On my own ship, over the years I have noticed a few revs... and they are few... where the motor tends to be smoother. At some revs, the ship vibrates and the compass pedestal in particular notices it. I have learned to live with it. It's hard to tell the degree of vibration you complain about.
Take your motor revs up until it is at its worst, then send a man below to see what might be doing it. You might see something then.
Here's the good news. Your vibration problem can be fixed.
The bad news that there are a myriad of things that could be causing it.
As others have mentioned, the most likely causes of the problem could be something as simple as a bent or foul prop. It could be a prop out of balance. It could be caused by a bad cutlass bearing or a bent prop shaft. A worn out or bent prop strut could be the cause. Damaged or loose motor mounts could also be the problem. As you can see, there is no simple answer.
My suggestion is to start with the things you can look at easily, the motor mounts. Also, check the bolts that hold the prop shaft to the engine for play
If you are unable to find the problem that way, depending on your location, you may have to get your boat hauled. In warmer waters, you could just put you mask on and go over the side to do a visual inspection and cleaning. For example, I have to scrub the crap off my prop a couple times a year or it vibrates like hell.
Over the years, I've had noticeable vibration problems several times. The first was caused by a bent prop. The second, loose motor mounts, and the last time, a broken prop strut.
Good luck! I hope you find a cheap and easy solution.
i had this same problem this past season. i thought it was the cutless or a bent shaft. when i pulled the boat this fall, i noticed a big chunk of barnacles on the prop & shaft. i'm assuming this is the cause since the vibration got worse as the summer went along.
Trantor has the best idea for a first step. That mot likely will tell you if it's a problem at the engine, or somewhere else.
You don't say which engine and prop you have. Just to throw it out there, what are the max RPM for your engine? Some engines are slow turning and they top out around 1900 or 2000 RPM. If yours is one of those, the answer could be as simple as you are pinning the engine at full throttle.
Another possibility could be engine alignment. I doubt that's it because you would experience symptons even at the lower RPM, but you never know.
Nasty looking shaft, but why the clean prop and strut?
Watched Summerfield's in Ft. Lauderdale haul a older 45 ft. motor yacht a few years back. Seem that the boat had been kept on a canal behind the owner's home for many years and he had finally decided to sell it.
The entire bottom of the boat looked like your prop shaft, except worse, with clumps of oysters hanging down like grapes. It took them over a week to get the bottom clean enough so they could paint it. Then, to top it off, the guy complained about how much the cleaning cost and refused to pay his bill. Summerfield's slapped a lien against the boat and eventually got paid.
Some people just don't deserve to own a nice boat.
Hey Poker...great photo. It occurs to me that surveyors like you and Paul might have some great photo archives on problems with boats/construction that could provide instructive. Please keep sharing those goodies!