips - perhaps having the tips 5m down...
As you know only too well, the energy of a stream varies with the cube of the flow, and my impression is that blue water flows such as the Gulf Stream are much slower than the strongest tidal currents... perhaps 0.5 m/s on average compared to 2 m/s.... that means only 1/64 of the energy...! And yes, I think average mean speeds will vary over this range in a similar way to wind, so one-size-fits-all not so good... What is your impression?
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by John Armstrong