Cutlass Bearing Wearing Out Quickly

[DonCarlos]

Hi, my boat was lifted out today and the mechanic said my cutlass bearing should be replaced. He showed me the movement and I accept that it does need to be replaced (and I have noticed some vibration / roughness when motoring at 2500 RPM and above). The thing is that the cutlass bearing was replaced a year ago (by a different yard) and has only done about 150 engine hours since. The mechanic said that the shaft shows no signs of being bent. It also rotates very freely indeed (when the prop is turned by hand), which he insists means that alignment is not an issue. The alignment has not been adjusted in the 5 years that I have owned the boat.

My specific question is does the fact that the shaft rotates very freely mean that alignment is not an issue? If it does mean that alignment is not an issue has anyone got any suggestions as to where I should check next.

Some background: the engine is a Volvo Penta TMD31B, the gearbox is a MS4B and there is a Gori 3 blade folding prop. The boat is a 1992 Najad 420.

[a64pilot]

Easy to rotate may help indicate alignment is good, but only in a new Cutlass, once worn it may be easy due to wear

It could be an out of balance prop or a prop with a slightly bent blade etc.

I also have a vibration, but on mine it’s above 2000 RPM, especially above 2200. On mine I believe it’s due to having a large amount of dead wood in front of the prop, and that blocks water flow, and causes vibration.

I’m going to try a “sailor” prop and if that doesn’t fix it, then I guess I just have to stay at or below 2000 RPM, cause I’ve tried everything except a new prop.

Start with checking run out of the shaft, if that is good, of course check alignment, it’s just too easy to eliminate it as a problem, if everything checks out, it’s likely the prop.

[jkishel]

1. It’s been five years, you should check the alignment anyway. Loosen coupler bolts, use .003 feeler gauge to check that coupler face is parallel to output flange within .003 inches as the shaft is rotated. Align by adjusting engine mounts.

2. If the system cannot be aligned either the shaft is bent or the coupler is no longer perpendicular to the shaft. Removing the coupler and shaft is the last thing you want to have to do.

3. The shaft could be damaged (scored, pitted) at the cutlass bearing and tore up the bearing rubber, you should be able to inspect this with the bearing removed.

4. As mentioned, the prop could be out of balance. Pull it and have the prop shop check and balance.

5. The previous yard may have installed the wrong bearing, if everything else checks out, install the right bearing and try again.

6. Inspect the bearing itself, is the wear even fore and aft, side to side and top to bottom? This indicates good alignment and implicates the prop..

[a64pilot]

This may sound silly, but run it up in reverse to high RPM, and or tied to the dock.
If it still vibrates, then it’s a wobbly shaft or a prop.
If You can access the shaft, take a stick or other soft something and with it braced against the hull to hold it still move it towards the shaft until it just barely touches, and bend will be immediately apparent.
Shaft has to be turning of course, but idle speed is OK, no need to run it wide open.
A dial indicator is used if you need to put numbers to how much it’s bent.

Mine is smooth as glass in reverse and tied to the dock, which tells me that my vibration is from induced flow.

[kmacdonald]

Originally Posted by a64pilot

Easy to rotate may help indicate alignment is good, but only in a new Cutlass, once worn it may be easy due to wear

Exactly. Look for cutless wear on one side of bearing and opposite side at other end of bearing. (oval wear)

If it is worn out in 150 hours there is a problem. Was it aligned when last replaced? Two or three blade prop? Lap prop to shaft and make sure key doesn't bottom out on prop key way. Measure shaft run out. (bent shaft) Are the engine mounts worn out? Are they the correct mounts? Most importantly is how much shaft is between cutlass and prop. It shouldn't be much more that the diameter of the shaft. All that is easy to check. If all that is OK then there is a problem with the gear or drive plate causing excessive vibration.

If you ever beat the prop off with a hammer, the bearings in the gear are damaged and causing vibration. You may not notice it but it's enough to wear a cutless much faster. Use a prop puller.

[wsmurdoch]

As a64pilot suggested...

I watched a mechanic tape a ruler to the keel of my boat and then measure the distance to the ends of the propeller blades as he rotated the shaft looking for a bent propeller blade as a cause of my vibration and cutless bearing wear.

Also, when I sail with the prop rotating and the alignment is good, the engine hardly moves on its rubber mounts. When something is off, it moves.

[a64pilot]

Originally Posted by wsmurdoch

As a64pilot suggested...

I watched a mechanic tape a ruler to the keel of my boat and then measure the distance to the ends of the propeller blades as he rotated the shaft looking for a bent propeller blade as a cause of my vibration and cutless bearing wear.

You may laugh, but that is exactly how blade track is checked on an airplane, I’ll use a chair, place it so it just barely touches one blade, then rotate the prop around and see if it just touches the other blades.

He has I think a folding prop, which due to my ignorance of them would make me more suspect of one over a fixed prop, more stuff to go wrong, but eliminate the cheap and easy stuff first, like shaft run out and alignment.
I think alignment needs checking at every Cutlass replacement anyway, as it will change, let one engine mount sag just a little and alignment is out.

[boden36]

How much movement is there? A cutlass bearing can tolerate a lot of slop without becoming dangerous. You may find after the initial wear it settles in and the wear rate diminishes. Unless it is dramatic, I would be inclined to watch and wait.


Regards,
Richard.

[DonCarlos]

Thank you all very much for your input - most helpful. The question was asked about the type of prop - it is a Gori 3 blade folding prop. I think I will replace the cutlass bearing and examine its wear for clues. I will then re-launch, test the trueness of the shaft as suggested, align engine and then take it from there. Thanks again.

[NYSail]

A mechanic cannot tell if the shaft is not true. I would pull the shaft and bring both shaft and prop to a reputable shop. They can tell exactly what’s going on and true out both prop and shaft.
Then I would reinstall and align engine with the new cutlass bearing in.

Good luck
Greg

[pcmm]

Originally Posted by DonCarlos

Hi, my boat was lifted out today and the mechanic said my cutlass bearing should be replaced. He showed me the movement and I accept that it does need to be replaced (and I have noticed some vibration / roughness when motoring at 2500 RPM and above). The thing is that the cutlass bearing was replaced a year ago (by a different yard) and has only done about 150 engine hours since. The mechanic said that the shaft shows no signs of being bent. It also rotates very freely indeed (when the prop is turned by hand), which he insists means that alignment is not an issue. The alignment has not been adjusted in the 5 years that I have owned the boat.

My specific question is does the fact that the shaft rotates very freely mean that alignment is not an issue? If it does mean that alignment is not an issue has anyone got any suggestions as to where I should check next.

Some background: the engine is a Volvo Penta TMD31B, the gearbox is a MS4B and there is a Gori 3 blade folding prop. The boat is a 1992 Najad 420.

The fact that the shaft "rotates freely" in the bearing is a start of it being worn out. Is it possible the wrong bearing was installed last year? maybe a metric bearing instead of imperial (or other way around) I've replaced a few cutlass bearings in my years and every time I install a new bearing its a snug fit on the shaft. I.E. the shaft will not spin freely with a new bearing installed, there is always some drag. I've just completely disassembled my stern tube and the 3 year old bearing is still this way.

[pcmm]

Originally Posted by NYSail

A mechanic cannot tell if the shaft is not true. I would pull the shaft and bring both shaft and prop to a reputable shop. They can tell exactly what’s going on and true out both prop and shaft.
Then I would reinstall and align engine with the new cutlass bearing in.

Good luck
Greg

Shaft, flange and prop. but the shaft and flange are most important to ensure are true and perpendicular

[singlespeed]

How many hours did you run the engine after you first noticed the vibrations at 2500 RPM? Did you dive on the prop/shaft assembly on the first opportunity? We burned out a cutlass bearing in less than an hour after noticing a subtle vibration we attributed to a sea running counter to the wind. Dove on it when we reached our anchorage and found bits of string from some sort of woven bag around the shaft where it enters the cutlass bearing. This blocked the going flow of the cooling water and burned the bearing.

Using a elastomer dampener plate in the coupling can prevent a lot problems, especially if you cruise in remote areas or places where getting marine parts involves extended shipping times or expenses. Minor misalignment is ha flexibly the flexing of the elastomer. The downside is that you need to check the alignment at least yearly. The dampened plate only covers sins, it doesn't fix them. It does help prevent damage to the transmission if you wind something into the prop. Some types even have safety straps that come into play after the elastomer sheers, so you can carefully continue motoring after clearing the garbage off the prop.

[motion30]

Also be aware the shaft alignment cannot accurately be done until the boat is in the water.. you would be amazed how much your hull moves when on the hard

[Clivevon]

Hi. I had the same problem years ago on my boat with cutless bearings wearing out quickly. When queried, the yard said it was due to the water in the area being extra silty.... which turned out to be BS. The problem got worse with heavy vibration on a long trip & another yard found the front engine mounts were shot. The engine had tipped slightly forward making it impossible to align the shaft properly, hence heavy wear on the bearing. New engine mounts (plus another new cutless bearing) solved the problem.
How old are your engine mounts? Ever been changed? By the way I got the new ones from a 3rd party maker for about one third the price asked by Volvo for theirs.