Unbalanced shaft

[KINGFISHER 9]

Motoring new (to me ) boat from launch to mooring at the weekend - propshaft starts to make a hell of a banging noise at 1200 revs, as though its making a bid for freedom. More throttle eliminates the racket and vibration and the boat will happily potter on at 6 knots or so. What should I be checking? Is it a misaligned propshaft or a coupling somewhere? Couldn't get to look at it performing as I was on my own. Previous owner warned me there was a slight vibration - not the 3rd world war going on under the cockpit! Stitched up I suppose! I'm worried about possible damage to gland as well - what's the answer? Get an engineer? Haul her out again? New shaft? What to look for? Any help much appreciated!

 

[Csail]

Damaged coupling, bent shaft, damaged prop, sterngland worn or even engine mounts loose. could be as simple as rope around the prop.

 

[johnalison]

Could be anything in the line, but I think you're going to need to check the P-bracket first if you've got one.

 

[Aja]

I would say that that would be the last to check.

First of all get someone aboard to help who can operate engine confidently from Cockpit.

First take engine up to revs where the vibrations started. This time in NEUTRAL. Do you still get the knocking? If so nothing to do with gearbox, stern gland prop shaft or P bracket. It might be worthwhile - while still in neutral to increase revs beyond the 1200 revs to see if still all ok.

This leaves possibilities like engine mounts. If you are getting knocking noises check movement of engine at each of the mounts. You will see movement.

If no knocking of engine while in neutral, time to have a go in gear. Cast off and take boat for trip. Again, with friend at controls go through the rev bands checking the areas most likely to be affected.

Again it could still be engine mounts whilst under load, engine alignment, prop shaft, bashed prop or P bracket.

If you get the problem again - next step is to check engine alignment. One checked re-try.

Once you have got to this stage if problem still exists, I would say its time for a haul out - but try these things first.

Donald

 

[Csail]

You don't even need a friend to do the checks....i taught my 2 1/2 year old son to operate the throttle! he is very good now when i shout more or less noise!

 

[KINGFISHER 9]

P-bracket renewed a couple of years ago - prop ok - no problems when out of gear, will rev away quite happily. Engine supposedly on flexi/rubber (?) mounts. I think there is some sort of coupling aft of the box ... not sure about this. Had the engine serviced while ashore but the guy failed to turn up for a check when she was launched! Oddly enough the problem didn't manifest itself when I first left the marina .. I must have slowly increased the revs ... motored on for 20 minutes, throttled back for a moment, increased the revs again and then it started - it was then playing up for the rest of the day - frightened the life out of me till I remembered the previous owner's tale of a 'slight vibration'! Thanks for your advice ... I'll have to find someone to come out with me for a jolly round Portsmouth Harbour! Any engineers out there who charge less than boatyards?

 

[Aja]

It would be very difficult to identify if flexible mounts were OK on land if the engine is not switched on. Did your engineer start the engine up?

My bet is

Alignment (take off the flexible coupling re-align and reconnect flexible coupling.
Engine mounts
Bent propshaft

That is assuming you havn't picked something up round the prop, or your shaft anode (is there one?) hasn't been tightened..... not unknown..........???

Going back to original post - does the sound it makes sound as if the prop shaft is hitting the stern tube...? Very difficult to identify, but narrows it down to alignment or bent shaft.

Only other consideration. When the P bracket was renewed for whatever reason - it didn't throw shaft alignement out??? If P bracket got bent was the shaft replaced as a matter of course???

Donald

 

[LadyInBed]

Sums it up nicely.

 

[KINGFISHER 9]

Alas I don't know what a shaft hitting a stern tube sounds like! This is just a very loud banging, with vibrations, like someone bashing the cockpit floor with a big rubber hammer! Discounted anything round the prop because everything returns to normal with more revs. I'll have to get someone to look at it ... neither my engineering skills or my ability to get myself into the engine space via the hatch in the aft cabin are up to it. It sounds important and expensive though! Thanks again.

 

[billy_bros]

Check your engine mounts. I had similar problem some years ago and one had failed and the protective cover was sitting on the engine bed.

 

[MoodySabre]

No need for a jolly round the harbour. Tie the boat up well and run in gear whilst attached to the pontoon. Good luck.

 

[KINGFISHER 9]

Many thanks for your suggestions - all worthwhile - much appreciated.
With the right measuring device, the name of which escapes me for now but my oily mate will know, how easy would it be to tell if the prop was misaligned? Also would it be possible to rebalance a shaft with weights, like a car wheel? Only wondering!
I think I'll try the engine mount suggestion first.

 

[Aja]

Kingsley

The only way to tell if the propshaft is damaged - that I know of - is to roll the shaft on a guaranteed flat surface. You would never be able to tell if is out of true in situ.

I've never heard of rebalancing a shaft. The price is not that prohibitive!

Donald

 

[Poignard]

[ QUOTE ]
With the right measuring device, the name of which escapes me

[/ QUOTE ]

Dial Gauge

 

[KINGFISHER 9]

Dial gauge ... that'll be it. The weight idea was just thinking aloud really ... probably quite unrealistic! I'm not bothered about the price of a new shaft too much if the problem can be fixed ... its the £300 round trip with the crane etc which is annoying having just launched last week (£200 one way!) I'll go for the mounts first I think - should be able to fix that afloat!

 

[bendyone]

I would have a look at the mounts first, but you will have to lift the engine right off them to see. They can look OK with the weight of the engine on them but can be totally broken.