During the refit in the 1990s I installed a heavy duty prop shaft to cope with the then petrol engine. A 6mm dia shaft in a 8mm dia tube. This had always exhibited a rattle at certain revs which is both annoying and inefficient. Maybe I induced a curve in the shaft when I fitted it. You can see scoring halfway down the shaft.
With the tube fibreglassed in, removal is not an option. However, a 6mm dia tube running a 4mm dia shaft could be fitted inside it. A 450mm long CAP Maquettes shaft did the job; though I am not impressed with the play between shaft and bearings. This outer tube, flush at the prop end stuck out about 5mm into the boat. I cut a suitable collar from a 6mm plain coupling fitting and soldered it to a plate that locks over the original grease nipple. This allows the new shaft to be locked in position and be removable (if those lose bearings prove a problem).
There was sufficient length on the shaft to extend the thread at the prop end and allow the use of a lock nut. Two new alloy propeller's, bought for the new 4mm shaft, were threaded right through and so it seemed a good idea. I also drilled some old fibreglass props through for the same. Now fear of loosing a prop in reverse.
I also came across a company called motion.co that do various shaft connectors and thought I would try the one in the photo. There is no end float which could be a disadvantage unless you set up the propeller etc very accurately.
Without moving the motor the coupling gap is smaller and I have trimmed down a standard plain coupling to fit as a fall back.
So, with the power problem resolved, and a larger motor fitted, it needs some sea trials.
David