is my propshaft really bent?

[pcatterall]

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.

[vyv_cox]

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.

[savageseadog]

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.

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.

[davidej]

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.

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

Try T Norris of Isleworth.

[Ex-SolentBoy]

I think you must have clipped an underwater obstruction so you should be sorting this out with your insurers.

[RivalRedwing]

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

It is normally the fitting of it that stings .......

[davidej]

It is normally the fitting of it that stings .......

True - but an easy DIY job. Your post suggests the boat is out of the water. If not, can you find some posts to dry out against?

[pcatterall]

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.

[Tradewinds]

For a 5' shaft at 1.5" dia £300 doesn't seem too far out to me.

[Neil_Y]

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.