Vibration Problem

[wndsr]

I have had a nagging noticable vibration on the Port side for a while now. So far the shafts, props have been checked and straightened. The trans coupling faces have been resurfaced and the engine aligned and the rear mounts replaced. The vibration is still there. This is a reputable yard that I do trust. There was about 10 thousands of run-out in the long shaft. after correcting vibration is still there. Others have told me that the coupling between the long and short shafts can be troublesome? It has also been suggested to replace the two piece shaft with one solid one. Is this a good idea? Why was a two piece shaft used in the first place? Has anyone experienced any of these conditions? Some other points. The vibration is very pronounced in the aft stateroom and the you can see the engine actuallt rock in the mounts-I know this seems strange-you would think if it is that noticable-the fix should be obvious.

 

[StratPlan61]

Are you absolutely certain every strut is completely secured to the hull? I think this is best tested with the shafts completely out of the boat. A simple 'hand check' may not reveal a strut with a broken bolt.

 

[Nonchalant1]

Just an idea - Maybe your description in the post answers it:

"You can see the engine actually rock in the mounts."

Maybe you have an engine mount problem and the engine rocking is transmitted through the coupling, which makes the shaft vibrate. The length of the shaft and spinning prop multiply the effect. If your mounts are bad, no matter how much you align it, you'll have problems. Check for sagging mounts or ones that have been drilled and through-bolted to hold them up.

Also, did they do the alignment in the water? A boat hull changes shape in-water versus blocked or straps on the hard.

Doug Shuman

 

[Genesis]

Alignment has to be done in the water or its worth nothing. You can do the rough (and must to be able to bolt the coupler!) on the hard but you can't do the final dry.

Second, if you can actually SEE THE ENGINE MOVE the mounts or beds are no good or an alignment or "bent something" problem is EXTREMELY severe. This is a serious situation; something is either worn out (e.g. mounts) or BROKEN (welds on the engine beds, etc) The "something" can be a shaft, wheel, or strut.

An engine that decides it doesn't want to be securely mounted to the vessel at the wrong time is a very bad thing.

 

[Brian Degulis]

The coupling conecting the short and long shafts doeas cause some run out but not the problem your describing. Assuming your engine mounts are in tact and assumuing you have no vibration in neutral here is what I would do. Get yourself a dial indicator set it up and slowly rotate the shaft by hand and check for run out I bet you will find some. Then set up your dial indicator to measure movement of the engine itself. Doen't put the base on an engine stringer put it on something else with the plunger at the transmission mount again slowly rotate the shaft and check for movement I bet you find some there should be no movement at all.

Then seperate the couplings and let the prop shaft lower and rest on the shaft log then make up something to measure from the bottom of the shaft to something on the boats structure then raise the shaft and prop it up at it's highest point and measure again. The purpose of this is to find the center of the shaft log and prop up the shaft at that point. Do the same thing side to side. If everthing was aligned corectly your shaft should be able to slide forward on the props and the couplings should fit together smoothly.

I bet it doesn't most yards align shafts without first centering the shaft in the shaft log if it's out severly enough it will cause the problem your describing. When doing an alignment It's possable really pretty easy to get good readings on your feeler guages with the shaft completly miss aligned to the boat. Many mechanics simply seperate the couplings slightly then adjust the engine for even spacing they don't first check the alignment of the shaft to the boat.

What I have described is a rough way to do this to do it completly the boat would have to be hauled and the shafts removed. Then both struts and the shaft log would be checked with a string or a laser. The method above can be done pretty easily and will be acurate enough to correct your problem if that's whats causing it.

Good Luck
Brian

 

[Maynard Rupp]

I would not look at struts, shafts, or props. If the engine is moving you have a broken mount. I really do not understand how they could have alligned the coupling faces if the engine is in a different place every cycle. Do take a hard look at the mounts and the welded pads and braces that support the engine mounts, and hurry please.

 

[motoryacht lover]

I used to own an aft cabin MY and had a vibration problem. When I bought the boat it had a vibration and after doing all that you did we found that it was the bore of one prop. Then another time I had a vibration that progressively got worse and that ended up being a cracked shaft at the taper. It was very hard to see and you had to remove the prop to see it. I now own another aft cabin MY and all aft cabin boats that I have been on or owned seem to make alot of noise in the aft cabin. I am not suggesting that I think you don't have a problem and tend to agree with the others that if you can see the engine moving then that is atleast part of the problem. If the engine movement is not the problem and you have to go back to the beginning see if you can get someone else to drive take the boat on a sea trial and put your hands on top of the struts on the inside of the boat and see if you can atleat isolate the problem side. Look for occilation of the spinning shafts in the engine room while underway. Dive under the boat and try to see if the prop has any movement fore and aft and is it tight to the shaft with no play, in other words I have seen props move a little without the shaft rotating. Just some simple ideas to help you solve the problem. It has been my experience when you have boating/mechanical mysterys you need to be hands on to get the problem solved in a timely fashion. Good luck, I know how frustrating this can be.

 

[Trojan]

Maynard is spot on. Engines are not to move. Start there and your vibration should go away.

BILL

 

[wndsr]

All of the suggestions are greatly appreciated and the frustrating thing is-we have checked all you suggested and the mounts have been replaced because they were indeed sagged. Tha is why we were so sure we had it! I will keep everyone posted as to our progress!!

 

[67hat34c]

were the props properly lapped and the the key honed? Do not trust that the yard did this right. We had vibration problem for a couple of years until you guys were talking about riding the key and lapping. Well when i changed props , i found one riding the key. Lapped and honed the keys and now no more vibration.

 

[Will]

Is there any vibration when the engine is reve'd up while out of gear?

With the boat in the water, how about pulling the prop off and test running the engine in gear at high RPMs?

As a last resort I'd probably pull the shafts and take them with the prop, coupling etc. to a prop shop who can do a Prop Scan and check everything as a unit. They can lap tapers and check keys and keyways too.
Will

 

[mike]

See info on boatdiesel.com ; Installing Shafts, Logs, Struts and Rudders along w/ your favorite Engine and Transmission: by Tony Athens

 

[Genesis]

Well here's the thing.

With a properly fitted shaft and wheel you could leave the key OUT and it wouldn't matter a bit.

If the key is BINDING its trouble, and a prop that doesn't properly seat on the taper, or where the taper or bore is "off" (and it DOES happen) will cause hideous vibration problems that are impossible to run down without pulling both and taking them to a shop.

However, if the engine is moving, something's stuffed and not in a good way. Find that and quick. Engines are HEAVY - if its visibly moving under load you've got major problems - possibly structural - with the mount system.

If someone put some of those newfangled "soft mounts" in there they used the wrong ones. The only mounts that should be used for engines like these in boats like these are the big heavy-duty ACE mounts. They're beefy, they're expensive, and they hold the engine exactly where you put it.

Shafts of this diameter are quite rigid and they will BIND if far enough out. If you've got "mush mounts" you've got real trouble as they can move enough to bind the shaft which then causes the engine to "jump" and also creates some really nasty vibration problems.

You've got something ugly going on here and it sounds like the yard you've been working with isn't doing you any favors. These sorts of things are not that hard to run down but if the mounts that were used to replace the old ones are the wrong ones you're never going to fix it until you get those out of there and put something in there that holds the engine solidly in place.

Alignment is pointless unless what you align stays where you put it!

 

[krush]

Also, a wiggling wheel on a shaft is asking for a nice fatigue failure. Stress concentrations will ruin any body's day.

 

[wndsr]

All of the mount suggestions are also appreciated. As I said before, the mounts were indeed sagged. Thats why they were replaced with the original correct style.

 

[captddis]

Were the props rechecked using Propscan? How about trying a different prop shop that uses propscan. Or try another set. It sounds like props to me. A bad prop will transmit vibration up the shaft and cause the engine to bounce. See it all the time. Usually prop vibration will get worse with speed and a alignment problem will be more evident at slower speeds. YMMV Dave