cyclical vibration under power

Jun 7, 2016
315
Catalina C30 Warwick, RI
I'm not sure if cyclical is the correct word, but here is what I am experiencing. Quick back story. This winter I removed the engine, replaced the engine mounts, cutlass bearing, and prop shaft. I aligned the engine in the water and got it within .030 all around. I cruise comfortably around 2100 rpm with no vibration. When I push it harder to above 2400 to red line there is a vibration /buzzing in the hull that I start to feel. The weird thing about it is, it is not constant but goes in 2-3 second intervals. Vibration for 2 seconds, then smooth for 2 seconds, wash rinse and repeat. If you vary the rpm it doesn't seem to change the duration but more changes the strength of the vibration.

I would understand that something is off alignment wise if it was constant, but where it is on and off, I'm not sure what to make of it.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
I aligned the engine in the water and got it within .030 all around
I trust the 0.030" is the radial alignment. The angular adjustment is 0.001" per inch of coupling diameter in which I think yours is a 4" coupling.

Even with this misalignment, the vibration should be constant and not cycling as you describe.
 

Tedd

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Jul 25, 2013
745
TES 246 Versus near Vancouver, BC
That's most likely a balance issue of some kind, probably aggravated by looseness in the drivetrain somewhere, such as a coupling or shaft bearing. Or an engine mount! Are you sure you have the correct engine mounts, correctly installed and tightened?
 
Mar 20, 2016
594
Beneteau 351 WYC Whitby
I have done a few shafts , the coupling needs to be fitted to the drive shaft and indicated in a 4 jaw chuck in a lathe and have no more than .0015 runout and then a cut across the face giving you .000 runout on the face of the coupling.Drive shaft is put in lathe and indicated to .000 ,then coupling is added with key ,then checked
Adding a R&D coupling will help and will also allow you to align the engine perfectly.
Also check that your drip seal is not too tight ,if your using that style
 

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Jun 7, 2016
315
Catalina C30 Warwick, RI
So I am not sure if this means anything but, some things I observed this past weekend out on the boat. First, I was working on the engine and fired it up at dock. I revved it up a bit and noticed a vibration even in neutral so I am thinking that the engine may be a significant portion of what I am feeling. Secondly, while sailing in 20 kts of wind doing between 6.5 to 7.5 kts the prop was free spinning and I did not feel one bit of vibration through the hull. I'm not sure if you would feel it without the transmission driving the prop but it gave me something to think about.
 
Feb 2, 2006
464
Hunter Legend 35 Kingston
I've noticed a "cyclic" change in the "sound/feel" before. Usually seems related to going through waves and the motor swaying a bit on it's mounts. For me, I wouldn't call it vibration. Boats are big resonant collections of parts and I'm always finding things that make noise.