[aerohead]

Originally Posted by Phase

https://shareok.org/handle/11244/316319

found this. inflatable boat tail. diff study. up to 80 percent drag reduction. interesting idea but doesnt seem like it will ever be a production item

Unless the setup was tested at supercritical Reynolds number, the Cds are of no help to us.
And I would argue that, when it was tested, the unbaffled tail would have the lower drag.
Whether or not this sort of thing ever made it into production, while of great interest, wouldn't affect us crazies, who could run with it in lieu of cultural imprimatur.
Kamm and Fachsenfeld tested a virtually identical setup at the FKFS, around 1935. Boat-tailing reduced drag up to 892%, depending on fineness ratio ( from Cd 0.58,to Cd 0.22- Cd 0.065 ).