Vibration

P

plewis

Well-known member
Joined
Aug 12, 2016
Messages
59
Fluid Motion Model
C-288 C
Vessel Name
Last Chance
I have a 2013 29' with the Volvo D4 engine, which I bought a year ago. I have always had some vibration at around 2300 rpm, enough to shake the hand rails on the bow - at higher or lower speeds all seems normal. I did check out the prop over the winter and there was not any oblivious deformity The 2300 rpm is about the speed at which the boat begins to plane, so I have always assumed that the engine was under much stress at this point and complains by lugging/vibrating. Is this a case of wishful thinking on my part or do I have a problem?
 
It's hard to say whether what you describe is meaningful in regard to the health of your propulsion system(engine, transmission, alignment, etc.). Does the engine itself vibrate more at that speed or is it just the railing that you describe? What you are describing sounds like there is a critical/harmonic frequency that gets excited when the engine is at 2300 RPM. The question is what is it that gets excited at that speed? If it's causing problems it could take some time to isolate the source. But since it occurs as a transient while you are accelerating it's probably not worth worrying about. If 2300 is a desirable operating speed then something should be done to resolve it. Having the shaft alignment and engine mounts checked is a good place to start.

The engine mounts on our boat have a critical at about 1000 RPM. The whole boat starts a low frequency "wobble". It was really bad when I bought the boat and improving the shaft alignment helped but it's still bad enough that I avoid extended operation at that speed. Which is annoying when maneuvering inside a marina/anchorage. But correcting it would likely require work to the engine mounts. And even then the excitation frequency would simply shift, possibly to a more useful RPM range. Which would be REALLY annoying.
 
While you looked at your propeller and saw no obvious dings, you may want to pull the propeller and take it to a proper propeller shop and have it balanced and trued. The gauges and machines are much better than our eyes.
 
Thanks to all for your suggestions.

Today to my surprise when I started to check out the alignment I found all four of the bolts securing the collars were loose a turn or two.( I'm hoping this is the source and not the result of the vibration.) The alignment was out ~.010 port and starboard which I understand is bad but doesn't seem excessive. I would correct it to spec but getting to and understanding those engine mounts will require a younger guy, so for now I'll tighten the bolts and see if there is an improvement. Stay tuned...
 
10 mils is not good but not horrible. But certainly induces more vibration. The loose bolts are much more of an issue. That is certainly a source of vibration. They must not have been properly torqued.

Boats...
 
plewis":234102b4 said:
Thanks to all for your suggestions.

Today to my surprise when I started to check out the alignment I found all four of the bolts securing the collars were loose a turn or two.( I'm hoping this is the source and not the result of the vibration.) The alignment was out ~.010 port and starboard which I understand is bad but doesn't seem excessive. I would correct it to spec but getting to and understanding those engine mounts will require a younger guy, so for now I'll tighten the bolts and see if there is an improvement. Stay tuned...
Click to expand...

Just to make sure I am not assuming something here. The alignment must be checked and set when the boat is in the water.
 
HURRAY!!! Tightening the bolts significantly reduced the vibration. I imagine the remaining slight vibration is caused by .010" misalignment, which I intend put on my lower priority my todo list. I guess the loosening of the bolts was caused by either, not tightened to spec at installation or perhaps the working of the coupling/bolts due to the current misalignment.

knotflying you assumed correctly the alignment test was done while in the water.

Thanks all.
 
I'm curious. How is this alignment test performed?
 
You loosen the bolts and insert a feeler gauge on each side and then bottom and top. Loosen the bolts enough to get,say a .025 in on one side. If the resistance is the same all around you are good. If one side is looser or tighter see what gauge you can fit in, the difference between the two is the important factor. For the life of me I can't remember what the max tolerance is, but on our engines with a small shaft the tolerance can be quite a bit. I am sure someone will chime in with the definitive tolerance. The usual adjustment is up and down because the rubber will initially compress.
 
Thanks Mike. And why does it have to be done on the water?
 
Angular misalignment good is .003 out. Usually anything over .006 is in need of alignment. An offset alignment should also be checked to confirm shaft to shaft center. The reason for alignment being done in the water is the boat is evenly supported. A boat tends to get slight twist or bends in the hull while sitting on the trailer or blocked on stands. Rough alignments can be done on the hard but after the boat is launched it should sit in the water for a few days and then Alignment rechecked.
Brian Brown
Cutwater 26
PORT-A-Gee
 
Red Raven":7gk51w1z said:
... And why does it have to be done on the water?
Click to expand...
Like many other things with boats this is an historical "one size fits all" practice that is unnecessarily applied in many cases. It's not a bad idea for the professional mechanic who warrants his work and isn't likely to be familiar with the design/construction of every boat that he works on. But particularly for the smaller models in the Ranger fleet on a reasonably well set up trailer it's unlikely that there is any benefit.

I guess it begs the question, does the Ranger factory do their final alignment prior to delivery with the boats afloat? Or is there a curious DIY type owner following this thread who has taken readings in both conditions and compared results?
 
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