Vibrating propeller

Dear All,

We have a three bladed Maxprop, that has a very annoying vibration.

It starts vibrating at just over 5knots; just before cruising speed.  Oddly, and very annoyingly, it does not go away if you increase the revs and try and power through it.  Which one might expect it to do.

The vibration is sufficient for it to be clearly felt and heard on deck; to the point that I think it would damage the gearbox and engine.

It’s always vibrated to a certain extent since new about 5 years ago, but would come and go and could generally be powered through.  About two years ago we had some electrolysis problems which lead to two of the blades needing to be re-built due to corrosion.  It was rebalanced, statically, at the time but continues to vibrate.

We have tried new cutlass bearings, but that make no difference.  We have also considered cavitation due to incorrect pitch, but checked the pitch settings with the distributor and have no signs of erosion on the blade tips, or of the anti-fouling on the hull above the prop.

Several people far more experienced than I am  have sailed on Tiderace and are all at a bit of a lose to explain what is going on.

My next course is to see if I can get the prop dynamically balanced, but nobody I have spoken to knows of anybody that could do this on a feathering prop.

Has anybody experienced anything similar or can offer an explanation?  Your thoughts would be much appreciated.

Many thanks in anticipation
Adrian

(TIDERACE II)

Hi Adrian,
If your propeller shaft extrudes more than one and a half times it's diameter this may be where the problem your excessive vibration arises from.
By this I mean the amount of exposed shaft aft of the cutlass bearing housing and forward of the prop boss.
I've a Gori 3-bladed folder fitted to Seqamróige (36/021) & had to have her cutlass bearing replaced & 2" trimmed off her prop shaft on my surveyor's recommendation when having her pre-purchase survey done - excessively extruding shaft length the cause of 'mild vibration' and an excessively worn cutlass bearing on a 2008 installation.

Hope this helps,

Best regards & fair winds,

Brian.

Thanks Brian.  I will double check the shaft length this winter but think it is OK.  We dropped a new engine in last winter and the engine fitter had to shorten the shaft as it pushed it too far aft.  Would certainly cause/contribute to the problem though.

Cheers

Adrian

Adrian
Here some more possible reasons for the vibration:
- The propeller is off center on the shaft because the key is too high. Propeller to be removed for checking the key.
- The shaft is bent. To be taken to a machine shop for checking
- The engine and shaft is out of alignment, but one would assume that when the new engine was installed the alignment was checked.

If the propeller geometry is not right - blades have uneven pitch or shape, or are not evenly located around the hub - this is difficult to verify without advanced measuring equipment. An alternative would be to temporarily put on another propeller with verified proper behaviour. If the vibrations stop, the reason is the propeller, if they continue it is elsewhere.
Would appreciate to hear what you find
What engine is there now?
Best regards
Lars

Lars,

Thank you for those suggestions.  We will go through those this winter and report back. A couple I have not thought off and also some I had assumed would be OK, but should be checked out as you suggest.

The engine is a Beta 50.  New last winter, and a marked difference over the Perkins which was original to the boat.  We have done about 250 hours this year with it (two long delivery trips) and its been very good; much reduced fuel consumption and quieter.  (If anybody is interested I'll post a thread on it, with some pics.)

Cheers

Adrian

We also have a problem with engine vibrations, starting at higher speeds, and being exacerbated by higher seas.

Today we found out that two silentblocks were broken, which was not visible from the outside. Replacing them seems to have fixed the problem as far as we can tell, but sea trials tomorrow will show for sure.

Before we had already fixed a bent shaft, changed strut bearings, shaft log and coupling, and were astonished about continued vibrations after the reparations. The theory now is that the bent shaft led to vibrations which in turn destroyed the silent blocks.

Christain 411/28 IF