vibration

[pandos]

I have noticed that when the boat is in gear there is vibration in ther shaft with a good bit of movement in the shaft. I understand that this is caused by at least 2 different problems.

1. Bent shaft.
2. out of position engine due to engine mounts being shot.

How do I tell which of the above is the cause, or what else could cause it.

Thanks in advance.

 

[Slipstream 34]

Other possibilities

Worn cutless bearing.

Unbalanced prop.

 

[webcraft]

A couple of suggestions for a preliminary rudimentary investigation:

1/ To check for loose engine mountings grab the engine by something solid (eg the flywheel) and try to rock it backwards and forwards. If one of the mountings is loose you will feel something much 'clunkier' than if it is just rocking on its rubber mountings.

(One of the rubbers may also be missing or worn away)

2/ If there is no untoward movement of the engine on its mountings after substantial heaving then check for shaft misalignment lightly touch a finger to the shaft when it is running - if you can feel eccentric movement then the shaft may be bent.

There are lots of other possible reasons for the vibration, and I am sure that this thread will shortly contain a bunch of useful observations and suggestions - just thought I would kick it off.

- Nick

 

[Topcat47]

In my case, last season, it proved to be long bolts on the flexible coupling. I shaved half a mm of each one and, hey presto, no vibration! The shaft wasn't moving though. I only found out after checking the engine mounts and the cutless bearing during the winter haul-out.

 

[pandos]

Bearing in mind what Nick said:

When I looked at it yesterday, when the boat was in reverse the gland ( volvo deepsea seal) spat a drop of water now and again, and also the action appeared to be a little excentric, almost as if the shaft was trying to ream the seal into a bigger size. ( it was only a very small movement but it was decernable.)

This suggests a bent shaft!.

What is the likely cause of this?

Is the only cure a new shaft?

How much excentricity can a shaftseal take before sinking?

Thanks in advance.

 

[macd]

Most likely cause is entanglement with rope or net.
Shafts can be straightened, but only if bend is slight and gradual (rather than something more like a kink). But it wouldn't be cheap and a new shaft is probably less than you'd expect, anyway. My last one cost around £90 from Norris Marine (sales@tnorrismarine.co.uk)
Bear in mind that any force sufficient to bend the shaft may well have also moved the engine on its mountings, which would need re-aligning. However, since the movement is probably lateral, not up and down, this isn't hugely demanding.

 

[pandos]

Could it bend without my noticing it, it was not like this when I bought it and I never noticed a dramatic event that would have bent it.

Could it snag a line and bend itself without an attentive ear hearing it ?.

I suspect the cost of the shaft will not be huge and I can take it out myself but would like to do it between tides so it will take a little organising.

Thanks.

 

[pandos]

just found this seems interesting http://www.yachtsurvey.com/Alignment2.htm

 

[Topcat47]

Having read the article, I'd have the engine mountings checked were I in your position. I made sure my engine mountings and cutless bearing were in good condition before looking elsewhere, and my shaft coupling was within 0.002" of perfect alignment.

I don't agree with the idea that accurate allignment is unnecessary. If the engine moves +/- 0.010" while running and your allignment is out by a similar amount then the maximum misallignment is 0.020".