Hmm. Another hour gone! As is the way of the web, you go looking for answer to A, end up getting answers to B, C and a lot of other stuff you didn't know you wanted to know. (I love looking in tool catalogues - just to see the tools for jobs I never knew needed doing.)
Anyway, back to topic, I have acquired (by default really) Raboesch 'single' universal joints, which having read all the above I will probably convert to doubles. However the splined brass inserts to go to the shafts are not a very tight fit in the couplings. I'm thinking from some of the above that they are meant to allow for the half mm of longitudinal play in the prop shaft (between ahead and astern) and so the motor should be installed with a little bit of clearance between the shaft ends and the coupling parts. Or should one or both of the splines be glued in place?
If I do double them I'm thinking it would be a good idea to glue at least the link insert into the centres of the couplings, to ensure it's a solid shaft there. Opinions?