Propshaft / cutless bearing clearance.

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jb185

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I am having a new cutlass bearing made up out of Tufnol for my Albin 25. My request is, can anyone tell me the correct clearance required to suit the 25mm shaft?
Any advice would be appreciated.


Cheers
John
 
Welcome to the forum

Can't answer your question directly, but is it possible to convert your housing to take a cutless bearing.

You might want to talk to TigaWave a poster on here. His company is called H4 Marine and he is the bearing guru.
 
Thanks Tranoma,

Guru or not we do have a machining calculator for our phenolic cutless bearings which will give ID and OD clearances for carrier and shaft.
Maritex Aquarius calculator


Just enter your figures for carrier ID and shaft OD in mm and it will give clearances. Maritex Aquarius has marine growth inhibitors within the bearing structure, and a surface finish that can remove marine growth build up on the shafts, it has been specifically developed for ship shaft applications from arctic to tropical waters running close shaft tolerances.

However, be aware that different composites demonstrate significant differences in performance which can effect clearances. Moisture absorption in phenolics using cotton fibres can cause bearing seizure and failure. So I would only advise using the calculator as a very rough guide if you choose another material.
 
I was wondering as before you gave he reference to an active web page as opposed to an Excell file....No excell in my surfing laptop:( Now on the house puter and you still seem to be in there with hammers and spanners fixing the web page:)
 
Tigawave has put up his excell file for this so if you put in your housing and shaft dimensions then you will ge the figures which are about what I would expect


You will note that these figures relate not only to required clearance but take into acount swelling as Tufnol absorbs water when submerged. One or two ships couldn t start their engines when it was first used /forums/images/graemlins/smile.gif Your actual clearance will be less than the bored figure once fitted especially if using an interference fit which I would recommend for this size.

Don't forget the water channels in the bore!!!!
 
Sounds like you're familiar with Tufnol! Maritex was developed to overcome some of these aborption issues around 15 years ago, and has lower moisture absorption than other composite bearings, what is also useful is tat the absorption is accurately known and it's not linear, hence the calculator with some very involved maths based on 400days soak test results.

Please don't use this calculator for other materials such as tufnol if you plan to run the bearings at the minimum clearance.

We would still advise, even on small shafts, that bearings are a clearance fit in the carrier bedded on epoxy, as soon as you press them they distort and there is bore closure which unless the carrier is perfect will be to some extent unknown. With a clearance fit you can confirm that the carrier is aligned, as the bearing should easily spin in the carrier whilst on the shaft and you will know the internal diamater and clearance are as finished in the factory.

In practice on small shafts with small hp you won't really notice a loose or badly worn bearing as quickly as you will when the shaft is 6" and the engine is 4000hp!
 
We would still advise, even on small shafts, that bearings are a clearance fit in the carrier bedded on epoxy, as soon as you press them they distort and there is bore closure which unless the carrier is perfect will be to some extent unknown. With a clearance fit you can confirm that the carrier is aligned, as the bearing should easily spin in the carrier whilst on the shaft and you will know the internal diamater and clearance are as finished in the factory.

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I was at sea for a number of years and ours was one of the first companies to specify Tufnol staves back in the 50'3 . It took a while to get the clearance figures right avter using Lignum vitae but it certainly worked well .

I am still a bit old fashioned /forums/images/graemlins/smile.gif so prefer interference fits...I have come out of the steam recip age JUST!

I can see that epoxying bearings in is an alternative but my concern would be the danger of someone not taking all the right precautions and the bearing coming loose as was mentioned in another thread here. But then that was hardly an engineered yacht with the stern tub cutless just bedded into a fibre glass hull . Oh dear I am definitely old fashioned .

/forums/images/graemlins/smile.gif
 
The "Cutless" bearing (note spelling) is a proprietory bearing marketed by Countrose. It has a rubber bearibg surface and the bearing is encased in a brass shell (or phenolic resin compound for steel shaft tubes.) Note that the bearing rubbing on the shaft is rubber and it is water lubricated. I would not be tempted to try a tufnol bearing for several reasons.

1. The material swells in water. OK, an allowance can be made for this, but it does mean that the shaft is a rattling fit when launched.
2. The material is inflexible and will wear the shaft. The rubber does this only slowly. After 30 years, my shaft was less than .001" down in diameter in the way of the bearing.
3. Tufnol (and other plastics have uses for bearings such as the ruddershaft, but NOT the prop shaft. The rudder shaft is not doing 1500 rpm hour after hour.
 
[ QUOTE ]
I am having a new cutlass bearing made up out of Tufnol for my Albin 25. My request is, can anyone tell me the correct clearance required to suit the 25mm shaft?
Any advice would be appreciated.


Cheers
John

[/ QUOTE ]

Perhaps Tigawave forgot to mention, but I wouldent advice using Tufnol as a cutlas bearing material, perhaps you should look at using Vesconite or something similar.
C_W
 
I think you will find that the OPs boat was fitted with a tufnol bearing from new, which is why he is seeking the same thing to replace it. IIR - going back many years now - it was a top hat shaped sleeve with grooves machined inside for water to flow. Because the material swells in water it is important to machine a clearance - which is why he is asking. I don't recall what the bearing carrier was so I don't know whether it is possible to convert it to a conventional cutless.
 
The old converted fishing boat, that I ran for 30 years, originally had a white metal outer bearing in its stern tube. It was badly worn, and I got a tufnol bush made to replace it. The new tufnol bush was made similar to a conventional Cutless bearing with channels for water lubrication. The shaft was 3.5", but it was only doing 300rpm. I had it for 30 years, and when sold, it was still going strong, with no obvious lift on the shaft.
 
This is a very complex but interesting subject, here is my perspective on your comments. Rubber does work as a bearing and that is what the industry has become comfortable with, change is often difficult.

We generally talk about cutless or cutlass as a bearing for a propeller shaft that is lubricated by water, I wasn't aware that cutless was a brand or trade name of Countrose. Countrose don't seem to use it on their website.

Shaft rattling? how much movement will a rubber bearing allow on a 2" shaft? We run minimum radial displacement of 0.20mm on a 2" shaft. As far as I can see the rubber bearings don't show clearances on the shaft, but even if it is the same then the rubber will allow greater movement as it is a softer material. Yes the material swells in water bore closure (on 2") 0.06mm, bore closure thermal expansion (beyond ambient water temperature) 0.09mm. It is well known and still alows close running clearances.

The material is inflexible, very true, that is why it is such a good bearing material, it holds the shaft in line. Why will it wear the shaft as the shaft is running on a film of water, the same as with a rubber bearing. At start up and run down it will be dry but composites have dry running lubricants so there is still a lubricating film.

Shaft wear is the result of abrasives in the water film, so your low shaft wear would probably be the same with a composite.

Tufnol and other plastics differ widely in their performance as bearings.

I'll refer you to Lloyds who have approved composites for shaft bearings (as well as rudders)

Boat builders and users, especially navies don't take on new technology lightly, it has to show improvements in service and performance, is extensively tested and usually then trailed on a number of vessels. So far to my knowledge none have gone back to rubber.

We've had reports of more performance and less noise/vibration from recent trials and users, which would agree with our belief that a bearing is there to hold the shaft as straight as is possible and prevent movement whilst offering good dry running performance.

But rubber works and if you are happy with rubber then there is no reason to think about changing, however this conversation started with someone considering tufnol, as this is also a phenolic resin composite (as Maritex is although with different properties) I felt I was in a position to give an informed opinion on clearances in tis type of material. Tufnol I believe has greater closure than Maritex but this should be checked with the manufacturer for running clearances in water. We are happy to publish our figures in the calculator but please only use these for Maritex.
 
With so many boat engines mounted on relatively flexible mounts, would this not suggest that a rubber cutless bearing is a better choice to allow for the engine movement?
 
There is plenty of opportunity for engine movement as the shaft is flexible and will sag under its own weight if not supported and the clearance in the bearing (0.20mm on a 25mm shaft 100mm long bearing) allows for shaft displacement at a rate of 0.2 mm per 100mm shaft length between bearing and engine.

Allowing the heavy propeller to orbit rather than just rotate just looses power and wears components. Hold the shaft more inline and you will have a more efficient drive with less noise and vibration.

In answer to the statement that composites aren't suitable for fast spinning shafts.

There is a race boat running in unlimted 8 litre class with a 454 chevy engine producing 740hp at 6500rpm. The shaft turns at 10,582rpm at full tilt. It holds several records and has been using Maritex bearings as shaft bearings for a number of years. They were wearing out bronze bearings every oter race. Maritex was still running within tolerance after over 4 years racing.
 
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