BTW; I copied your above massive text block into a document file and split it up into paragraphs so I could see where you're at!
My conclusion: so far so good BUT!
You made the one classic mistake of many model boat / ship builders ๐ค
You continued the prop shaft tube right back to the propeller and hence you had to make oversize struts to support them.
This is fundamental wrong and creates unnecessary work.๐
On real ships, including the Schnellboote, the so called 'stuffing tube' is JUST THAT, it 'stuffs' the shaft through the hull and includes stuffing glands to prevent the ingress of sea water.
Outside the hull ONLY the rotating shaft itself continues on through the
bearing
in the support strut and to the prop.
See attached pics of my HMS Belfast as an example.
There was actually no reason for you to make oversize strut
bearing
s, simply bushes to match your prop SHAFT not the tube would have been correct.
Inside the real ship there is also NO TUBE, only
bearing
s at suitable intervals. They look like gigantic versions of the big ends in your car.
Imagine on really big ships, carriers, container ships, bulk tankers etc, with shaft diameters of 1metre or so how big the 'tube' would be, how much weight that would add and how difficult it would be to service and maintain!
I've often noticed in posts here that folk confuse shaft and tube, often referring to the whole assembly as 'the shaft'.
For convenience we modellers use prop tubes, who wants to fiddle about making a row of internal shaft
bearing
s no one will ever see and will most likely never be really concentric?
The downside is that continuing this 'convenience' outside the hull is wrong, adds weight and detracts from the scale appearance of the model. ๐ญ
OK, it's 3am here now so - orf me 'obby 'orse and up (in my case down!)
the wooden stairs to Bedfordshire, G'night all, cheers, Doug๐
Re shaft length: What fits fits, what don't don't!
Such a question is like asking 'How long is a piece of string?'!
If all three motors abreast won't fit you have to decide if the central motor should / will fit fore or aft of the outer motors. Then measure / adjust the shaft length accordingly. Before you start fitting the centre motor check what length shafts are commercially available and adjust your motor fit to suit.
Otherwise make your own shafts and tubes to fit as required, as I've started doing cos I got fed up with 'standard sizes' wot don' wanna fit my ship. ๐ค G'night All, cheers, Doug ๐