Vibration puzzle

[byron]

OK! here's one to test you. At about 1700 revs a terrible vibration sets up. This steadily gets better as the revs rise but never entirely go away.
It is not the shaft(s) as far as I can see, the Props are refurbished, the alignment is OK, the engine mounts seem fine... any suggestions warmly welcomed. The boat is a Princess 435 with Volvo (ugh!) 306s.

©2001

 

[peterg]

Re: Vibration puzzle....could be...

one or more cylinders not firing properly due to :

a) slight fuel blockage (injector?)

b) head gasket leaking internally

Does it rev out and/or idle OK otherwise?

can't think of any other reasons if alignment etc is OK as you say but no doubt someone else may be able to come up with others

 

[alanhanson]

sounds serious to me just to be clear you say
the vibration sets in & gets better as things rise but its not your shaft!
you are sure you are aligned ok
perhaps you should ask the wife what she thinks or she should take a look
cheers alan

 

[longjohnsilver]

Have you checked your cuttlass bearings. I had a similar problem and changed these bearings (a bastard of a job!) but it stopped the vibration.
Good luck!

 

[byron]

Re: Vibration puzzle....could be...

The Cutlass Bearing is a possibility although I did give the shafts a good tug or two when she was out and they seemed firm enough, maybe I should change them anyway when she's out next year and maybe fit new props too just in case one is out of sync. I feel (in my bladder) that this is not an engine problem.

©2001

 

[ArthurWood]

Re: Vibration puzzle....could be...

Other possibilities:
-something caught around prop. Do both engines vibrate?
-slightly bent prop. Is the vibration present when revving in neutral?

Did the vibration begin suddenly or gradually?
Get down under and have a look!!!

 

[stuartw]

Some additional thoughts & checks.

1/ Has this vibration suddenlly come on? If yes then you can more or less discount bearings. Could be rope round prop, or broken blade.
2/ Is the vibration dependant on a particular engine? Keep one engine @ the worst point and vary the other.
3/ Feel both shaft glands while underway at the worst revs (hopefull somebody else on the bridge). Any prop or shaft imbalance will easily be felt.
4/ Inspect very carefully the flexible coupling on both shafts. A cracked web in the plastic or rubber could have serious effects.
Thats all I can think of for the moment.
good luck
Stuart

 

[ianphillips]

You have my sympathies!

We have an identical longstanding problem with our boat (single engine). In our case the vibration comes and goes, not only at different revs; but for a few days it will be barely noticeable then suddenly the next day it will return. We think the shaft is causing the boat to vibrate - you can see it! But we don't know what causes the shaft to vibrate and have done everything short of replacing shaft and prop; we have had a new R&D coupling, repacked the Stern Gland, replaced the shaft key/flange bolts and had shaft alignment checked several times, ie. between each job. Unfortunately since the R&D Coupling and the Shaft Key were very badly worn, there was indeed quite a lot of play in the shaft, but in fact now everything is nice and tight, if anything the underlying vibration is slightly worse!

The engine runs sweetly and revving out of gear causes no noticeable vibration, so everyone is now convinced the problem is sterngear related.

Get another engineer I hear you say - yeah right - we are on our third!!! and each time they are hellbent on checking the work of the previous. We must have the most aligned shaft on the south coast! Our current "expert" said it must be the cutlass bearing; so out of the water she came. I would have been delighted to find a badly worn bearing, but it was as good as new (only two seasons old anyway). No visible signs of any other sterngear problems, so put her straight back in. This engineer is now saying that maybe the amount of vibration we get is normal for this type of boat (Aquabell 33).

It is still possible that the prop is out of balance or the shaft is bent (unlikely as shaft is very short) or may be it's the rope cutter, nobody seems to know. I have this sneaking suspicion that even if we were to replace the shaft, prop and cutlass bearing the problem would still be there.

As this has bugged us ever since we've had the boat (several seasons) and we can quite easily avoid the offending rev ranges we have decided to carry on living with it!

My point here is that these problems are not always as easy to track down as "Marine Engineers" make out and when you add the price of the lifts to what appears to be a very experimental nature of resolving this type of problem - it can get very expensive!

So if anyone knows of a good sterngear specialist, based on the South Coast, I'd be interested in speaking to them.

Best of luck.

 

[peterg]

ooooh matron.....

nothing like giving the shaft a good tug is there?

Seriously, I hope your bladder is working correctly and it's just a bearing prob

 

[traceyandsteve]

We had a similar problem on our Princess 385 with 306's when it was carrying a heavy load. We had a lot of kids on board (day out for underpriviledged kids) plus we had full fuel and water tanks, and a new RIB hanging off the davits, so we put it down to a weight problem.

Are you carrying a lot of weight? (If that's not too personal a question........)

Tracey

 

[byron]

lack of weight more like, don't carry a dinghy, dump all water before setting off, single handed so no wife's clothes to carry.
Princess have given me one or two good ideas though.

©2001

 

[longjohnsilver]

Just out of interest what have Princess suggested?

 

[longjohnsilver]

You say that you have checked the cutlass bearing - are you sure that there's only one - I'd be surprised if there aren't at least 2, one in the P bracket and the other somewhere in the stern tube.

 

[byron]

The following:-

It could be that the props were not rebalanced correctly.

Rubber in the engine feet could be wearing out.

Injectors may need cleaning or replacing.

Timing on the engines may need adjusting.

Have you checked the cutlass bearings in the P brackets *these could be the problem if they have not been changed



©2001

 

[longjohnsilver]

Sorry should also have said that you really should have the prop balanced as well, that could be the cause of your troubles. It should only cost £50 or so and can be done when you next come out of the water. A place i have used for all my sterngear balancing and replacement is C & O Engineering in Newton Abbott, Devon tel 01626 367782. I have found them helpful and efficient as well as not too expensive.

 

[longjohnsilver]

Byron
I forgot to mention that the actual cause of most of my vibration troubles turned out to be a worn and slightly bent rudder stock. It was of course the last thing that I looked at. Injectors and timing sound a little unlikely unless you are having trouble starting or the engines are smoking. The advice from all my mechanic friends is to leave these areas well alone unless you have problems starting or smoking.
Hope you sort it out soon,
Regards
John

 

[byron]

The guy who services my engines will be coming Monday AM to 'do' the boat. Just to be safe side I will get all the Injectors done. When the boat is out next March I will get everything else (inc. rudder stocks) checked. As from here on in my boating is Thames restricted so the problem will not be rearing its ugly head.

©2001

 

[david_bagshaw]

We had a strange hum/ harmonic once props looked fine , no dings etc, but eventually had the blade pitches checked, one blade was 1/2 " out, end of problem

David ps Take a look at my site www.yachtman.co.uk

 

[ArthurWood]

Had a similar experience to david B's. Port engine has not revved out to 4400 rpm for years in spite of prop balancing. No vibration, however. During recent haulout, I noticed one blade neatly curved about 1/2" out and only saw this by looking from the side and turning the prop whilst referencing a fixed point eg my finger. Had it trued( the prop!) and bingo all is well.

 

[suzanne]

Re: Vibration puzzle....could be...

Hi Byron, nice to see you back! Gonna give you my theory but remember just a female so could be talking complete rubbish. Went away on a Humber 40 to Holland for 3 weeks (bloody nightmare don't ask,see pauline b posting) anyway we got a vibration doing about 25 knots one day, turned out to be a lack of oil in the gear box if that makes sense, revs jumped around and everything found a leak. Have you checked that?

Suzanne xXx