Vibration only at certain rpm

[cmedsailor]

My engine has rated maximum 3500rpm which I can obtain easily. I always had some vibrations (I had a similar thead in the past) but yesterday I paid some more attention and found out this:
Up to 2000rpm there are no vibrations. Then from 2000-2500rpm there are very little vibrations (especially towards 2500rpm) but really they are not annoying at all. From 2600 to 3000rpm there are more vibrations (especially at 2700-2800rpm) which are annoying. Above 3000rpm and especially at full 3500rpm there are no vibrations. Yesterday I run the engine at 3500rpm for some minutes and it sound really sweet (no vibrations, no smoke, no anything).
Why is this happening? If the shaft needs alignment, or the cutlass bearing needs replacing or anything else why vibrations dissappear at high rpm?

 

[scottie]

This is not unusual and is almost expected with some engines .
When engines changed from having max revs in low 2000 to 3000 plus and used smaller flywheels it became more of a problem . It is also more noticeable with larger undamped GRP panels than wood .
If its a new problem then you need to find out why it occurred ie failed engine mount etc

 

[savageseadog]

I used to work on vibration monitoring of rotating machinery. You don't say if you are saildrive or shaft but there will be a number of modal frequencies on any rotating machinery and shafts have more than most. I cannot of course be sure your setup is OK but if just at one or two rpm speeds and OK otherwise, merely an rpm to avoid, not a problem.

 

[Bilgediver]

This is not unusual and is almost expected with some engines .
When engines changed from having max revs in low 2000 to 3000 plus and used smaller flywheels it became more of a problem . It is also more noticeable with larger undamped GRP panels than wood .
If its a new problem then you need to find out why it occurred ie failed engine mount etc

Rotating masses have harmonic frequencies. This means that at certain frequencies any out of balance forces may be amplified. On big ships this can be a problem and these critical speeds are avoided for any continuous running. They are usually clearly marked on the control panel. However are usually torsional unlike yours which may be more radial than torsional.

Your vibrations could be due to out of balance propeller, slightly bent shaft, worn cutless and damaged engine mounts in addition to one of above.


It will need some further inspection to see what is causing yours would be necessary.

 

[ostell]

Could be a singing propeller. Resonance in much the same way as a wine glass when a finger rubbed round the edge. Search round with Google.

I've got that, it comes and goes depending on engine speed, and disappears if there is dirt on the prop (and comes back when the dirt falls off!).

 

[jordanbasset]

With us the vibration is when we are on low revs, goes away when we increase revs above 1500. Not worried about it as we are seldom on revs lower than 1500

 

[Heckler]

Rotating masses have harmonic frequencies. This means that at certain frequencies any out of balance forces may be amplified. On big ships this can be a problem and these critical speeds are avoided for any continuous running. They are usually clearly marked on the control panel. However are usually torsional unlike yours which may be more radial than torsional.

Your vibrations could be due to out of balance propeller, slightly bent shaft, worn cutless and damaged engine mounts in addition to one of above.


It will need some further inspection to see what is causing yours would be necessary.

He has posted twice on this already.
Stu

 

[lustyd]

I think it's crabs holding on to the prop for a laugh. They do it as a competition to see who can hold on longest. When you reach the higher rpm they can no longer hold on and hence the vibration stops.

 

[Skylark]

My engine has rated maximum 3500rpm which I can obtain easily. I always had some vibrations (I had a similar thead in the past) but yesterday I paid some more attention and found out this:
Up to 2000rpm there are no vibrations. Then from 2000-2500rpm there are very little vibrations (especially towards 2500rpm) but really they are not annoying at all. From 2600 to 3000rpm there are more vibrations (especially at 2700-2800rpm) which are annoying. Above 3000rpm and especially at full 3500rpm there are no vibrations. Yesterday I run the engine at 3500rpm for some minutes and it sound really sweet (no vibrations, no smoke, no anything).
Why is this happening? If the shaft needs alignment, or the cutlass bearing needs replacing or anything else why vibrations dissappear at high rpm?

You really do need to try another, similar vessel to compared vibrations against yours.

I have the same boat with the same engine and mine, too, resonnates at certain frequencies (engine speeds). I normally avoid these speeds.

There are many reasons for a vibration. The root cause is often traced to an out of round or imbalance but finding it is anything but straightforward. In any rotating system, it's quite remarkable how one component can excite and resonnate another.

 

[cmedsailor]

You really do need to try another, similar vessel to compared vibrations against yours.
.

I have. Same boat, same engine, same age, same more or less engine hours but no vibrations at all. The only difference is the 3-blade fixed propeller whereas mine is a 2-blade prop.
I am planning to also change to a 3-blade fixed prop when I get the boat out of the water and at the same time replace the cutlass bearing (I hope Stus' method will work!) and do an engine/shaft alignment thereafter. It's the absence of vibrations at high rpm that looked strange to me. Something I never noticed in the past.
Thanks anyway for all the answers.

 

[emandvee44]

Vibration

Same problem.
After only 5 months in the water this year, we went out for our final trip of the season and as it was a very light wind we had to motor a lot. It was immediately obvious that we had a vibration problem at certain revs., which was a situation new to us after three seasons of ownership.
We hauled out shortly afterwards and the photos show what was almost certainly the problem. Lots of small limpets causing an imbalance.
The prop. had been treated with a special propeller anti-fouling, which obviously did not work, but in fairness to the manufacturers I do not think I applied it correctly. Anode depletion was much less than last year, but about what it should be.