Drive shaft vibration problem.

[Allanrichard]

Had a similar problem on a Moody 33 Eclipse with 43hp Volvo. OK at lower rpm but as speed increased then heavy vibration occurred. Finally traced it to rope stripper. Part of the heads of the smaller Allen screws on the inner cutter stand proud and they appear to clear the fixed cutter block on the P bracket. However under forward power the shaft moved forward slightly forward and was fouling the block causing a very bad knocking as the screw heads banged against the block. Only noticed it under heavy load and it cleared as the revs dropped.

 

[PaulJ]

Perhaps the clue is that both you and Andrew Bray experience the problem in rough water but not in smooth. How far is your prop below the surface? Could it be that in the rougher conditions the water near the surface becomes more aerated and as each blade in turn comes nearer the surface it draws in air, ie cavitates and the knocking is caused by the cavitation varying as each blade "gulps air" and then plunges into solid water. The knocking effect is possibly exacerbated by the Aquadrive...... when I was choosing a flexible coupling for my installation I was advised against an Aquadrive on the grounds that it has no torsional flexibility. In the end I bought a coupling from Centaflex which has two rubber "doughnuts" which have some "give" and absorb torsional shocks........ 'haven't tried it yet though as the boat still isn't finished!

It could also be that you didn't have the problem with the old (smaller) engine because either it drove a lower pitched prop or it didn't have enough oomph to drive the same prop fast enough to cause cavitation.

 

[david_bagshaw]

Sorry to hear of your drive train probs, my thoughts are


1 is the prop in the same place, ie not nearer to the dead wood,

2 could the blade tips be nearer something , that they wernt before

3 is there some flexibility that allows either of the above when under load

4 could a blade have been damaged , slightly bent etc by pushing shaft out during the works

Good luck

David
www.euroboating.net

 

[chippie]

I knew an old timer who used to track down stray vibrations by filling a glass of water 2/3 full and hold it on various parts of the driveline.It became quickly apparent which part was out of synch with the others.
Be very careful of the rotating parts!
Worth a try perhaps.

 

[vyv_cox]

Andrew and other posters,

Thanks for this. My P-bracket is straight, judged by the wear witness marks on the old cutless bearing. I heaved it about when the boat was ashore to try to detect any movement or looseness but none was found. The noise on the water gives the very strong impression of coming from this area. My stern gland has been unchanged in the life of the boat (as was the old prop) so I don't suspect either of these as the source. I only installed the rope cutter this winter, and since the original knocking noises occurred before then, I don't suspect it as the source. However, one point mentioned regarding movement of the prop and cutting gear under load - this cannot happen with an Aquadrive (or only as far as compression of the stiff rubber bushes will allow, say 1-2 mm) as thrust is transferred directly to the fixed plate.

The fourth one is very interesting but I can think of two contrasting viewpoints. BMC Mini driveshafts (exactly the same as an Aquadrive) used to knock on full lock when worn, even though they were quiet in the wheel straight ahead position. However, there was probably a permanent misalignment in the vertical sense, so this could still be valid. The other thought is something of a tangent but might be interesting. Gear-type couplings in rotating equipment rely on internal grease for lubrication. Since the advent of laser alignment equipment we are seeing failures of these couplings due to poor lubrication. It seems that these couplings need a small amount of misalignment to allow teeth to slide over each other and draw lubricant in. This may have nothing to do with my boat, but the whole train is in pretty much perfect alignment. It should be quite easy to lift the back of the engine on the jacking bolts to induce some deliberate misalignment.

Thanks to all for their enthusiastic responses. I have ordered a replacement flanged coupling from the Aquadrive thrust bearing to the prop shaft, as there seems to be some movement there and I suspect that the machining may not have been perfectly concentric. I will check that the thrust bearing is not touching the thrust plate. I will run the engine and Aquadrive disconnected from the shaft to check for any unexpected noise. I am away now for two weekends, but will report back.

Vyv.

 

[jfkal]

Is the problem still there? I had exactly the same problem and was going in circles due to wrong assumptions as well. I was assuming it cannot be X because.
Well month later it turned out to be alignment (shaft not centre stern tube).
I still have a rough spot around 2,200 rpm where the shaft gets thrown about (must be a harmonic). I am fine below and fine above. Other possible reasons:
Barnacle growth, shaft slightly bent, engine to shaft alignment (unlikely due to aqua drive), shaft seal, unbalanced prop, bent prop blades, loose zincs.
Is the shaft oscillating when the knocking occurs? Is there are difference going astern, is there a difference if tied to the dock when running. Is there a sound when sailing and letting the shaft turn?

 

[Sammy]

I am facinated by this thread which I picked up off the Sadler forum.
Have you found the answer?
I have exactly the same set up on my Sadler 34 and would agree that there is not much movement from the hull in any weather. The engine will move around on the soft mounts quite a bit which is why I fitted an Aqua Drive. The only thought that has occoured to me that no one has mentioned is the distance between the output flange of the gear box and the position of the thrust race bulkhead if it is too close then the small shaft in the aqua drive will possibly bottom out and knock as the engine moves around and transmit the noise down the shaft. From memory it should be put in the middle of it's travel. Just a thought. Incidently I have had no trouble with my set up at all except when the bottom is fouled then the prop cavitates because I swing a 17" prop with not a lot of blade area.
Hope you get it sorted,
Chris.

www.BoatsThames.com