Cutlass Bearing Wearing Out Quickly

[sailcrazy]

On our boat the lifting strap goes just before the saft leaves the hull and goes to the "P" strut. Just after buying we hauled out for a small job, and had a rotation-connected vibration issues after. Long story short....we ended up pulling the shaft and putting it on a lathe in a machine shop; statically...it looked straight, but under rotation there was a small, slight bend in the shaft. We had a new (straight) shaft made, and have never had an issue since. It doesn't take much!

[Mike Banks]

The quickest way to wear out a cutlass bearing when everything is correctly installed and aligned, is to motor through water containing suspended sediments. That brown tinge in a river in spate, or fine sand or mud in or near a tidal race is highly abrasive to water-lubricated bearings.

[sesmith]

With the shaft out, you should probably check that the new cutlass bearing lines up with the center of shaft log and the center of the engine by sighting it with a laser. Until you can prove that all three line up, you can't really assume that aligning the engine to the coupling with feeler gauges is actually aligning the shaft and not stressing the cutlass in some way. Then you can do the final alignment in the water with the feeler gauges.

You should also get the shaft checked at a machine shop, while it's out, and also have them fit and face a new coupling.

[sailu2]

Hi
I had the same problem with a new cutless bearing every six months. Then I bought the cutless myself, US made, not Chinese. More expensive but cheaper in the long run now get four /five years before renewal. Also I check the water seal on the shaft with a quick squeeze to expell any air on a regular basis. It is possible that this is the cause as in my case. Good luck

[SwellGuy]

*cutless

A cutlass is a sword.

[Oceansailor]

The Najad 420 is a center cockpit. I would imagine that the shaft is quite long? If so it could be that the engine tilts forward when motoring hard. If the engine mounts are getting soft it tilts more and you will wear down the cutlass bearing quicker.

[hamburking]

Once the new cutlass bearing is installed, test again to see if you can rotate the shaft with no resistance.

My shaft was bent on haulout once. The prop/shaft would not move at all.

[sy_gilana]

A64Pilot nailed it I think.


It could be shaft whip. IF one of the blades on the Gori does not rest on the stopper the same as the other/s, it will induce shaft whip. Let me explain that in an analogy.

Imagine a flexible shaft for the purpose of this explanation, perhaps a pvc conduit pipe, representing the shaft, stuck in the ground vertically. On its top end is a flange, a disc of plywood simulating the plane of propeller rotation. Now, place three beers on the edges of the disc, at 120 degrees separation, simulating the thrust developed by each blade.

The support PVC tube is straight, as the forces are even and balanced. Now, add a fourth can of beer on top of one of the three. There is more push on one blade (or less) and the support will bend, its called whip, and will get worse as rpm increases and centrifugal force adds to the eccentricity.

Have your prop scanned, or carefully check that they stop on their stoppers in a perfect virtual disc.

[Minggat]

Originally Posted by sailu2

Hi
I had the same problem with a new cutless bearing every six months. Then I bought the cutless myself, US made, not Chinese. More expensive but cheaper in the long run now get four /five years before renewal.

Interesting comment.

My haulouts are usually about 5 years apart.

There are so many ways to be vulnerable.

In researching the subject here on the forum, I read about some bearings going into their teens.

In my case, the prop strut had been out of alignment with the shaft log since the boat was new, 43 years earlier. I won't go into that long story, other than to say that there was never any play in the cutless for the 15 years I had owned the boat. I cannot imaging the bearing ever having been changed out, with the strut misalignment being what it was.

Did some major work getting the engine to stop sliding down the stringers so alignment would be of value.

Anchored down the engine, resolved the strut misalignment issue. The engine is radically more quiet and the prop shaft freewheels at much lower speed through the water.

Sailed Mexico to Hawaii. At my last haulout, the cutless wear was obvious (which my bottom cleaner had been telling me).

So somewhere in the miles at sea, in probably less than 6 month-but a couple of thousands of miles later, the cutless needs replacement. I am hoping that prop balance is the culprit, after all the other work I did.

I am wondering how much freewheeling taxes the cutless.

I am planning on swapping my fixed 3 blade out for a folding prop, with hopefully longer cutless life being one of the benefits.

[DonCarlos]

Just to follow up here for the record. Covid meant that I was only able to lift and investigate the issue recently. I believe that I have got to the bottom of my issue, although only time will tell for sure.

The shaft appeared to be straight, as far as any test in the boat could ascertain. I disassembled and serviced the Gori 3-blade and it appeared perfect, as far as I could tell. I got the engine mounts and alignment checked by a mechanic and he said that all was fine. However there were two water intakes on the stern tube and these were very severely / almost totally clogged up with solidified salt. Thus very little water could reach the cutless, which I assume would result in greatly increased wear. I cleaned these intakes and the flow of water to and across the cutless should now be hugely increased. The transmission now sounds and feels much smoother (although in previous years it also felt smoother after the cutless was replaced).

I bought the boat 7 years ago and since then I have had the professionals (in 2 different yards) change the cutless and service the prop each year. Disappointing that neither of these looked at or cleaned those intakes.

Thanks to all who contributed to this discussion - it really helped my zoom in on what I needed to check and do.

[4HMainer]

Wear rates vary hugely, as there are many factors that can effect wear, use in abrasive silt in shallow rivers can be as bad as lack of use and marine growth.


Alignment issues are not always engine mounts they can be caused by stern tube, strut or P bracket being out of true.


If you buy bearings made as a clearance fit you can have a dry run assembly and with shaft supported you should be able to slide bearing into carrier and rotate it with only 0.10 - 0.15mm clearance. That will confirm alignment of drive set up.


As a guide for a new bearing 2" shaft 2.5" carrier ID
New clearance on shaft is 6 thou or 0.15mm
We advise changing when clearance approaches 50 thou or 1.27mm

[nuku34]

Could be the yard installed a cheap “generic” cutless bearing. Read this site to find out from a pro that quality does matter:
https://marinehowto.com/replacing-a-...leeve-bearing/