S-vawts are hard to break down with numbers. I don't know of any way to sit down with a piece of paper and figure out exactly what size/configuration of an s-vawt you need to build get a given amount of power out. I think it's ok to go by 'feel' though...
You can't really say an s-vawt made from 6" pipe will spin this fast in this wind. Besides the height, you've got weight, air gap, bearings, general build quality, load, etc....lots of things play a part, even with a simple-S design.
Then, are you really talking about a simple Savonius? To me that means each blade is a half circle, the gap is about 18%, the blades don't overlap, there's likely a center rod and you've got end plates. Or are you talking about a modified simple S?..
I consider my quick vawt to be a modified simple S...the blades aren't half circles (though they could be), they do overlap a little, there's no center rod, the gap is totally configurable and a bottom plate is all that's really mandatory (to protect the bottom bearings from the weather).
Past about that point you get into the more complicated designs/problems and all hell breaks loose. Then you start pondering the aspect ratio..wider is better for low winds but is lower rpm, tall and skinny for high winds and high rpms. Oh, should I use those "J" looking blades?..those are more efficient. Etc.
There's just so many variables.
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