resonating shaft

[Birdseye]

odd one this. my boat is our of the water sat on her keels. the engine (volvo 2003) runs very smoothly when disconnected from the shaft, but with the shaft connected but not turning, there is a pronounced vibration. looking under the boat shows the shaft vibrating quite markedly.

now the mounts are OK and are taking equal weight on all 4 as best I can tell. the shaft alignment is within 2 thou but then I wouldnt think that would matter with the shaft not rotating.

it seems to be some sort of resonance effect - vibration at idle but then most boats do. then pretty smooth to 2500 rpm then several vibration periods.

any suggestions? any engineers here who can tell me how to calculate the resonant frequency of a shaft held at one end? and perhaps more usefully, how to move the frequency up the range.

 

[eyehavit]

Assuming a rigid connection, the shaft is just a long extension of the engine/gearbox. Relatively tiny engine movements will cause an unrestricted shaft to move a proportionally greater distance. Add node resonance at the 'right' frequency and the vibration could be severe.
Check shaft bearings for wear, especially the fwd bearing. Bearing wear could exacerbate the problem.

 

[Birdseye]

The cutlass bearing in the P bracket is good - the only other support is at the gearbox output. There's a seal about 6 inch further down from the box but thats all.

Insofar as I can measure, the vibration at the box drive flange is much greater with the shaft connected but not rotating than it is without the shaft connected . So it seems very likely that its a resonance problem.

If it is, how do I modify the frequency upwards?

 

[eyehavit]

Was this happening when the boat was in the water or only since on the hard?
Has anything been done to the drive train before the vibration was noticed?
If it was previously free of the vibration currently being experienced and no work/modification to the drive occurred, the vibration may disappear once the boat is back in the water. Water in the stern tube would help dampen things down (yes, bad pun).
As to changing the resonance a larger diameter shaft might help.

 

[pampas]

Tube de-zinced and sheared behind the cutlass bearing.

 

[Bilgediver]

This is something that is present to a degree on all boats where the engine has flexible mounts and is solidly connected to a shaft wich is firmly held in place at the P bracket.

When the engine is idling there are all sorts of out of balance couples taking place in the engine which cause it not only to want to bounce up and down on the mounts but also oscillate and sway etc. These are caused by mechanical forces as well as accelerations not totally damped by the modern light flywheels.

Some engines are worse than others and if you were to disconnect your shaft and run the engine you will see how much the engine moves around on its mounts at some speeds, particularly idling.

The connected shaft is picking this up and trying to resrict it by force and will also resonate in the process. This resonanace is no doubt damped when the boat is in the water.

The situation can be improved by installing one of those nylon type flexible mounts between the shaft and engine though no doubt if you do then next year you will send in a post " SHould my engine move as much as this?" /forums/images/graemlins/laugh.gif

It is normal with Yanmars and BEtas etc fit these couplings and the amount the engines sway and woble at idle can be alarming.

There is an expensive option which includes fitting an inner shaft bearing and a short universally jointed stub shaft however the flexible coupling is OK for most smaller applications. There is usually enough clearance between the prop and rudder to allow for the shaft to be moved back to accomodate this sandwich type flexible coupling.


John

 

[Birdseye]

Thanks for the thoughtful reply. As I indicated, I have run the engine with the shaft disconnected. Its difficult then to guage the vibration levels - certainly it doesnt feel great. And there is no resonance at speeds above idle

With the shaft in place but not rotating, there is a resonance at 2600 rpm and you can hear the vibration as a drumming noise let alone see it looking at the shaft under the boat. Clamping hold of the shaft by hand doesnt stop it totally either.

The boat is relatively new to me so its difficult to say nwhether it happened before this winters program of work. I suspect it did. Certainly there was a noisy engine speed when last in the water.

 

[bendyone]

Have you taken the mounts off and looked to make sure that they are OK? The rubber could have become detatched from the metal but with the weight of the engine it would be hard to tell.