On our way back from our LI Sound Cruise, the prop shaft started vibrating. It was fine below 17K RPM but above that the shaft vibrated quite violently.
The great thing about the sailing community is you only have to ask and you get great advice. I emailed a bunch of sailboat owners who know their onions and I got back some great advice. Here are two examples:
From Tim Fletcher:
The wonderful thing about the engine train is that there are so many things to vibrate!