I am really curious how you manage to get to single shell with a gap in between. I wouldn't know how to do that ;-) Setting spiral vase generates only one perimeter and that is what you need - only one shell.
The assembling of the sterntube is better to see at the other boat: http://www.thingiverse.com/thing:790998
Whatever you take, you need a stern tube which is glued into the printed tube to protect the PLA of getting hot (frictional heat).
Are you sure, you can modify the sterntube with 123D?
Otherwise tell me what OD you need and I can upload a separated part for testing (only sterntube with motor mount)