is my propshaft really bent?

P

pcatterall

Well-Known Member
Joined
2 Aug 2004
Messages
5,507
Location
Home East Lancashire boat Spain
Visit site
Our cruise along the south coast has come to an end in Poole where the boat is moored awaiting haul out and repairs.
Back in Newlyn we had an incident where a bolt 'failed' in the prop to gearbox connection.
The prop thrashed about a bit until we realised what had happened and we were concerned that the shaft may have been bent.
When reconnected we tried to ascertain if the prop had, indeed , suffered.
At slow revs it did seem uneven but we were not sure if it had been like that before.
At about 1000 (engine) rpm it was hard to see any eccentricity, we tried to measure it with a screwdriver which though crude suggested that it was not out by more than around 3mm. We tried her out and there did not seem to be any vibration. We continued the trip checking all the time for vibration and overheating at the stern area.
All seemed well until we were leaving Poole where the cutlass bearing seems to have failed.
The local mechanic says the prop is 'horribly bent' by about 10mm but we thought that this was mainly due to the lack of the cutlass bearing ( it was far worse than what we had observed before the bearing went)
Reading up on the issue ( what we always do 'after the event'!) I understand that my shaft ( about 5 feet long and 1 1/2" diameter ) may 'sag' slightly at the centre until it is spinning at some revs.
Question is.... does it sound as if my shaft is really really bent beyond the limits at which it may be straightened. Fingers crossed.
 
You need to measure it at zero revs, barring the shaft over. Ideally you should use a clock gauge but with care you can see a bend against a reference point to give a small clearance. Either 3 mm or 10 mm would be calamitous, the value should be no more than a few thousandths of an inch (sorry for mixed units, I still have problems thinking metric for small values).

A 1.5 inch shaft 5 ft long will barely sag at all under its own weight.

I doubt that a shaft can be straightened sufficiently well. Almost certainly a new shaft.
 
vyv_cox said:
You need to measure it at zero revs, barring the shaft over. Ideally you should use a clock gauge but with care you can see a bend against a reference point to give a small clearance. Either 3 mm or 10 mm would be calamitous, the value should be no more than a few thousandths of an inch (sorry for mixed units, I still have problems thinking metric for small values).

A 1.5 inch shaft 5 ft long will barely sag at all under its own weight.

I doubt that a shaft can be straightened sufficiently well. Almost certainly a new shaft.
Click to expand...

I was going to chip in with my thoughts about the "minor" matter of 3mm but this sums it up well. Best way to tell if a shaft is straight or not is to remove it and roll it on a flat surface. The propeller is more difficult and you would need to send it to a company like Steel Developments for a check.
 
savageseadog said:
I was going to chip in with my thoughts about the "minor" matter of 3mm but this sums it up well. Best way to tell if a shaft is straight or not is to remove it and roll it on a flat surface. The propeller is more difficult and you would need to send it to a company like Steel Developments for a check.
Click to expand...

I just want to add that a new shaft is not an expensive item by boating standards - say £150.

Try T Norris of Isleworth.
 
I looked up the Norris site before and it looked more like £300!!. Boat will be coming out in the RYMC yard and their contract engineer is going to sort her out with my assistance.
The 3mm I refered to was aguess at the accuracy I could measure to in situ I think it needs to be around 0.006" but will need to be out to measure it.
Thanks all.
 
The bent prop will have tried to shake the shaft but as it is restrained by the shaft bearing this will have taken extra loads causing the water film to collapse and the shaft to run directly on the bearing material. rubber bearings wont last that long in those circumstances.

I suspect all of your problems are from the prop inbalance from being bent, the shaft can be bent but it needs some force. Some long shafts can get a bend set in if the boat is on the hard for some time but we've only seen this in much bigger shafts.

I've never tested the theory bu I imagine the vibration from a bent shaft will be quite extreme, the bent prop will be trying to vibrate but the bearing will take all the loads (until it fails) so there will be limited movement. A bent shaft will shake the whole transmission.

You can see if it's sag by applying support in the centre (work out the weight) and see if it matches the force to take it to the horizontal. Or take it out and roll it as others have said.
 
I shelled out almost £1000 for a new shaft, bearing, prop and all the labour to fit test etc.
This from an engineer whom I trust as his quotes are routinely below big marina costs.

I'd rather you didn't tell me that I've been shafted... I have got over the shock and would rather not have to relive it.

It is running much more smoothly now.

(It did involve changing from LH to RH prop and allowing the gearbox to run the right way round for the first time in its life.)
 
Once it is out of the boat, if it is not obviously bent the way to test it is not to roll it on a bench, which will only reveal a bend you can probably see anyway. The method is to support the shaft in two vee blocks at opposite ends, and use a dial gauge near the centre of the shaft to measure total run out, which should be no more than a few thousandths of an inch, whilst turning the shaft by hand. The vee blocks and the stand for the dial gauge need to be clamped down to a substantial bench.
 
I priced a new shaft from TNorris and Lake Engineering. There was little to choose between them, both around £150, 1 inch, about 4 ft long. Then I was told of a small engineering shop in Bangor, who did it for £60.

Bangor is probably not much help to you but the message is to find a local, non marine shop if possible.
 
pcatterall said:
Question is.... does it sound as if my shaft is really really bent beyond the limits at which it may be straightened. Fingers crossed.
Click to expand...

No. All stainless bar comes out of the rolling mill a bit bent and its then straightened by passing through rollers. So your shaft can be straightened - question is whether your mechanic has the skill and kit to do so.

Your description of what happened is not clear but guessing that the bolt that "failed" ( why the parenthesis?) was in the gearblox flange / shaft coupling connection, I do not see how one of four bolts would cause the shaft to thrash around enough to bend it. TBH I would have thought that sufficient thrashing about to bend a 1.5 "shaft by 10mm would have done damage elsewhere anyway- for example to the stern tube.

In an ideal world you would pull the shaft, put it in a lathe and use a dial guage to measure straightness but depending on facilities and your mechanics skill it might well be cheaper simply to replace it. But if it is really that bent then with some care check everything round the shaft between gearbox and prop including the gearbox outer bearing
 
Last edited:
He said the prop was bent by 10mm "The local mechanic says the prop is 'horribly bent' by about 10mm" which would account for vibration and bearing wear.
 
Slight correction!! Following the first 'incident' the prop was checked as best we could and was no more than 3mm out ( possibly much less but our measurement technique was too crude) The stern gland remained steady.
After the second incident ( where it seems the cutlass bearing has failed) the mechanic said it was more like 10mm but we thought this was because there was now no bearing so the whole shaft was moving. The stern gland was moving about.
Anyway it will all come out and then we will know! I am braced for £1000 plus!!
 
pcatterall said:
I looked up the Norris site before and it looked more like £300!!. Boat will be coming out in the RYMC yard and their contract engineer is going to sort her out with my assistance.
The 3mm I refered to was aguess at the accuracy I could measure to in situ I think it needs to be around 0.006" but will need to be out to measure it.
Thanks all.
Click to expand...

Be wary of Norris. They sold me a brass stern tube and weren't embarrassed.
 
From another thread, I am presently "re-shafting"
I am reliably informed that a long shaft should have bearings, no closer than 22" and no further apart than 57"

My 2 metre 1" shaft comes in at £219 plus vat, the cheapest price I got (Versatile Marine) was £100 for the machining and £39 per metre for 316. Duplex obviously costs more. The company are going to bore match the prop
 
If it's any help to you I work in Shaftesbury, about an hour from Poole, I'm often straightening cam shafts etc and feel confident I could straighten your shaft ( must re-phrase that :o ). PM if interseted. A freebie BTW,
 
You must log in or register to reply here.

Other threads that may be of interest

P
Essential pre purchase or even pre survey checks
2
Replies
28
Views
895
Refueler
Refueler

Share this page

Top