On 20/10/2010 20:25, andrew wrote:
I fancy playing with a screw device in our weir but cannot see how it would
trap a fraction of the flow let alone work at 90% conversion. Even if it
were in a tube with minor losses around the periphery why should it get
more out than a simple propeller type turbine in the same tube?

Both the Archimedes Screw Turbine and the Kaplan Turbine are supposed to
work at a very low head. A quick google suggests that both can exceed
80% efficiency but I didn't find anything that gave a direct
relationship between head and efficiency.