Engine Itself Also Vibration Source - Maybe Nothing is Wrong
Something else to consider about vibration that I didn't see mentioned yet. Besides the prop and condition of the drive train, the engine itself can be a major source of vibration. I looked up the Hunter 25.5 on yachtworld.com saw an offering listed as having a one cylinder Yanmar. One cylinder and even a two cylinder diesel engines can vibrate very noticeably. Just their nature. The ~1979 made two cylinder Yanmar 2QM20 on my boat can cause my anchor to start resonating against the anchor roller fitting at certain rpms even in neutral without the prop turning. (It has a nice bell-like tone though.) When motoring, the trick is to learn the rpm ranges where the vibrations tend to cancel out. For example the smoothest spots on my boat are 1600 rpm (= about 3.5-4.0 kts) and 2200 rpm (= about 5-5.5 kts). If the motor mounts are getting worn and no longer provide adequate support and vibration dampening, that can also be a cause for increased vibrations.
From my Seloc repair manual:
"Smaller engines with 1, 2, or 3 cylinders will often vibrate violently at slower rpm. This is particularly noticeable with lighter engines whose softer mounts are optimized for the smoothest running at operating rpm. All engines have rpm bands where the vibration is greater. Often the vibration is more pronounced because of poor installation."
