Prop Shaft Wear - Does this need replacing?

[Ruffles]

I'm sure you've thought of this but...
On Moodys the shaft is very slightly offset so that the shaft will come out, round the steg, with the cutless bearing removed. Not easily though. Needs a midwife. You have to use the full width of the stern tube having removed the shaft seal.

 

[awol]

Just imagine what would happen if that snapped.

Well, it won't fall out, will it?

 

[vyv_cox]

Some good advice above - one solution not so far offered, or if it was I missed it, would be to heat and shrink a tube onto the outside. Would restore the torsional strength but would require enough space to get it on and the stern gland would not come off again without surgery.

I presume surgery on the skeg is out of the question?

I suspect that if you calculated the temperature required to expand a tube that would shrink back to the diameter of the worn part you would find it to be several thousand degrees C! Shrinking a tube onto it would do absolutely nothing for the torque transmission, the power would simply pass through the remaining solid metal.

 

[vyv_cox]

Why not use a mic and ask Vyv? or another reasonably competent engineer.

Nice to know I am considered reasonably competent. Praise indeed.

 

[awol]

I suspect that if you calculated the temperature required to expand a tube that would shrink back to the diameter of the worn part you would find it to be several thousand degrees C! Shrinking a tube onto it would do absolutely nothing for the torque transmission, the power would simply pass through the remaining solid metal.

The tube is required to be long enough to bridge the worn part. A deltaT of 300K would give an expansion of >0.2mm on a 40mm od 304 stainless tube . Would require careful measurement, calculation and machining of the internal diameter and 0.1mm (say) interference fit on the shaft would certainly provide sufficient connection to transmit torque.
It was the method used to retain half-shafts in Ford Anglias and Cortinas and they only rarely lost them(!). Problem was cooling before the retaining ring was seated hard against the bearing and then you had to grind it off and start again!
My suggestion is a total pig of a job to do with opportunities for setting fire to the boat, the tube cooling too early and jamming and it makes certain the shaft won't come out backwards, but it is a solution.

 

[awol]

I suspect that if you calculated the temperature required to expand a tube that would shrink back to the diameter of the worn part you would find it to be several thousand degrees C! Shrinking a tube onto it would do absolutely nothing for the torque transmission, the power would simply pass through the remaining solid metal.

The tube is required to be long enough to bridge the worn part. A deltaT of 300K would give an expansion of >0.2mm on a 40mm od 304 stainless tube . Would require careful measurement, calculation and machining of the internal diameter and 0.1mm (say) interference fit on the shaft would certainly provide sufficient connection to transmit torque.
It was the method used to retain half-shafts in Ford Anglias and Cortinas and they only rarely lost them(!). Problem was cooling before the retaining ring was seated hard against the bearing and then you had to grind it off and start again!
My suggestion is a total pig of a job to do with opportunities for setting fire to the boat, the tube cooling too early and jamming and it makes certain the shaft won't come out backwards, but it is a solution.

 

[vyv_cox]

A deltaT of 300K would give an expansion of >0.2mm on a 40mm od 304 stainless tube . .

Based on the photo the wear scar is several millimetres deep. Hence my comment.

 

[GrahamM376]

I'm sure you've thought of this but...
On Moodys the shaft is very slightly offset so that the shaft will come out, round the steg, with the cutless bearing removed. Not easily though. Needs a midwife. You have to use the full width of the stern tube having removed the shaft seal.

Not sure which particular Moody you refer to but my shaft clears the skeg & rudder with ease and has to be removed to change the cutless bearing, it's out at the moment.

 

[GrahamM376]

If you use an angle grinder be very careful where the sparks go. A pad of wet cloth can help, but make sure it stays wet and have a fire extinguisher handy. I have cut a stainless shaft with an ordinary hacksaw and a good bi-metal blade. You just need slow steady cutting strokes using the full length of the blade.

These are the cutting discs to use, will go through the shaft in a minute or two. Far less sparks and effort compared to using grinding disc.

http://www.screwfix.com/p/erbauer-metal-cutting-discs-115-x-1-x-22-23mm-bore-pack-of-5/76482

 

[ribrage]

Sorry to hear of your problems , If it were me I'd change the shaft , chances are it won't shear from normal usage , if your engine was originally 80 hp and it's done several years general usage it's probably only 50 hp by now so the power output has been reduced.

its the time you give between forward and astern that you need to be concerned about, a quick shift while manoeuvring your boat will be the death of that shaft.

We have a 1.5 inch shaft similar to yours my shaft to gearbox coupling failed early 2014 , we weren't in a location to do a proper repair so did a temp fix in Gijon Northern Spain it lasted maybe 500 miles of motoring and similarly failed going astern into a berth in Lisbon the taper on the shaft sheared off at the gearbox end, we couldn't get hauled in the marina yard due to restrictions, we couldn't get towed as port authority wouldn't allow it .

So I stick welded the coupling directly to the shaft as a get out of the marina fix with intentions of making a short coastal hop , there were zero vibrations and the quick fix became a permanent fix until winter haul out , I have just had the shaft repaired a few weeks ago, we cut through the shaft using cutting discs on an angle grinder

similar to you it wouldn't come past the rudder , we had the engine out anyway for a different job so slid it inside the engine compartment it JUST had enough space with removing the stern greaser to come out that way , the old shaft was machined to remove the damage and a Volvo flange clamped to the shaft with bolts acting as the locking keys in machined grooves.

Torsional stress will shear that shaft when you need it most - manoeuvring - it will just play on your mind until it's properly fixed, every time I went into a marina I was on edge , now it's fixed I've even let the mrs practice stern too berthing in the marina !


How how many sleepless nights do you want ?

some pics here : http://www.sailblogs.com/member/blown-away/?xjMsgID=344105

 

[macd]

These are the cutting discs to use, will go through the shaft in a minute or two. Far less sparks and effort compared to using grinding disc.

http://www.screwfix.com/p/erbauer-metal-cutting-discs-115-x-1-x-22-23mm-bore-pack-of-5/76482

+1 Cheap as chips and the best thing for the job.

Northcave: I alluded to this in a post on your metal spraying thread:
a. Cut the shaft, get it out.
b. work out what is the longest shaft it is physically possible to insert without major work.
c. work out the difference between that and the length of shaft required.
d. ask a sterngear specialist (I suggested T Norris, but there are others) about a properly engineered means of using the shorter shaft: perhaps an extra long coupling.
Job done, and you then have a shaft you can easily get out again if needed.

 

[Tim Good]

d. ask a sterngear specialist (I suggested T Norris, but there are others) about a properly engineered means of using the shorter shaft: perhaps an extra long coupling.
Job done, and you then have a shaft you can easily get out again if needed.

I assume that, given no one has suggested it, I can't just cut a new shaft in two between the P Bracket and Stern Tube and have a heavy stainless coupling installed there. If that were possible and a good solution then I suppose everyone would do it.

 

[ribrage]

I assume that, given no one has suggested it, I can't just cut a new shaft in two between the P Bracket and Stern Tube and have a heavy stainless coupling installed there. If that were possible and a good solution then I suppose everyone would do it.

Part of our repair when the new engine went in was a shaft extension , but the coupling joint is internal , don't think I'd be happy with it outside ?

 

[Tim Good]

Part of our repair when the new engine went in was a shaft extension , but the coupling joint is internal , don't think I'd be happy with it outside ?

Given the distance from the stern glad to the aqua drive in the photo below, even if it was cut in the middle and then a coupling made, it still wouldn't be short enough to come our of the stern tube. Close but not enough.

 

[Piers]

Hasn't it been shown that a windmilling prop creates less drag than a braked prop? Am I missing something here?

 

[Tim Good]

Hasn't it been shown that a windmilling prop creates less drag than a braked prop? Am I missing something here?

Yes. I have a hydraulic box and it is not preferable to have it continually turning in terms of both noise and wear. Besides I have a folding prop now so the only issue is the worn shaft.

 

[macd]

Hasn't it been shown that a windmilling prop creates less drag than a braked prop? Am I missing something here?

I think you might be. The shaft brake isn't the issue (except that it's trashed the shaft) and, anyway, seems to have been discarded. The damaged shaft and the difficulty of removing it is the issue.

 

[GrahamM376]

2. Any clever ideas on getting it out AND getting a new one in without raising the engine? No i haven't removed the cutlass bearing yet but I doubt it will make much difference. Once retracted up against the skeg there is about 12 inches of shaft still in the boat.

I'm not sure whether that's correct. The cutless is probably half an inch larger OD than the ID so, with that removed, you will be able to angle the shaft by a quarter of an inch at the P bracket. With the distance to the skeg, the shaft end will obviously move even further over which just may allow the end to clear. Before cutting anything, I would (if possible) use a split pipe tool to extract the bearing with the shaft in place.

 

[Tim Good]

I'm not sure whether that's correct. The cutless is probably half an inch larger OD than the ID so, with that removed, you will be able to angle the shaft by a quarter of an inch at the P bracket. With the distance to the skeg, the shaft end will obviously move even further over which just may allow the end to clear. Before cutting anything, I would (if possible) use a split pipe tool to extract the bearing with the shaft in place.

Yeah I'm going to give it a try. The engine said that it is often possible on some boats but mine is so heavily laid up that she's stiffer than a dead camel and unlikely we'll be able to squeeze it past. I'm going to give it a dam good try though!

 

[Jassira]

I suspect that if you calculated the temperature required to expand a tube that would shrink back to the diameter of the worn part you would find it to be several thousand degrees C! Shrinking a tube onto it would do absolutely nothing for the torque transmission, the power would simply pass through the remaining solid metal.

I would of thought that when you heat a tube it's the material that expands, I would expect the outside diameter to increases and the inside diameter to reduce.

For what it's worth, I wouldn't leave the harbour with a shaft like that, I have an 80hp engine with a 1 3/4" shalt, when I changed it, I cut it in half with an angle grinder and took both bits to T Norris who made me a new one.

aren't boats fun