For a 1 foot diameter rotor the circumference will be 3.14 feet so in a windspeed of 10mph, the outer edge of the rotor will be going 52800 feet/hour or 88 feet/minute.

Dividing the circumferencial speed by the actual circumference gives you 88/3.14=28rpm

I believe optimum loading for a savonius is when it's running at a tsr of 1/3 so to extract maximum power you'd want your alternator to hold it down to a little over 9rpm in a 10mph wind.

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.