Did the boat, with the original prop, achieve full RPM? If it did I would have to say it's an over-prop issue. In a gasoline engine you could burn a hole through a piston over time with this issue, in a diesel it will smoke especially at low RPM. If you can run the engine to WOT (wide open throttle) in neutral as per the specs, your cable is not a problem. I have a diver go down to clean my bottom every three weeks and he has removed my prop more than once for different reasons. He charges $90 to remove a prop and install a new one.