I too have been thinking about propeller shaft coupling and its alignment.
Can the expert mechanics and machinist comment why a universal joint is not a practical solution on a boat?. With this type of coupling there will be no need for alignment and all that vibration.
Am I thinking wrong? It is a revolutionary idea.
If there were room for a universal joint, I think you would also need a steady bearing holding the front end of the propeller shaft to keep the shaft in the center of the log. More hardware and space needed than a coupling.
Dwain
1997 Catalina 36 - Coastin'
Ingleside, TX