Engine or prop shaft Vibration

[Billboy]

I have a Westerbeke 30 B diesel engine. 27 horsepower.
7 years old in a 1995 Catalina 320 sail boat.
It has developed a nasty vibration at cruising revs.
Approx 2500 rpm.
I had the cutlass bearing changed two years ago and this has developed since then.
The people that changed it have come back several times now and assured me that the new vibration has Nothing to do with the new prop shaft or cutlass bearing they installed.
They have lined up the engine and checked everything several times.
They say it must be the engine or transmission that is causing this.

Slowing the engine revs down makes the vibration go away but under power when rev back up the vibration comes back.
It usually starts when it all warms up.
Maybe an hour.
The vibration can come on as many times as 8 in half an hour.

We have lifted the engine and the engine mounts still seem supple not hardened.
The engine seems in line with the prop shaft.
The cutlass bearing replacers are competent people and they say it must be the transmission.
The mechanical company have checked it all over and say the transmission is fine,
It doesn't need re building.

Should I re build the transmission anyway?
its driving me nuts and stopping me from going to far from home.

Any ideas?

[pitlaw]

I have a strange vibration too which occurs in seas 2' and higher at 2200 rpm and above. It does not occur in flat water. I have a Westerbeke 55 B. I also have a Max Prop. Its being investigated now by techs. I suspect its my prop or cutlass bearing. Its sort of a deep rumble from aft the engine as the vessel rises up on a wave. If I find out what it is I will let you know. I assume your techs have actually heard it and am surprised they can't pin point the source while it occurs. I would not rebuild anything until they can. I have thought about a stethoscope to find my source. Why not?


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[Kokanee]

I'd be checking the prop. For growth, and for tightness on the shaft.
I've seen them get flogged out internally from galvanic corrosion, allowing them move slightly into an out of round position causing vibration.

[dlymn]

Harmonic vibrations are rarely the motor. I would go for a damaged prop. You might have touched something and one blade might have a slight distortion. This gets the vibration going and then it might progressively build and then fade as it interacts with a very slightly misaligned or possibly bent shaft or a loose bearing. You might consider getting the prop rebalanced as an option. As well, check the cutless bearing holder to see how firmly it's attached to the hull. A little looseness there would really encourage harmonics. some of the bearing holders are quite long and thin and will exaggerate a minute imbalance. Solving that could be interesting. Do you have zinc anodes on the shaft? They do not always corrode uniformly. Do a test run with them removed.

I replaced my cutless bearing 3 years ago because of an annoying vibration which has now returned.

[fstbttms]

Originally Posted by dlymn

Do you have zinc anodes on the shaft? They do not always corrode uniformly. Do a test run with them removed.

20 years in the hull cleaning biz and many thousands of anodes installed on prop shafts, and I have never come across this as a cause of unwanted vibrations in the drive train.

[Mojadito]

Keep in mind the angle tolerance between transmission and propeller shaft flange is .003" (approx the thickness of a dollar bill). I would bet that the engine is out of line with the shaft. Simple test, unbolt the flanges from each other, rotate the prop shaft once by hand, if you cannot rotate it easily, the strut or the bearing is not installed properly. If it moves OK, slide the flanges back together. Then try to insert a dollar along the edges, if you can get it in anywhere around the circumference, the engine is misaligned with the shaft and needs to be adjusted. If not, rotate the propshaft 90 degrees and try again. If it goes in now, you have bent couplings. If the dollar will not slide in, in either position, the engine alignment is good.

[JPA Cate]

If you've wrapped a line around the prop, that could cause a change. If you changed the prop, and there is too little tip clearance, the turbulence can cause vibration.

If the shaft is proper aligned, and the cutlass bearing in good condition, it's a puzzler.

Could be the engine mounts, but they're pretty new.

You might actually try consulting with a different group about the problem.

Order of suspicions,

shaft alignment

cutlass bearing

engine mounts

transmission--highly unlikely Does the vibration occur in neutral at the same revs as when in gear?

Has someone else driven your boat who might have wrapped a line of some sort around the prop by accident and not told you?

Ann

[Billboy]

Thanks for the replies they are all helpful.
I had a mechanic look at the engine last week and he has identified that the centre cylinder is week.
Not pulling its fair share of the load.
When running at high idle we cracked the injectors on at a time and the first and third cylinder significantly slowed the revs but the centre one didn't.
A smaller effect than the others.
Mechanic says this would explain why the vibration only shows up under load.
2 foot seas or more, head wind and three quarters revs.
He is suggesting either re build this engine or replace.
Ow. That hurts financially.
I am surprised as I thought the engine is low in hours and as its maintained in a charter fleet it has been well maintained.
Regular oil changes etc.
I thought that diesel engines ran for ever if well looked after.

He has checked the shaft alignment and the transmission plate connection.
I have not been able to get it to vibrate at the dock or just outside in the bay to show the mechanic.
I am taking the boat out for a few days and the mechanic told me to crack the injectors one at a time as we did at the dock if the vibration occurs again under load.
I will try that.

I have asked the mechanic to do a compression test to prove the centre cylinder is weak.
He says this is time consuming.
Is it worth it?

If his analysis is correct whats the best solution here.
Re build or replace the engine?

[SoundOfSilence]

I have the same problem. Volvo Penta from 91', also MaxProp. I will get it checked on next haul out, probably in March, and let you know. Until then I'm cruising below 2000 rpm without vibration.

[DeepFrz]

Before doing a rebuild certainly check compression and also rotate the injectors to eliminate a bad/dirty injector. Could also be valves or piston rings of course.

[model 10]

Try and figure out the freq. The shaft is one thing and the engine is another. You probably have at least a 2:1 ratio reduction, maybe more for the shaft.

[Sandero]

How much engine vibration is normal? And if it is... what is the cause of it? Obviously there are some asymmetries such as the alternator etc. which is located off axis connected by a V belt.

[jimbunyard]

If you know that the center cylinder is not firing, it makes little sense to chase anything else until that is sorted out. Check the valve clearances first (all three cylinders), and then, as DeepFrz suggests, swap two injectors and see if the problem follows the injectors.


I'd be leery of a mechanic whose first recommendation at a misfire in a low hour engine is 'rebuild or replace'.

[SoundOfSilence]

Originally Posted by SoundOfSilence

I have the same problem. Volvo Penta from 91', also MaxProp. I will get it checked on next haul out, probably in March, and let you know. Until then I'm cruising below 2000 rpm without vibration.

Just to follow up on this one. I took the shaft out to be checked, nothing wrong was found. I changed all bearings and also adjusted the Maxprop propeller. The vibration is now gone.

[fstbttms]

What do you mean by "I adjusted the Max Prop"?