Prop shaft vibration

[jtinterland]

We have a 1981 Tartan T-33, 33'8" long, 1981. I have rebuilt the engine (Universal 24 hp) 3 years ago, installed a new cutlass, straightened the prop shaft (out only .015), balanced the prop (negligible deviation), installed new motor mounts (all previous listed items over the last 3 years), and aligned the prop shaft to the engine to within .003 to .005, and at the intermediate RPM, we experience a hunting vibration. The boat drive assemble seems to knock at the hull. The cutlass support is tight, and doesn't budge when pushed or pulled. Today, when I checked the engine alignment, we tied off the boat securely, and reved up the engine to 2,000 RPM with the engine in either forward or reverse, but did not experience any outstanding noise or vibration. The engine runs great (Stays at the designated RPM, and does not fluctuate). I cleaned the bottom today, and there is not any fouling on the shaft or prop, and I took off the anode. This vibration happens 75% of the time. Any ideas?

[Lake-Effect]

Originally Posted by jtinterland

...straightened the prop shaft (out only .015)

How long is your propshaft, and at what point is the maximum runout? Also, I have never heard about someone successfully straightening a propshaft, but I'm not a machinist.

But I did spend a few months apprenticing for a boat diesel mechanic, and we would not accept an engine alignment til it was less than 4 mils out at the coupling.

So between the shaft runout and the coupling... enough error there to set up an imbalance, maybe.

Also how are your engine mounts - saggy or torn? How old?

[Free as a Bird]

I am doing the same exercise at the moment.
I have a two section driveshaft. The prop section is supported each end but for its diameter and length is approaching the limit without requiring a bearing in the middle: a bit hard as I have a timber boat with an inaccessible shaft log.
I believe that a shaft this long can set up harmonics at certain rpm's.
Alignment issues are critical.
May I refer you to an article which outlines a very comprehensive method of getting great alignment. It is a bit tedious but very good.
Written by Jack Harden it is on marinegears.com Or Google Get in Line jack harden.
Good luck and look forward to hearing progress.
Peter.

[40 South]

0.003" to 0.005" is too much out of alignment. It should be within 0.001".

Flexible (rubber/hard plastic) engine mounts? Flexible or solid gearbox to shaft coupling?

Flexible mounts require a flexible coupling as the engine is free to move but the coupling/shaft is not if a solid coupling.

The other issue could be hull to prop clearance which can set up a harmonic vibration at certain RPM.

[SailPenelope]

We ended up installing a second bearing at the inboard end of the shaft tube......one at each end solved the problem.......just too much distance from cutlass bearing to engine coupling and the rotating shaft was vibrating enough to hit the tube. The experts at the shipyard did the job.....too much for us.

[rocksculpter]

With that boat the rudder is nowhere near. I assume the prop has proper clearance. Is it two or three bladed or been changed? Was it a problem before the new mounts and where they sized to that engine?

The shaft in my Norsea was pretty long and would set up harmonics at certain revs. The Yanmar has really soft mounts. The local chandlery owner looked at it and told me if it were him he'd "chain that sucker down". In the end I created a bulkhead with tapered roller bearings to take thrust and an automotive u-joint and spline to isolate the engine. No more alignment problems and much quieter.

[Rapanui]

Spent a few years sorting these problems on small to large patrol boats and fast strike craft.
If the vibration occurs over a wider range of speeds, then out of balance or misalignment are the usual sources, the cure for which is self evident.
If the vibration is concentrated into narrow band of running speed then its usual source is some form of resonance, possibilities are torsional vibration, whirling/lateral vibration or a hull structural resonance being exited.
For torsional resonance, assuming it was ok before then the only possible cause are the new engine mounts and possible a new resilient coupling if that was changed.
True whirling tends not to happen because the stiffnesses of say P brackets are different in vertical and horizontal planes, so you tend to get a lateral vibration. Again, the only significant change which would affect this are the new engine mounts and cutless bearing.
From what you say, the hull structure is the same so should not be the cause of a difference.
The rev range over which the vibration occurs would be useful. What are the differences when the vibration does not occur at the critical speed?

[jtinterland]

My prop shaft is about 30" long, was .015" out of round, which was bent straight, the cutlass is about 2" from the prop, and my coupling is a universal ftg packed in neoprene.
I can easily take deflection in the .1, .2" range. My motor mounts are new, speced for the engine and are 3 years old, good shape. When I pull the boat I am going to replace the cutlass bearing. I think that is my problem, even though it is tight. That's the only thing left to do. Thanks for the info.

[jtinterland]

My (3) friends have the same design boat, make and model, and do not have any problems with vibrations.

[OceanSeaSpray]

Shaft vibration can be a real hassle. One thing not stated very often is that you should not use a flexible coupling if your engine is rubber mounted and your shaft is not supported at the shaft seal, like when using a cutless bearing at the back of the tube, a dripless seal at the front and nothing else.
Sometimes you can get away with it (short shafts), but Volvo for example specifically advises against it. Otherwise the engine vibrates and wriggles around, shaking the rotating shaft and this can initiate whirling on a perfectly straight shaft. Clamp it hard at the coupling and it is now held straight, in line and moving with the engine, instead of floating around.
I have solved quite a few vibration issues this way, including mine for a start. Even installations that don't significantly vibrate can run quieter when direct-coupled.

However, if you have a second cutless just behind your shaft seal or if you use some kind of packing or seal that supports the shaft inside the boat, then you must use a flexible coupling at the engine if it is rubber-mounted.

[banjoship]

This is the first time I have come across this point of view. there is some logic to it, and may explain an annoying vibration problem at certain revs on Banjo. I fitted a flexible coupling on a recent repower (on all advice), but otherwise its simply a pss and cutless bearing. do you have more references, explanation?
thanks
lee

Originally Posted by OceanSeaSpray

Shaft vibration can be a real hassle. One thing not stated very often is that you should not use a flexible coupling if your engine is rubber mounted and your shaft is not supported at the shaft seal, like when using a cutless bearing at the back of the tube, a dripless seal at the front and nothing else.
Sometimes you can get away with it (short shafts), but Volvo for example specifically advises against it. Otherwise the engine vibrates and wriggles around, shaking the rotating shaft and this can initiate whirling on a perfectly straight shaft. Clamp it hard at the coupling and it is now held straight, in line and moving with the engine, instead of floating around.
I have solved quite a few vibration issues this way, including mine for a start. Even installations that don't significantly vibrate can run quieter when direct-coupled.

However, if you have a second cutless just behind your shaft seal or if you use some kind of packing or seal that supports the shaft inside the boat, then you must use a flexible coupling at the engine if it is rubber-mounted.

[OceanSeaSpray]

Yes, I have been involved with marine engineering for 25 years, but the way my shaft initially behaved really baffled me. The flexible coupling was an R&D unit, supplied by the engine dealer even though I hadn't requested it. It is a good product, but it was the wrong application.
When all else fails... RTFM, Read The Fine Manual, and I suddenly noted this recommendation from Volvo, which is quite categorical. I have attached the page for you here.

When you think about it, it makes complete sense. Coupling the shaft hard also holds it straight behind the engine, not only centred. If it is only supported at the cutless bearing, you can't place any significant bending load on it anyway.
All what the flexible coupling does is allowing it to misalign, because it allows the perfectly parallel flanges to become non-parallel.

The myth of the "flexible coupling" or "drive saver" is so engrained that it seems to fly into the face of best practice, but the guys at Volvo are 100% spot on. I have seen quite a few boats noticeably improved now by doing just that.
On my boat I machined a solid spacer with a spigot to replace the flexible coupling. It went from the shaft hitting the inside of the tube to smooth.

[banjoship]

Thanks for the ideas. I had a solid spacer machined up when I did the repower. as you know, a solid spacer in place of the flexible coupling is necessary for engine alignement. I will do a few experiments replacing the flexible coupling with the solid spacer and see what happens. I really hadn't thought along these lines, suspecting prop imbalance etc. When I repowered, everything was done properly...shaft tested for true, new cutless, etc etc.
the only thing new at repower was the flexible coupling, which is supposed to solve problems!!. Even WM Diesel who supplied the repower, recommended a flexible coupling.

Have many projects on hand, so it will be a couple of weeks before I get back with results.
Lee

Originally Posted by OceanSeaSpray

Yes, I have been involved with marine engineering for 25 years, but the way my shaft initially behaved really baffled me. The flexible coupling was an R&D unit, supplied by the engine dealer even though I hadn't requested it. It is a good product, but it was the wrong application.
When all else fails... RTFM, Read The Fine Manual, and I suddenly noted this recommendation from Volvo, which is quite categorical. I have attached the page for you here.

When you think about it, it makes complete sense. Coupling the shaft hard also holds it straight behind the engine, not only centred. If it is only supported at the cutless bearing, you can't place any significant bending load on it anyway.
All what the flexible coupling does is allowing it to misalign, because it allows the perfectly parallel flanges to become non-parallel.

The myth of the "flexible coupling" or "drive saver" is so engrained that it seems to fly into the face of best practice, but the guys at Volvo are 100% spot on. I have seen quite a few boats noticeably improved now by doing just that.
On my boat I machined a solid spacer with a spigot to replace the flexible coupling. It went from the shaft hitting the inside of the tube to smooth.

[darylat8750]

This is an old thread but it saved my bacon.

I've been fighting with a vibration on my starboard drive train for more than a year. I did some under water agricultural work and damaged my starboard factory installed coupling that looked like a universal joint with rubber installed around the cross. Parts were unavailable for 30+ year old trawler so I replaced it with modern flexible coupling. Had a bad engine mount so I replaced all of them. Still had a vibration. Had the props tuned.
Still starboard side noticeably rougher than port. Put a dial indicator on every place that i could get to and starboard side has less runout than port. I was all set to pull the boat again and take the shaft to a machine shop while I replace the sleeve bearings and did the Cruiser's Forum Google search. Found this thread and OceanSeaSprays comment. Removed flexible coupling and Bingo! Problem solved!

Thank you Cruiser's Forum and OceanSeaSpray!

[JPA Cate]

daryl@8750,

Hey, good on ya, mate. Glad ye got your answer.

A.