I was just about to suggest cavitation, but see that I've been beaten to the punch. If your prop is bent (new shaft, so that would seem an unlikely cause) then you would be getting some vibration even at lower revs.
Most sail boat engine/prop setups are a compromise design, aiming for minimum drag under sail, often with less than optimum clearances from the hull or from skegs etc, and the pitch/design characteristics of the prop itself are usually aimed at providing reasonable drive at cruising revs, with highest pitch
possible to reduce drag when not powered (I'm assuming yours is a fixed bladed prop). They are not designed to operate well at higher revs, cavitation is often a very real issue (I get a bit of it in my Cavalier 32 when pushing hard), but no one sits for long on max revs, so it isn't generally a major problem. Also, I see your boat is a Catalina 30
(is that right?), I guess your static LWL would be about 25'...so your hull speed
(theoretical) is about 6.7 knots. When you're pushing along under engine power, the actual LWL will increase somewhat, but your theoretical hull speed
still won't be much over 7 knots. When you try going above that speed, the drag increases dramatically, so the prop will be forced to bite/work even harder, which also helps promote cavitation. The boat will do it, but under protest. Do you get the sense that there are slight RPM fluctuations when you are experiencing the vibration(s) (like the prop is biting, slipping, biting etc)? If so, that's typical of cavitation. If you find that your engine is not quite able to achieve its max rated RPM at full throttle in forward gear
, which is quite common in many sail boats (assuming the engine is ok)...that's a pointer that the prop is pitched up a bit higher than it ideally should be, so as to minimise its drag when under sail. If that's the case, some cavitation is likely to occur at very high revs. It isn't a problem just something to be aware of.
Your question regarding the prop turning freely without vibration or noticeable shaft movement when you're under sail, in neutral...that proves nothing really, if there were any wobbling/oscillations/vibrations in that situation, then you would have a significant problem, in which case you would experience vibrations under power even at low revs.
My apols at the wordiness of this response, hope it makes a bit of sense.