Cutless bearing - Shaft Wear

[Ammonite]

I've just removed the prop shaft and there's some wear in the area of the cut less bearing. The shaft is 25mm dropping to 24.9 in the cutless area. The area is also slightly ribbed to the touch but we're talking fractions of a mm. Just replace the cut less and be done with it or time for a new shaft?

 

[30boat]

In your position I would bite the bullet and replace the shaft.

 

[Norman_E]

If the shaft is ribbed in the lengthwise direction it will quickly wear the cutless bearing, but if the ribbed effect is the other way it may have little adverse effect, particularly if the shaft is highly polished where it ran in the bearing. One question is how badly worn is the existing cutless bearing? If it is only slightly worn, or has been in for a long time consistent with reasonable wear then I suggest that only bearing replacement may be needed.

 

[jakeroyd]

0.1mm is only 4thou (0.004")
In your case I would not replace the shaft unless bent or badly corropded etc.
I would possibly get some fine emery paper and make sure the shaft was smooth and even where it passes through the new bearing.

 

[Ammonite]

The ribbed effect is not length ways and is very smooth. The cut less was replaced last year and I think the guy that did it overtightened the grub screws which hold the bearing - which crushed the phenolic casing - the shaft will move up and down in the bearing just under a mm but there is zero play left and right. The design of the stern tube means the cut less has to be cut down from 100mm to about 70mm. I was thinking of going for a naval brass jobbie this time around

 

[Tranona]

As it is a non standard bearing size you might consider having a composite bearing rather than a cutless. Try www.h4marine.co.uk who will custom make a bearing for you which is epoxied into place.

 

[Chris_robb_ndota]

I can think of 2 questions I'd be asking. Is the reduction in shaft diameter causing otherwise unexpected vibrations/movement/play, and does it induce water ingress if the bearing cannot be tightened enough to compensate. If yes to either, then it would be best to replace it. If neither, then there is not a problem.

 

[vyv_cox]

I can think of 2 questions I'd be asking. Is the reduction in shaft diameter causing otherwise unexpected vibrations/movement/play, and does it induce water ingress if the bearing cannot be tightened enough to compensate. If yes to either, then it would be best to replace it. If neither, then there is not a problem.

+1 (the first one) I changed everything except the shaft over a long period of trying to solve vibration and noise problems - Cutless bearing, P-bracket, propeller, added an Aquadrive, none was successful. I then replaced the shaft, which was worn maybe a little more than yours, and the vibration disappeared. It was such a tiny amount of wear that I had never considered that it might be the cause of the problem, but it was.

 

[Ammonite]

Thanks guys. There's some vibration flat out but its fine at cruising revs @ 6 knots. I think I will replace the cut less this year and see what happens (making sure not to over tighten the grub screws!)

 

[Neil_Y]

A new bearing clearance on a 25.40 shaft is only 0.13mm, so you've nearly doubled the clearance. The shaft needs to be completely smooth and also concentric, it's possible with shaft wear that this isn't the case and then the vibration this causes accelerates any wear. The latest shaft bearing materials do not seem to show any shaft wear at all in a wide range of marine applications. Having said that there are too many variables to be sure what will happen with any particular boat/shaft/type of use situation.

We can supply any size, length, thickness of bearing to suit your boat as Tranoma (thanks) said.

 

[Modulation]

Thanks guys. There's some vibration flat out but its fine at cruising revs @ 6 knots. I think I will replace the cut less this year and see what happens (making sure not to over tighten the grub screws!)

That's what I did in similar circs 3 years ago and no problem since when using at cruising speed. I do get a bit of vibration when I take her up to flat out but I do that so infrequently i don't care