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I guess it depends on how long you want to make it. If you get a gentle enough slope (and good enough conditions), you can have up to a 22degree angle before there is separation. People generally say 15degrees is a good number to stick to. Mine is between 10 and 13 (10 on top, 13 on bottom) and there is good attached flow. I would like to build a boat tail, and would try to make it steeper, like 15degrees. This would help keep the tail shorter.
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