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Have you ever seen such a thing do more than power a lightbulb though?
Propellors beat a Savonius design every time, for any load bigger than
a spinning advertising sign

Those spinning advertising signs are sodding dangerous. I saw a toddler
get his head seriously smashed by one once. Just toddled into it on a
windy day. Some of them are very solidly made and have considerable
rotating mass and no apparent overspeed braking.

Come to think of it, I haven't seen one for quite a while.

--
Skipweasel
Pay no attention to that man behind the curtain.