Cutlass Bearing Lubrication

[BobnLesley]

Our yacht's fitted with a 10hp engine, traditional gearbox and a fixed three-bladed prop on the end of a 25mm/1" shaft; the shaft's clamped into the rear of the gearbox, passes though an oil-filled seal on white-metal bearings and then a white-plastic cutlass bearing at the stern - I can't remember what the material of this bearing's called, but it's supposedly 'self-lubricating'?

This cutlass bearing is OK, but needs replacing every other year and after six years, the shaft now shows significant wear at the bearing face and might well need replacing next time around: To be fair, the shaft/bearing are probably worked harder than most, as we might well average 200+ engine hours each year.

During a recent conversation with a chap from the Netherlands, who I believe to be a seriously good engineer - the 68' steel ketch that he's designed, built from scratch and lived on for the last fifteen years is testament to that - gave me some interesting points/advice:

•To work effectively, these self-lubricating bearings still ought to have some sea-water to help keep them cool, so they should at the very least have a few grooves filed into their inner face (mine are smooth) to ease the water's path into/out of the propellor tube.
•Even these grooves won't help much as the propellor's action causes a low-pressure area in the water immediately astern of the bearing, as a result most of the water which has percolated into the stern tube whilst sat on anchor will be slowly sucked out once the propellor begins turning and not replaced until the engine next stops.
•He suggested that active sea-water lubrication might help: - fit a 6mm/1/4" T junction into the flexible engine cooling water pipe just before the point where it connects into the exhaust elbow and tap a 6mm/1/4" connection into the propellor tube where it emerges into the engine bay (just astern of the existing oil-filled seal) then connect a hose between these new fittings which will then take a proportion of the expended engine cooling water out via the propellor bearing. Any comments/opinions on the suggestion? Whilst I can see the logic of the water lubrication, my biggest concern is that the modification might build up sufficient water pressure within the propellor tube, to force this injected engine cooling water forward too, past the the forward oil/water seals and into the oil filled shaft seal?
•Peter's other (and preferred) suggestion was to install a shaft bearing about 8mm shorter than the existing one and preferably of bronze, with a lipped oil seal installed astern of it, make the tap-in to the propellor tube as described above, but instead of connecting to the engine cooling system, fit a remote oil-filled header tank (similar to the one which pressurises the existing oil-filled shaft seal ahead of it) and then fill the whole propellor tube with oil. Any comments/opinions? My main concern with this proposal is that unless the shaft alignment is perfect (whilst Peter's undoubtably is, I doubt I could manage it) then any 'lash' in the propellor shaft would quickly destroy the new oil-seal at the stern leaving me with a bigger/more regular maintenance task than I have already; my own preference would be to continue using the nylon self-lubricating material still, that way, if the seal did fail, then I am not obliged to haul out for a replacement immediately.

Has anyone made/tried/considered any of these suggestions?

BobnLesley

 

[boatmike]

Our yacht's fitted with a 10hp engine, traditional gearbox and a fixed three-bladed prop on the end of a 25mm/1" shaft; the shaft's clamped into the rear of the gearbox, passes though an oil-filled seal on white-metal bearings and then a white-plastic cutlass bearing at the stern - I can't remember what the material of this bearing's called, but it's supposedly 'self-lubricating'?

This cutlass bearing is OK, but needs replacing every other year and after six years, the shaft now shows significant wear at the bearing face and might well need replacing next time around: To be fair, the shaft/bearing are probably worked harder than most, as we might well average 200+ engine hours each year.

During a recent conversation with a chap from the Netherlands, who I believe to be a seriously good engineer - the 68' steel ketch that he's designed, built from scratch and lived on for the last fifteen years is testament to that - gave me some interesting points/advice:

•To work effectively, these self-lubricating bearings still ought to have some sea-water to help keep them cool, so they should at the very least have a few grooves filed into their inner face (mine are smooth) to ease the water's path into/out of the propellor tube.
•Even these grooves won't help much as the propellor's action causes a low-pressure area in the water immediately astern of the bearing, as a result most of the water which has percolated into the stern tube whilst sat on anchor will be slowly sucked out once the propellor begins turning and not replaced until the engine next stops.
•He suggested that active sea-water lubrication might help: - fit a 6mm/1/4" T junction into the flexible engine cooling water pipe just before the point where it connects into the exhaust elbow and tap a 6mm/1/4" connection into the propellor tube where it emerges into the engine bay (just astern of the existing oil-filled seal) then connect a hose between these new fittings which will then take a proportion of the expended engine cooling water out via the propellor bearing. Any comments/opinions on the suggestion? Whilst I can see the logic of the water lubrication, my biggest concern is that the modification might build up sufficient water pressure within the propellor tube, to force this injected engine cooling water forward too, past the the forward oil/water seals and into the oil filled shaft seal?
•Peter's other (and preferred) suggestion was to install a shaft bearing about 8mm shorter than the existing one and preferably of bronze, with a lipped oil seal installed astern of it, make the tap-in to the propellor tube as described above, but instead of connecting to the engine cooling system, fit a remote oil-filled header tank (similar to the one which pressurises the existing oil-filled shaft seal ahead of it) and then fill the whole propellor tube with oil. Any comments/opinions? My main concern with this proposal is that unless the shaft alignment is perfect (whilst Peter's undoubtably is, I doubt I could manage it) then any 'lash' in the propellor shaft would quickly destroy the new oil-seal at the stern leaving me with a bigger/more regular maintenance task than I have already; my own preference would be to continue using the nylon self-lubricating material still, that way, if the seal did fail, then I am not obliged to haul out for a replacement immediately.

Has anyone made/tried/considered any of these suggestions?

BobnLesley

Yes indeed. In fact your advisor is obviously well experienced and right on every count. The only mistake in your post (and a hobbyhorse of mine) is it's a CUTLESS bearing not a CUTLASS bearing!
Firstly these work best in a situation where they are open both ends and can "pump" water through, like a P bracket. Where they are in the outer end of a stern tube they are usually improved by having water channels forward of the bearing where the water can enter, and exit normally at the rear. I am confused by the fact that you say yours is "smooth" as a proper cutless usually has spiral grooves in it to force the seawater through like an archimedian screw. I have seen those with parallel grooves, I don't think they work as well but there are a lot of them about. I have never seen one with no grooves. Frankly I don't think this is a cutless bearing at all and more likely just a plastic plain bearing. You could well consider replacing with a cutless bearing next time and providing a water flow to the front of it somehow.
The trick of tapping some water off the exhaust is an old one, and has the added advantage of oxygenating the shaft and preventing crevice corrosion. In a small vessel it is unusual to have a full oil bath tube but it's a good idea apart from the need to keep it topped up. If you don't the bearings will suffer quite rapidly.

 

[vyv_cox]

I agree with everything boatmike says. Conventional cutless bearings as used by many boats in P-brackets and shaft logs are usually either a bronze or tufnol sleeve lined with a nitrile or chloroprene rubber. These must be lubricated by a flow of water, as your adviser has told you. Your plastic bearing is rather less accommodating for dirt, which may be why it fails frequently and is leading to shaft wear.

I would try to fit a conventional cutless bearing in your log, with the seawater supply to the forward end. Oil lubricated shaft bearings work OK but will inevitably be more labour intensive, costly to run and probably more difficult to find spares. The only rider is that cutless bearings come in a range of sizes and you will need to find one that fits.

One other thing - if your self lubricating bearing is PTFE, which sounds possible, it may be glass filled to provide mechanical strength. The glass will wear stainless steel shafts, while remaining unworn itself.

 

[Tranona]

These plain bushes were standard fittings in Scandinavian boats of the period. They are water lubricated and probably made of an acetal such as delrin. However, as you have discovered they are not very satisfactory for long term use as they wear. A Cutless outer bearing would be better but you will probably find there is not room to fit one - either length or bore.

Two suggestions. The first is to consult the owners association - there are 3000 of these boats around and if there is a solution somebody would have found it by now. Second, contact NeilY who posts here regularly. His firm specialises in composite bearings for rudders and stern tubes and he may be able to recommend a bearing material with a longer life.

 

[stevebirch2002]

These plain bushes were standard fittings in Scandinavian boats of the period. They are water lubricated and probably made of an acetal such as delrin. However, as you have discovered they are not very satisfactory for long term use as they wear. A Cutless outer bearing would be better but you will probably find there is not room to fit one - either length or bore.

Two suggestions. The first is to consult the owners association - there are 3000 of these boats around and if there is a solution somebody would have found it by now. Second, contact NeilY who posts here regularly. His firm specialises in composite bearings for rudders and stern tubes and he may be able to recommend a bearing material with a longer life.

I agree, contact your Owner's Association, namely me! Hi ya Bob we now make the bearings from Vesconite, much less absorbtion but in the process of discussing with NeilY ref. another material that has even less absorbtion.... Certainly a lot harder wearing. I havent changed my bearing for 6 - 7 years and infact with a little slop in the bearing more than enough water seems to get in teh bearing.

Cheers

SB

 

[Neil_Y]

Lots has changed in bearings over the past ten or fifteen years.

Many commercial vessels stopped using rubber as cutless bearings, advanced polymer composite bearings now offer superb bearing properties.

The self lubricating properties are there for run up and run down or for slow cyclic movements such as goose necks and rudder bearings. Any rotating system requires a lubricating film which once the shaft is rotating supports the shaft and also the forces trying to bend it such as the thrust applied by the prop.

The dry running lubricants that are built into modern composite bearings are there to reduce wear and heat at run up and run down before a lubricating film is established.

We supply oil filled systems and they are very good with very low maintenance requirements and very easy to work on, but they do require accurately toleranced bearing systems to keep the shaft true to the lip seals. We have needle roller and plain journal bearings with pump circulated oil.

We are typically looking at many ,000's of hours on composite bearings, the bearings don't wear when the engine is running it is only as they start and stop that the shaft runs directly on the bearing material and causes wear.

The plastic you have is likely to be a melting polymer that will soften with heat, as it softens sand particles that get into the gap will wear the shaft as do some fibres introduced to the mix to give meachanical strength.

Its a big subject!

If you have any specific questions I'm happy to help.

 

[boatmike]

I agree with everything boatmike says.

Gawdn Bennett!!! Thats a first!!! I like you Mr Cox you are a Gentleman!
I shall now shut up for the rest of the day lest I spoil it.........

 

[vyv_cox]

I agree with everything boatmike says.

Gawdn Bennett!!! Thats a first!!! I like you Mr Cox you are a Gentleman!
I shall now shut up for the rest of the day lest I spoil it.........

I didn't mean everything for ever!

But I don't often disagree with most of your posts.

 

[alexrunic]

another thing that can cause cutlass bearing wear is misalignment of the shaft. it should be check every year when the boat is it the water.