How much bend is a bent prop shaft?

[wooslehunter]

My prop shaft is around 600mm long. I measured 0.4mm deflection in the middle. I do have a phantom vibration I'm trying to find although it's not bad & could easily be something else like exhaust.

Anyone think this mighht be enough to generate the vibration?

 

[david_bagshaw]

Is that measured while installed?

 

[Rossynant]

My prop shaft is around 600m long.

Quite a longboat You have
Rule of thumb I recall was for 1m (1 metre) 0,1mm should be max. Also depends on shaft diameter.
But then it's not so big, provided flexible couplings and flexible engine mounts are used there all machinery will vibrate more, anyway. This will cause more wear, so Your decision. Machine shop should be able to get it straighten, as it's not bent so much to be discarded.

 

[wooslehunter]

Wishful thinking in the extreme. Edited to 600mm.

I measured out of the boat.

 

[Iliade]

My prop shaft is around 600mm long. I measured 0.4mm deflection in the middle. I do have a phantom vibration I'm trying to find although it's not bad & could easily be something else like exhaust.

Anyone think this mighht be enough to generate the vibration?

I don't think you'd really notice the vibration from a 0.4mm deflection over a dirty great two cylinder diesel's vibration. However, that must have got there somehow, so I'd have a very good look at the propeller.

This deflection may have compromised the cutlass bearing & stuffing box*. A bent prop and a loose cutlass bearing would be noticeable.

* My C has a free-floating deep sea seal which would be fine.


- Check the bolts retaining the stern tube just aft of the seal.

- Check that the gearbox-prop shaft coupling has not worn the end of the shaft. (Mine did this & trashed the gearbox)

- Check engine mounts.

- Check rudder bearings.

- Check rudder secure on shaft.


If possible start at the beginning (boat in water) checking for the vibration at each step:

- Run engine out of gear.

- put in ahead & astern.

- Disconnect shaft coupling from gearbox & run in ahead & astern.

 

[wotayottie]

I had a bend in my shaft some years ago. I took it to the toolrom at work and after confirming that I had a bend the toolmaker did no more than put the shaft in supports at either end and lean on the middle to straighten it. I used to work in a steel plant making stainless steel bars amongst other things in the days when Sheffield had a steel industry, and that was the basic principle of the roller straighteners that we used to make shafting. I'd have a go - unless you go at it mob handed so to speak, you are more likely to fail to make any change than you are to make it worse.

 

[david_bagshaw]

Unusual to be bent in the middle, usually at the ends, resonance could be caused by a bent shaft, but more likely by a bent blade, or the design of the engine & transmission. NHV is something that boat building has a long way to go, compared to the motor manufacturers.

 

[davidej]

What is NHV ?

Tried googlig without success!

 

[Tranona]

What is NHV ?

Tried googlig without success!

Noise Vibration Harshness.

Buzz words from Ford about 30 years ago when their cars suffered from all three and they set up a department in Dunton to try and reduce it in new designs.

Got purloined by ad men who added mystique!

Pretty basic stuff now, but all three are difficult to deal with in boats when the hull is one big sounding board and very difficult to isolate such things from the structure.

 

[earlybird]

0.4mm TIR isn't too bad but could be better I suppose. If it was measured in-situ, it might not be all due to shaft bend, reasonable workshop equipment is really required. Workshop straightening should be possible.
If the possibility of vibration worries you, try the prop for out of balance. The shaft run-out might pale into insignificance.

 

[Iliade]

NVH analysis is used all volume car manufacturers and even some smaller ones. It could readily be applied to boats. The trouble is we *all* like donking low speed diesels so we can tell that all is well whilst asleep within the forepeak or being towed in the dinghy :0)

Actually, much of the process is pretty simple: Play a loud noise of a sweeping frequency and track down anything that gets excited. Add blobs of weight or coverings to stop the sympathetic vibration. Listen to closure & control operations and make sure they don't sound too 'cheap.' Add mass/damper systems if they do.

The manufacturing volumes of even the biggest boatbuilders barely exceed the number of prototypes the motor industry uses. Hence the cost would be prohibitive on an AWB. Something of 150m LOA from some fancy Italian yard may well have benefitted from a one-off NVH analysis.

It even gets used in ships etc as a means of predicting machinery failure: perform a spectrum analysis of the noise produced when new. Monitor for changes in the sound.