What do you think about using such an item (inox sailing / balcony cable tensor) as a coupler ? Of course replacing the left part of the below picture by the gear shaft, and the right part by the prop shaft…
There would not be any slippage and alignment should be fine ?
definitively no slippage but It will be very hard to make the threading on the shaft and the gearbox/motor. Keep in mind concentricity is also very important for the lip seals to work as intended also avoid vibrations.
I would rather use a rigid coupler like this one from ruland with Loctite 638
Hi and thanks @Hiorth
I was thinking about threading as it should provide a perfect alignment thus no vibration ? Maybe I am wrong…
What about a tendon joint from the windsurfing world ? Sustains misalignment and cancels vibrations.
Each axis to tendon link could be ensured by 2 hollow shaft maintained by 2 pins/screws.
10euros at https://www.decathlon.it/diabolo-bianco-piede-dalbero-id_8511178.html
At North/Duotone, they have replaced the traditional polyurethane tendon by an even stronger high-tech BASF polyamide.
That could work, would depend on the torque the plastic piece is able to transfer, I would also be a little concerned about concetricity. Ideally the shaft is “Independent”(jaw coupling ir similar) of the gearbox, but ideally that requires a dobble ballbearing that keeps the prop shaft from wabbeling. Using a plastic joint should be an ok compromise where some rigidityis left that account for the misalignment 
Our design with a rigid shaft coupler with only one ball bearing is a hack. It works (if put together very carfully), but not ideal. As a shaft coupler will never be 100% straight, there will always be some misalignment, even tho it’s not visible for the eye.
I was thinking about that too.
Another possibility would be 1 steel keyway on the whole length of the tendon cylinder. That would:
