Check you alignment...
We had a 2GM20F in our prior yacht, a 1976 Cal
2-29, swinging a 2-blade fixed prop and the engine
was always very smooth running. The only time we had any vibration, it proved to be a mis-alignment problem as a result of a mount lock-nut that had loosened somewhat.
"It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."