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Neil,
The first portion of your boat tail would allow the air to stay attached, but because the flare angle increases sharply towards the rear, I'm inclined to think that the air will most definitely detach before it reaches the end.
ASTM calibration orifices have a similar problem. These are orifices that are used for calibrating flow meters in the lab. We have them at work. Sonic nozzles have a limit on the maximum included angle that the exit cone can open up at.
I believe a total angle of 15° is considered the maximum that a cone could open up and still maintain attached flow.
The Honda Insight, as well as the Prius both has a maximum angle of about 20° or so. I gets correspondingly harder to keep the flow attached at angles steeper than this, although in some cases it *may* be possible.
If I add a boat tail to the Insight someday, I will be sure to keep the maximum angle at 20°, otherwise the downstream side of the tailboat is non-functional after the air separates.
Of course, you *could* build one to your specifications and do tuft testing to see just how far the air stays attached.
Aerohead has put quite a bit of information on site about this very issue. It may be worth a second review before proceeding.
Jim.
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