Vibration!

[DERF]

Help please anyone...

Just had cutlass bearings replaced and boat back in water.. props are balanced, shaft alignment good(can spin by hand easily) , rope cutters secure..

Anything over 20knts causes severe vibration...
Everything been checked and re checked.
Engineers lost for solution.

Anyone have any ideas?

Thanx

 

[TigaWave]

It must be an inbalance in the rotating part of the drive, so any part of the drive set up that rotates is suspect.
Bent prop unlikely
Bent shaft very unlikely
If its a rotating blade type of cutter bent or broken blade
Debris caught on prop or cutter
It doesn't take much to un balance the drive set up.
It does look like a mask and snorkel inspection with the bread knife...
The waters still pretty chilly for a swim, hope you sort it.

 

[boatmike]

Very difficult to say but considering your boat and trying to visualise the setup I would ask if the vibration was there before the "engineers" replaced the cutless bearing or it has appeared since. If it has only appeared since I would suspect the work that was done. Was the new bearing secured properly? Is the shaft bent? Is the prop damaged, bent? Stating the obvious perhaps but if the prop has been removed (pretty obvously the answer is yes) has it been replaced on the taper properly and the key and taper properly cleaned before re-assembly?
I had a guy with a Nelson last year that had had his prop off and a new bearing fitted and was getting vibration. when we hauled the boat we found the prop had been refitted over a distorted key and was jammed on at an angle......
Sounds like something obvious in any event. If you need more advice you will have to describe your system in a bit more detail as most people on here will probably identify with either a conventional drive on a sailboat or a saildrive.

 

[DERF]

Ok thanx Mike.

Twin Shafts running from Gearbox through hull(Bronze cutlass bearings replaced) through P brackets (bearings replaced) to prop. Strippers fitted between shaft end(after P beracket) and prop. Props are balanced and in excellent order.( At leat they were when they were removed). same for the shafts. no loose play on gearbox end. No problem with water ingress so alignment is OK.

The problem has only manifested itself after the work was carried out; there is no doubt that this is the cause of the problem.

Perhaps the props(or prop) are as you say, poorly located on the shafts.

What is surprising is that she runs without noticable vibration up to planning speed!

I guess the only solution s to remove and refit.

Thanx for your advice.

 

[boatmike]

Obviously there is no easy answer to this one. Sometimes if a shaft is bent, it runs OK in slack bearings but vibrates when held tight in new ones.... But I am clutching at straws now! There appears little you can do without hauling out again. I would try and mount a DTI on the shafts somehow and clock run out. Then clock the back face of the blades of the prop before taking anything apart.
As a matter of interest can you cut one engine at a time or throttle back to see if its one or the other or both that vibrates? I know you will loose speed but if you throttle back each one quickly while on the plane it might show a difference. Of course I am assuming twin engines and gearboxes rather than a splitter gearbox....

 

[silverseal]

Silly question .. if you have two shafts and two propellors, do they turn in same direction? If yes, has the wrong propellor been fitted on the wrong shaft? ( were they marked when taken off the shaft?)

 

[Anonymous]

[ QUOTE ]
The problem has only manifested itself after the work was carried out; there is no doubt that this is the cause of the problem.

[/ QUOTE ]Something might have been damaged during haul-out and launch. Do you have trim tabs or stabilisers? From what you have said, it doesn't follow that the problem is necessarily in the rotating machinery.

 

[DERF]

Ok thanx again all for your input.

Engineer said that he marked all parts and replaced in correct order. Trim tabs are ok and can't see how they would interfere or indeed cause vibration.

My worry is that the propshaft may be bent and new bearings have highligted a problem that already existed.... lets hope not! lead time for props and shaft from F'Line 9weeks.

I will try the shutting down each engine in turn... stupidly didn't think to do this on weekend!! May be clearer if one or both a problem.

 

[Thistle]

You don't say anything about the frequency of the vibration or if it changes when you make small alterations to the revs on one or other engine. Is it possible that all the individual components are fine but that there's a resonance being set up between the engines?

 

[graham]

I presume you have checked for something wrapped around one of the props? This is the most usual cause of vibration at speed and easiest to fix fortunately. Going hard astern immediately you notice it often works but probably a diver or lift out now.

Are the rope cutters new or did you have them before? Not everyone agrees they are a good idea on high speed launches.

 

[DERF]

Re: Vibration! Cause

[ QUOTE ]
Help please anyone...

Just had cutlass bearings replaced and boat back in water.. props are balanced, shaft alignment good(can spin by hand easily) , rope cutters secure..

Anything over 20knts causes severe vibration...
Everything been checked and re checked.
Engineers lost for solution.

Anyone have any ideas?

Thanx



[/ QUOTE ]

Well the boat came out the water and the problem...... Imperial bearings supplied and fitted to starboard.... metric bearings(supplied in Imperial box) and fitted to Port. Metric bearings too small and hence vibration.

Thanx for all the advice.