Engine vibration mystery part 3...

[rays]

New readers start here. Older readers are probably as confused as I am!
Boat launched yesterday. Vibration still evident after prop/shaft/alignment all checked out. What about the engine mounts? How do you know if they are worn? This isn't as stupid as it sounds (I hope). They look OK, but are quite soft. But we don't know what new ones would feel/act like.
Given the cost of replacing these, is there any way of knowing if they're buggered before buying new?

 

[sailorman]

ask volspec to give you a report & costs
then DIY

 

[richardm47]

On my boat we had bad engine vibration too. Was thought to be a cracked engine mount/bracket. Survived last season with careful use.

But on tackling the work this winter, it transpired that the bracket itself was ok. BUT all three bolts that secured the mounting bracket (and the alternator) to the engine casting had sheared. The breaks were hidden inside the casting. Lucky to get through last season!

Suggest you look at securing bolts carefully, maybe withdraw them to inspect. Can you observe where on the engine the vibration starts at all?

 

[Cliveshep]

Suggestions:

1. Get a crow/pry bar and lever up the engine to see if all your mounts are still attached or just there with the engine sitting on top of them. Basic one that!

2. Are we talking diesel engine here? A slower revving diesel will still transmit vibration through the engine mounts to a grp hull, all your plates cups and cooker grill will happily resonate in sympathy, fact of life for older, slower revving engines. However, aside from that, if your prop/shaft/bearings are all good, (presumably you've checked those?), what about your coupling from gearbox output flange - is that worn? If any of the Layrub bushes are getting knackered on the flexible coupling they'll cause vibration on every rotation where the knackered bush is supposed to do it's work and fails.

Other than that, dunno, knowing neither the boat nor engine set-up.

 

[AlanRD]

Also launched yesterday and had a similar problem although shaft alignment has not been checked. At low revs in gear the engine vibrates which causes the prop shaft to vibrate as well.

The big question is, as engine mounts are rubber is it normal for there to be a certain amount of vibration to the engine and consequently the shaft as well? It is not something I have looked at before.

 

[Tranona]

Have a look in the latest PBO. There is an article on replacing engine mounts - including diagnosis and recommendation of a different mount from the expensive (in this case Yanmar) OE units.

 

[Cliveshep]

Quite normal, what you have is a crank and flywheel turning, with the crank weight moving in a circle and action and reaction etc does the rest. That is why the more cylinders the better as each opposing segment of the crank tries to cancel or overcome the reaction moment of the preceding part. Inevitably this is going to transmit to everything touching the engine and rubber won't completely dampen it. Your engine moves on it's mountings and even with a flexible coupling is still attached to your prop shaft. Ergo - vibration is transmitted but nowhere near as much as if there were no mountings at all. For near zero vibration, use an electric motor or a sail, for all other forms of propulsion you'll get some transmission of noise/vibration.

 

[Wino]

When you have tried everything try this. Disconnect your drve flange from the engine, make sure it can spin without catching the other part, then start your engine and run it through the whole rev range,if your vibrations have stopped you know - or have a pretty good chance of knowing - that it must be on the shaft side. If its still there then liokely to be the engine. However always bear in mind either part may run OK without load but not when running light.
Have you checked your engine alignment fore and aft?
I once ran into a problem where new mounts had been fitted but the wrobng ones, they were much harder than the engine manufacturer recomended.
In the mean time - Best of Luck!

 

[Caer Urfa]

To check the engine isolation mounts you need to only measure the distance from the inside to inside of the brackets where each mount is.
The gap should be equal at least per pair ( two front ones nearest the bow and the two back ones nearest aft).
Idealy they should all be 'equal' but a mm or so is ok from the front pair to the aft pair but if diagonaly they are differant you have to correct them by adjusting the screw that retain them until they are the same.
Mike

 

[TigaWave]

Whilst we're on the vibration theme, we had one the other day that turned out to be a couple of injectors working intermitently at around 200-2200rpm. It took the MTU electronics and lap top to identify the problem.

from reading the posts so far it wasn't clear if you had checked the bearing alignments? this is often overlooked.