I am a bit confused about the “synchronize the crankshaft” idea then. That is not really possible for humans to do.
I can synch engines by ear and I can hit the button on the synchronizer to do the same, but the only way I have ever seen that could synch the actual relative positions of the crankshaft and props is to use a synchrophaser. I can’t recall ever seeing a marine version of these, but they probably exist somewhere.
For the airplane version the simplest ones have a knob you turn for the least noise and vibration.