@ Isaac – thanks
It is all too easy to put excessive pressure when gluing the blade skin parts and get an airfoil section similar to Saturn viewed from the side. Shaping before gluing is definitely recommended!
@ Erik – I confess not to care about how freely the propeller can rotate. With just a pin and paper parts, the friction is relatively high. With a brass pipe, it can be better
Regarding centering, the propeller hub has a tightly rolled paper part inside, with just the space for the pin to pass. This way, I assure that the pin is pretty perpendicular to the hub formers.
Well, 12 cylinders equate to 12 exhaust pipes. Tedious work, here, somewhat facilitated with inside paper bits and rolled parts.