worn propeller shaft at cutless bearing

[wee_niall]

Afternoon folks

After starting some minor jobs on our little marcon striker and it's aged 1gm, i noticed some play in the cutless bearing. After taking the shaft out I realised that it had worn about 1mm pretty much the whole length of the cutless bearing.

I've priced up a replacement shaft which is coming in at 200+ quid and I've also looked into metal spraying which is about the same.

So I'm now looking at finding a cheaper option and I was wondering if it was an option to get the shaft reduced down from 1inch to 7/8 at the cutless bearing and fitting a slightly smaller bearing?

The other option I was pondering was to use a bobbin on the shaft to push it aft so that fresh metal will be at the bearing but I'd need to recut the taper/thread and I'm not sire there is enough space before the worn bit to do that...

Any thoughts??

Cheers
Niall

 

[Ruffles]

I would get another quote for a new prop shaft. Have you already removed it? That's half the battle already won!

http://www.tnorrismarine.co.uk/documents/1TNPriceListFEBRUARY2014.pdf

 

[wee_niall]

Thanks Rob - shaft is out the boat, although the coupling is still attached and looks a swine to remove!

T Norris is about 200 quid as well (excluding postage) and I've had 3 quotes all around the same price - shaft is quite long at 54inches

I would get another quote for a new prop shaft. Have you already removed it? That's half the battle already won!

http://www.tnorrismarine.co.uk/documents/1TNPriceListFEBRUARY2014.pdf

 

[HunterWanderer]

Dont bother.

1mm of wear isn't a problem. These should have plenty of clearance as they are water lubricated.

 

[wee_niall]

Well that would be good but would 1mm play not knacker a new bearing pretty quickly? I'd be keen to avoid having to redo things as the engine has to come out to remove the shaft...

 

[Neil_Y]

Speaking as a bearing supplier a new bearing is finished to have a clearance of 0.13mm on a 1" shaft, more than 0.6mm and it's starting to wear rapidly. So 1.00mm is a problem, yes it is water lubricated but too large a gap and the water film can't be maintained or will not support the same pressures. The pressure on the bearing is the weight if the shaft/prop and also the forces trying to make the shaft bend with thrust and torque.

Thin water film = smoother running shaft and lower wear rates.

 

[wee_niall]

OK, so with 1mm clearance it looks like I'm going to be doing this again next year...is there any merit in my other options above or should I bite the bullet and order up a new shaft?

 

[Neil_Y]

Assuming the shaft is over spec'd then you may be able to get it turned down to say 24.40 from one end so a bearing for the new size can be passed over. Most rubber bearings will to standard sizes but some suppliers can finish bearings to any size, however that will cost more than an off the shelf 1" bearing. Swings and roundabouts you might spend a lot of time shopping and end up spending a £100 sorting what you have.

 

[pampas]

Have sent a pm for you, Once the shaft is out, find a welder then a turner to bring it down to size. but you say you have the means to cut a taper and thread? if so turning could be done by you.

 

[wee_niall]

Ah, I had forgotten that I need to get the shaft through the bearing so can't just cut down that section - might have to change the prop as well.

Any thoughts on using a bobbin to extend the shaft? I think I'm clutching at straws here! lol

 

[wee_niall]

Unfortunately I don't have that skill - was meaning a local engineering place would be able to though

Have sent a pm for you, Once the shaft is out, find a welder then a turner to bring it down to size. but you say you have the means to cut a taper and thread? if so turning could be done by you.

 

[Jcorstorphine]

Ah, I had forgotten that I need to get the shaft through the bearing so can't just cut down that section - might have to change the prop as well.

Any thoughts on using a bobbin to extend the shaft? I think I'm clutching at straws here! lol

I don't think you will need to change the prop as thinning the shaft down will only reduce the contact area of the internal taper of the prop by a few mm as it will not slide on any further.

 

[HunterWanderer]

Speaking as a bearing supplier a new bearing is finished to have a clearance of 0.13mm on a 1" shaft, more than 0.6mm and it's starting to wear rapidly. So 1.00mm is a problem, yes it is water lubricated but too large a gap and the water film can't be maintained or will not support the same pressures. The pressure on the bearing is the weight if the shaft/prop and also the forces trying to make the shaft bend with thrust and torque.

Thin water film = smoother running shaft and lower wear rates.

Speaking as a user my experience is the exact opposite of this. The wear on mine has not changed for years and Ive used the boat a lot.

Surely as the bearing wears the flat areas on the flutes increase reducing the pressure? And how will the water film not be maintained when its underwater?

 

[vyv_cox]

I would try some local engineering companies. In your area there are surely quite a few? I was quoted around £200 by T Norris and by Lake Engineering but a small firm in Bangor made one for £60.

I had a severe prop vibration problem that turned out to be due to less than 1 mm wear of the 25 mm shaft. Changing the shaft cured it.

 

[wee_niall]

I would try some local engineering companies. In your area there are surely quite a few? I was quoted around £200 by T Norris and by Lake Engineering but a small firm in Bangor made one for £60.

I had a severe prop vibration problem that turned out to be due to less than 1 mm wear of the 25 mm shaft. Changing the shaft cured it.

i thought that as well and went to a well respected automotive engineering place in dundee and they said that the standard stainless bar you get is never properly straight which would be a problem with the fairly long length and they'd have to turn down an oversized bar (which would take a full day!) - the other option was metal spraying the worn area but the cost was going to be around 240 for that.

I'm leaning towards getting the shaft turned down to 7/8" as that means i could still use (fairly) standard cutless bearings and the loss in strength would not be an issue for a 1GM! It also means i won't have the hassle of getting the coupling off the shaft....

 

[Neil_Y]

Speaking as a user my experience is the exact opposite of this. The wear on mine has not changed for years and Ive used the boat a lot.

Surely as the bearing wears the flat areas on the flutes increase reducing the pressure? And how will the water film not be maintained when its underwater?

The water film can only support a certain pressure dependant on clearance and also surface area and speed, so if you have big clearance and slow speed there will be no water film. Turn a shaft by hand and it will be running dry on the bottom of the bearing, turn it fast enough and a water film is established. The motion of the moving shaft drags a thin layer of liquid with it creating a pressure that supports the shaft and that pressure is inversely proportional to the increasing clearance. Pressure also increases with shaft speed, liquid viscosity.

The physics is all about running at minimum clearance for hydrodynamic bearings to establish and maintain a water film that will support a given pressure, surface area is important which is why we often supply marine bearings with two or three waterways. The physics is "Pressure within the fluid mass is greatest at some point approaching minimum clearance and lowest at the point of maximum clearance (due to divergence)" A shallow wedge establishes the water film faster than a steep wedge, so the closer the shaft OD and bearing ID the better.

To be fare many small boats have such small loads on the shaft that wear is only seen when something is wrong with the set up, but on commercial vessels with shafts of 2 - 8" there can be a huge benefit in smaller clearances.

 

[onesea]

Thanks Rob - shaft is out the boat, although the coupling is still attached and looks a swine to remove!

T Norris is about 200 quid as well (excluding postage) and I've had 3 quotes all around the same price - shaft is quite long at 54inches

1.4m that's nothing mine was 2.4m (1"dia)and I do not recall it being over 300 quid (I could be wrong, although it might of been with VAT). I bought to much from him and do not wish to recall all the prices!

http://www.lakesterngear.co.uk/sterngear.html

No link just a poorer customer...

 

[Jcorstorphine]

Try Neil Y

i thought that as well and went to a well respected automotive engineering place in dundee and they said that the standard stainless bar you get is never properly straight which would be a problem with the fairly long length and they'd have to turn down an oversized bar (which would take a full day!) - the other option was metal spraying the worn area but the cost was going to be around 240 for that.

I'm leaning towards getting the shaft turned down to 7/8" as that means i could still use (fairly) standard cutless bearings and the loss in strength would not be an issue for a 1GM! It also means i won't have the hassle of getting the coupling off the shaft....

You could also talk to "Neil_Y" who has posted here. He could machine you a custom diameter in composite material bearing. All you would need to do is get the machine shop to polish up the shaft and remove the step where the prop is fitted and get an actual final diameter of the reduced shaft so that "Neil_Y" could machine you a bearing. You would also need the ext diameter of the bearing which you removed.

 

[wee_niall]

Not a bad idea but I’m quite keen to keep things fairly standard and not get into having to get custom things made up. There’s also the problem that the worn bit hasn’t worn even so would need more than polishing and the rest of shaft aft of the bearing would need reduced. I’ve asked the engineers for a quote to reduce the diameter of the last foot so hopefully it will be nice and cheap

 

[Old Troll]

Purchase a 1140mm dia x 25mm dia s/s shaft from John Hood of Clydebank ( HM forces suppliers) for £30 inc vat. Another £30 got the shaft tapered/keyed/threaded and nut at a machine shop in Glasgow. Worth a thought if you are through to the West from the far East to keep the costs down.