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Discussion Starter #1
G'day,

ever since I brought my boat I have had a bit of a concern with the cutless bearing. Above 1,500rpm it vibrates and makes a noise that is noticable in the back cabin. At less than 1,200rpm it is not detectable at at 1,900RPM (normal cruising speed) it is fairly loud. The vibration appears to be at the end closest to the engine / away from the prop (I can reach in and grab the outside and there is almost no vibration at the prop end where it goes thru the hull, all the vibration is at the other end).

We have now had the boat for two and half years and have sailed approx 2,000 nautical miles and the noise does not appear to have changed.

I also understand that the PO replaced the prop shaft approximately six months before I purchased the boat. I believe he fitted the prop shaft and cutless bearing himself. I have found some of his work to be variable.

My inclination is "don't fix what's not broken". As it has lasted 2 1/2 years / 2,000NM I should leave it & spend my time / $ on other things that do need fixing (auto pilot, generating capacity, etc etc).

However wondering if their are any other opinions out there? Are noisy cutless bearings common or should I be ripping the the thing apart ASAP?

Ilenart
 

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Something else you may be experiencing is propeller shaft whip or maybe even cavitation.

However, as this noise has been constant and there has been no detriment, it is probably safe to leave well alone and continue using the boat without change as after 2,000 N Miles I would have expected the noise to have changed and possible problems with your shaft log or strut to occur. Just try and avoid using the rpm's that cause the vibration.
 

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how close was the shaft alignment the PO did? it should be less than .003 thousand " also did he rotate the shaft 180 & recheck to see if it was the same? if not the flange is not true on the shift. if you have not alined a shaft there a number of places you can find information about it.
 

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It could also be an out-of-balance problem with the prop. Since the cutless bearing was replaced with the shaft, I assume the old prop was put back on.
 

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My inclination is "don't fix what's not broken". As it has lasted 2 1/2 years / 2,000NM I should leave it & spend my time / $ on other things that do need fixing (auto pilot, generating capacity, etc etc).

However wondering if their are any other opinions out there? Are noisy cutless bearings common or should I be ripping the the thing apart ASAP?
My motto is somewhat different: If it seems that there is a small problem right now - it is definitely going to be easier to deal with than when it runs into a bigger problem. Because it INVARIABLY does turn into a bigger problem.

Perhaps your bearing is wearing, your prop has a flop or the shaft's a bit daft ( :) )

Wouldn't you rather spend a few hundred fixing it now than try to plug a huge leak when it tears loose a hundred miles from shore and rips a hole in the hull ???
 

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Discussion Starter #6
Something else you may be experiencing is propeller shaft whip or maybe even cavitation.
Some of the noise could be cavitation from the prop as the bottom blade is spinning approx 6 inches from the back of the keel. However most of the noise is comming from the vibrating cutless bearing, as the noise reduces considerably when you grab the outside of the bearing
 

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Discussion Starter #7
It could also be an out-of-balance problem with the prop. Since the cutless bearing was replaced with the shaft, I assume the old prop was put back on.
Yep, could be. The old prop was put back on. Was also just reading a summary on Drivetrain Basics and he listed causes of vibration as:

Number One. Dirty bottom and running gear.
Number Two. Propellers out of balance.
Number Three. Loose propeller.
Number Four. Improper alignment of coupling.

Number one is out as the noise does not change with a clean bottom. So number two or three could be the culprit.

Thanks for the suggestion.
 

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Some of the noise could be cavitation from the prop as the bottom blade is spinning approx 6 inches from the back of the keel. However most of the noise is comming from the vibrating cutless bearing, as the noise reduces considerably when you grab the outside of the bearing
On checking the drawings for the Roberts 45 Classic Roberts Classic 45 it has a long shaft log and I what you are putting your hand on is not the bearing it is the packing gland. It is possible that there are 2 bearings in this kind of design, but now seeing the layout I would check numbers 2, 3 and 4 in your last post.
 

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I had a similar problem that turned out the be loose motor mounts.
 
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