Prop Shaft Vibration - Advice please

[changeman]

Have recently been out to our boat in France and carried out a few tests to try and pin down a vibrating prop shaft problem.
It starts at about 800 revs and fades away above 1200 revs. This occurs in both forward and reverse gears.
I will now head for an appropriate yard to get it fixed.
However, the questions are now,
1. Am I likely to damage anything on the journey if I keep the revs below 800 on the canals or above 1200 on the rivers? I usually cruise at 1000 revs.
2. Is the vibration likely to be due to an unbalanced propellor or some other problem?

 

[vyv_cox]

Having suffered a similar problem at 1800 - 2000 revs, I don't think this is due to an imbalance. I believe it to be due to some factor that causes resonance at around the natural frequency of the shaft. For interest, what is your shaft diameter and distance between bearings?

Mine started when I changed engines from a Bukh to a Yanmar. I went through a long elimination process, changing P-bracket and cutless bearing, propeller, adding an Aquadrive, checking shaft straightness, none of which had any effect. There was a small amount of shaft wear in way of the cutless bearing, which I considered to be acceptable. In the end the shaft was the only unchanged component, so I replaced that. The resonance almost disappeared, although a trace of it remains.

If yours is resonance the only likely damage as a consequence of motoring is to the cutless bearing, which it might be wise to replace.

 

[Topcat47]

It's a huge queston and one that doesn't allow for short snappy answers. My boat has a short shaft and a three bladed prop. Vibration becomes noticeable when the cutless bearing is on the way out. I've not tried to run it for long with a worn cutless bearing and most people think my bearings would be good for another season when I change them, but I guess I'm an old woman about potential U/W failures.

Prop imbalance is a potential source.

If you've a modern fin keeler, shaft whip may be an issue (that often means the shaft is too thin or bent). Shaft anodes if fitted too far from the "p" bracket can compound such problems and I've seen a few fitted dead central in my time.

Shaft misalignment is another possibility. Also have a good look at your engine mountings. If they are tired, or one had "gone" this will encourage shaft vibration.

 

[jmh2929]

Some other problem?

I'm having a new propshaft made as the old has uneven wear at the bearing.
Today whilst cleaning up the prop and generally messing about, I noticed that the prop went onto the old shaft further without the key than with. Pushed on firmly but without the nut I could detect a rocking. I carefully reduced the hight of the key a few thou' at a time 'till rocking dissapeared, the prop was now 3mm further on and presumeably truer on the shaft.
I can only assume that the prop was off centre and throwing the shaft off line.
As this has been like it since I bought the boat I can't really comment on vibration as a 1 cylinder yanmar in an old boat shakes a bit anyway. So I assume.
Hopefully the old girl will be a bit smoother when afloat again.
This may have no bearing on your prob..

 

[AllanJ]

FWIW When I had a vibration I discovered that one three-blade prop had lost a whole blade. I have duo-props. Easy to check and eliminate as a possibility.

 

[Caer Urfa]

However, the questions are now,
1. Am I likely to damage anything on the journey if I keep the revs below 800 on the canals or above 1200 on the rivers? I usually cruise at 1000 revs.
2. Is the vibration likely to be due to an unbalanced propellor or some other problem?

One thing that has not been mentioned is have you checked your engine mounting rubbers and bolt tightness.

As Vyv says resonance can be the sleeping culprit and it is hard to find where it is picking up the frequency from that is causing you symptons.

Having spent 28 years in the vibratory equipment Industry detecting the source of the problem is not easy, generally if one part is loose or broken then the vibration/frequency looks for the next weekest link and gradually gets worse as the frequency changes.

The other suggestions made may well be the cause, but my advice is start at the basics before you start taking the boat apart!

Mike

 

[Tranona]

Have recently been out to our boat in France and carried out a few tests to try and pin down a vibrating prop shaft problem.
It starts at about 800 revs and fades away above 1200 revs. This occurs in both forward and reverse gears.
I will now head for an appropriate yard to get it fixed.
However, the questions are now,
1. Am I likely to damage anything on the journey if I keep the revs below 800 on the canals or above 1200 on the rivers? I usually cruise at 1000 revs.
2. Is the vibration likely to be due to an unbalanced propellor or some other problem?

Your description and the boat in you profile suggests you have a large slow running engine which would mean a big prop shaft. It is unlikely that the shaft is bent - lthough if it has excess overhang aft of the rear bearing it might get bent if you hit something.

Start with the easy bits which can be done in the water. Check all engine mounts for security and wear. Check alignment shaft to gearbox output - this can go out with collapsing or loose mounts or problems with the flexible coupling (if fitted). Finally if there is a long run of shaft from the gearbox to the inboard bearing check that it is properly supported by a bearing - although this is a design rather than a use issue.

If that is all OK then the final checks are out of the water for the outboard bearing and the prop.

 

[Neil_Y]

Much good advice above, unfortunately the decision on if it is bad enough to do damage will have to be yours, but have a close look at the shaft at its worst vibration at the through hull, if there is a chance it is touching stern tubes or flexing the coupling I'd go easy on it until you can get it checked out.

Firstly you can't be sure that it isn't the engine unless you have disconnected it, as engine mounts age get harder engines can have resonant frequencies, but lets assume thats not the source.

I would be looking at first prop, then shaft and finally bearing alignment. Many people will suggest shaft to engine alignment (at rest) but in practice this is constantly changing (unless you have an inline thrust bearing and cv joint) and the system is designed for this to be changing with shaft flex allowing the engine to move on rubber mounts without bending or breaking parts of the drive train.

The vibration is more likely (once prop and shaft have been checked out OK) to be coming from a worn bearing or a bearing which is not aligned to another bearing if you have two or more. As we supply hard bearings rather than rubber I'm a bit biased in the belief that rubber slightly worn or even new allows too much flex at the bearing in some cases and even when new it's difficult to confirm if the carrier is aligned. With a hard bearing running a fine clearance on shaft and carrier a dry assembly with shaft chocked allows an engineer to check carrier alignment as the bearing can be spun by hand in the carrier whilst on the shaft if all is true. Having said that some systems have such long and thin shafts a degree of shaft flex exists through lack of support which doesn't get better when you add thrust to the equation.

I wish you luck as some of these hunts for a cause can get quite protracted.