Cutless bearing life

30boat

30boat

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I'm fed up with replacing cutless bearings every year.My last boat had one for 8 years.Before you ask the alignment is done very carefuly.I have a Aquadrive and the cutless bearing is of a smaller diameter than the housing and it's always aligned by injecting epoxy resin in the housing so that it remains perfectly concentric with the shaft.The water is not silty and last season I didn't do all that much motoring.Maybe it's the material it's made of?
 
Assuming it starts off the right size for your shaft, I can only suggest that something is moving to change the alignment?
Could the P bracket flex under load?

Or is the prop too close to the end of the bearing so no water flows through it?
 
If you are bedding the bearing on epoxy as we advise with our bearings have you checked the carrier is aligned? chock shaft and a dry run assembly should allow the bearing to spin in carrier. What is the clearance/thickness of the epoxy? epoxy absorbs a lot of water and swells significantly, this could be giving rise to bore closure.

Finally you could always try a hard composite bearing rather than rubber as a close clearance fit in the carrier, we've seen wear rates of around 1/3rd of that of rubber.

Prop clearance to allow water exit should be minimum of 3.00mm (allowing for any shaft movement)

You can get the epoxy bedding clearance from here, it should be
In a 40.00mm ID carrier you should be looking at a max clearance of 0.30mm for epxoy or thickness of 0.15mm
http://www.h4marine.com/Downloads/Maritex AQUARIUS General Calculator REV 20100519.xls
 
Neil_Y said:
If you are bedding the bearing on epoxy as we advise with our bearings have you checked the carrier is aligned? chock shaft and a dry run assembly should allow the bearing to spin in carrier. What is the clearance/thickness of the epoxy? epoxy absorbs a lot of water and swells significantly, this could be giving rise to bore closure.

Finally you could always try a hard composite bearing rather than rubber as a close clearance fit in the carrier, we've seen wear rates of around 1/3rd of that of rubber.

Prop clearance to allow water exit should be minimum of 3.00mm (allowing for any shaft movement)

You can get the epoxy bedding clearance from here, it should be
In a 40.00mm ID carrier you should be looking at a max clearance of 0.30mm for epxoy or thickness of 0.15mm
http://www.h4marine.com/Downloads/Maritex AQUARIUS General Calculator REV 20100519.xls
Click to expand...

Thanks for your reply.There is quite a lot of clearance between the carrier and bearing,I haven't yet measured it.I usually chock the shaft to take the weight off the bearing,insert the bearing and after sealing both ends with silicone or similar I inject epoxy resin with a syringe.I wasn't aware that epoxy absorbed water,always thought it was pretty impervious.Your link doesn't seem to be working by the way.
 
You may likke this article which I found in Maritimre Reporter some time ago....

Something New, Something Traditional

C.S. Inter Marine’s shipyard manager Khun Patum Sardsiri is a practical man. He works with management to balance costs and benefits through out the company’s sizable fleet of tugs, barges and bulk cargo handling stations. In some cases this can result in relatively large expenditures, such as one made recently to swap engines on a one year old boat. The handy little steel tug had been launched with a 500 HP second hand engine that quickly began accumulating costs in down time and repairs.

When management made the decision to replace the engine with a brand new Cummins KTA19 M3 rated 640 HP at 1800 RPM, Khun Patum pulled the boat up on the bank at the company shipyard in the ancient Thai capital of Ayuthya some 50 miles up from the Gulf of Thailand. The old engine came out easily through purpose-built skylight. At the same time Khun Patum had the 7:1 gear pulled along with the shaft and four-blade 75x16-inch propeller for service and inspection.

With the shaft sitting out on a bench, he explained to a visitor the cost to benefit advantages of the Tamarind wood bearings that he uses in the company tugs. “A vinyl bearing costs about 90,000 baht ($2,500 US) while I can have these made for only 3,000 baht ($75.00 US)”

In America, lignum vitae wood was used for many years in shaft bearings. Writing in Power Transmission Design Magazine, Managing Editor Tom Hughes reported, “…early users of wood bearings (bearings with no impregnation of additional lubricant) were ship builders and woodworking shops making underwater shaft bearings for tugs and freighters. The natural resins inhibited water absorption and served as a base for a water film between bearing and shaft… Wood bearings operate well in abrasive environments. Gritty particles that manage to reach the bearing surface embed harmlessly in it.”

Demonstrating the good and minimal wear of the year-old bearing, Khun Patum explains that he carefully selects a tamarind log from a tree that is at least 20-years-old. The last one he purchased for 2,500 baht. After aging one week in the shade, he had eight (four sets of two) bearings turned at a local lumberyard. The two-foot long tail bearing for this shaft has an 8-inch interior diameter with 5/8-inch walls while the forward bearing keeps the same diameter but is 16 inches long. “I normally inspect the bearings every three years but they will last indefinitely if we keep a good water flow over them to lubricate and remove the river’s sand,” he explained.

In doing a cost benefit analysis it may be that new machinery is the best investment, but in others it may be that staying with the tried and true is the best course.
 
I read somewhere that submarines used to use wooden shaft bearings.The problem with a sailing boat bearing is that the wall would have to be too thin otherwise I'm convinced it would work.
 
Many boats used to use wooden bearings, but there have been significant improvements. Composites can offer very low wear rates whilst being tolerant to dry running and temperature change.

Maritex is used on one of the most recent submarine shafts, with dry running capability.

The link is a download? I'll check it tomorrow, thanks.
 
Neil_Y said:
Many boats used to use wooden bearings, but there have been significant improvements. Composites can offer very low wear rates whilst being tolerant to dry running and temperature change.

Maritex is used on one of the most recent submarine shafts, with dry running capability.

The link is a download? I'll check it tomorrow, thanks.
Click to expand...

This could be the solution,where is it available from?
 
This just has to be an alignment problem, or maybe a bent propshaft. I'm also very uncomfortable with the idea of using epoxy resin to hold the bearing in position. Sounds suspiciously like there has been excessive wear on the housing in the past, to make it loose. Maybe it needs replacing.

Lignum vitae is a red herring. You are just trying to solve the symptom, not the problem.
 
On my last two boats I fitted composite bearings. The wear on the first-a Gibsea 96-was not inspected as just after a large expenditure of cash and time First Mate and I had a very senior moment and fell in love and purchased the boat we have now. The current boat's cutlass is as good as the day it was fitted after two years and we expect many more years before it requires replacing. I did spend time checking the alignment of the shaft as the complete stern tube had to be pulled to press out the old brass/rubber bearing. Tips on the owners association website said to leave 1/4 inch of the new bearing protruding from the stern tube as this allows a pair of mole grips to be clipped to the bearing which can then be pulled out. I have done this so in future the stern tube may stay in place should it require a new bearing. As another poster said, the cause of premature wear may be alignment or possibly engine mountings allowing excess movement of the engine/transmission and so causing the shaft to apply uneven side loads on the bearing. Whatever the cause, a cutlass bearing should last more than a couple of years normal use.
 
I think you'll find the point about wooden bearings is that the shafts turn really slowly. Mind you, if everyone started using wood we'd have no hardwood trees left. But then, are there any hardwood trees left?
 
AndrewB said:
This just has to be an alignment problem, or maybe a bent propshaft. I'm also very uncomfortable with the idea of using epoxy resin to hold the bearing in position. Sounds suspiciously like there has been excessive wear on the housing in the past, to make it loose. Maybe it needs replacing.

Lignum vitae is a red herring. You are just trying to solve the symptom, not the problem.
Click to expand...

I agree with the solving symptom not problem but as far as fixing on epoxy is concerned we have been supplying Lloyds approved shaft bearings for class vessels for many years and the recommended installation is clearance fit on epoxy, but not thick epoxy to take up misalignment. As a guide (it varies on bearing wall thickness) a 25mm shaft and a 40.00mmID carrier gives a bearing of (25.13-25.23) ID x (39.70 - 39.80) OD

To clarify, as there is always some confusion a composite bearing means a composite bearing surface, they are one piece made from the same material so not a liner or bearing surface in a tube of metal or composite.

Many boats still use nitrile rubber bearings which will can be plain rubber or rubber glued/bonded inside a metal or composite tube carrier.
 
rotrax said:
On my last two boats I fitted composite bearings. The wear on the first-a Gibsea 96-was not inspected as just after a large expenditure of cash and time First Mate and I had a very senior moment and fell in love and purchased the boat we have now. The current boat's cutlass is as good as the day it was fitted after two years and we expect many more years before it requires replacing. I did spend time checking the alignment of the shaft as the complete stern tube had to be pulled to press out the old brass/rubber bearing. Tips on the owners association website said to leave 1/4 inch of the new bearing protruding from the stern tube as this allows a pair of mole grips to be clipped to the bearing which can then be pulled out. I have done this so in future the stern tube may stay in place should it require a new bearing. As another poster said, the cause of premature wear may be alignment or possibly engine mountings allowing excess movement of the engine/transmission and so causing the shaft to apply uneven side loads on the bearing. Whatever the cause, a cutlass bearing should last more than a couple of years normal use.
Click to expand...

The housing is not worn just oversize.It's perfectly round and smooth.I don't know, my boat being the prototype Westerly Fulmar perhaps they just grabbed the first P bracket they could find.There's no shaft movement because there is an Aquadrive that keeps everything straight and aligned.Setting the bearing in epoxy is not a bad thing.I can ensure perfect alignment this way by placing the bearing on the shaft and then freezing everything with resin.Theoretically it should work.On my last boat the cutless bearing lasted for at least 8 years.
 
Unless the bearings you are buying are very unusual, from a small supplier(?) i doubt very much whether their hardness or material is defective. Even with fairly poor alignment I would expect to get more than a year from one. The only components left are the P-bracket itself and the shaft. Is the bracket firmly attached to the boat, no movement? Assuming it is, I would check the shaft in way of the bearing for small pits or other defects. This is assuming the drive runs smoothly without lots of imbalance or vibration that might suggest a bent shaft or damaged prop.
 
Have you looked at the Aquadrive unit whilst the prop is spinning?
Does it look like it's running smoothly?
I had the main thrust bearing go in my Aquadrive unit. (My mistake caused it.)
 
Another interesting wear inducing problem I know, is if the boat in an area where small barnicle style creatures are abundant. The small ones love the dark spaces in the bearing, attach themselves and start to grow. When the propshaft turns it's like coarse wet and dry for a while until they are worn away.
 
perhaps I have missed something ? How is the OP's bearing lubricated ? Is it 'natural' water flow via shaped inlets, or a pressure feed off the engine coolant side ?

It's almost as if there is no lubrication causing the wear if the rest of the system is aligned and tight.

Could the epoxy injection be blocking water pathways ?
 
I'm leaning towards barnacles .There's a lot of them here and they're quite abrasive.The Aquadrive is in good shape and the shaft the same.The P bracket is not loose and the bearings are usually bought from Trafalgar Yacht Services .
The water flow could be an issue.I have a rope cutter that is mounted by necessity only a few mm away from the P bracket.Maby that's not enough for a good water flow.
 
I made Vesconite shaft bearings for both my last two boats - turned them on a lathe using the calculator on the Vesconite web site, added a helical groove to allow for a bit of water lubrication, and they both lasted for at least 5 years (ie during my ownership). The second was also fitted with an Aquadrive - almost impossible to get alignment problems with a 900mm long shaft I'd have thought, unless your P bracket is wildly out of alignment.
 
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