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-   -   Aggressive engine wobble out of nowhere (https://www.cruisersforum.com/forums/f54/aggressive-engine-wobble-out-of-nowhere-227554.html)

tommycook 09-12-2019 13:25

Aggressive engine wobble out of nowhere
 
My wife and I are on our way down the ICW in Jupiter Florida. As we slowed to wait for a bridge opening we heard a clunk and the engine started shaking aggressively. It runs perfectly in neutral at all RPMs, but as soon as I put it into gear (forward and reverse) the engine starts shaking very hard.

We are now being towed (all systems off) and I can see in the engine bay that the engine is still shaking as the drive shaft turns while we're being towed. I tried to get a look at the prop off the back of the boat, and it did not appear to be fouled.

Any ideas for troubleshooting would be greatly appreciated!

Cheers!
Tom Cook

Paul L 09-12-2019 13:27

Re: Aggressive engine wobble out of nowhere
 
Cracked engine mount?

MV Wanderlust 09-12-2019 13:37

Re: Aggressive engine wobble out of nowhere
 
Following. I'm interested in what you find was the root cause.

a64pilot 09-12-2019 13:51

Aggressive engine wobble out of nowhere
 
I’m thinking drive line damage of some sort, and that maybe broke an engine mount.

What kind of prop do you have?
This may sound silly but if your the typical sailboat with a small engine , trying to rock the engine by hand should let you see a broken mount.

DougR 09-12-2019 14:01

Re: Aggressive engine wobble out of nowhere
 
As stated, engine mount or drive line damage....or something wrapped around the shaft.

tommycook 09-12-2019 14:50

Re: Aggressive engine wobble out of nowhere
 
Well, I definitely found the cause, but the damage done remains to be seen. As I was poking around I found 2 whole bolts, 1 washer, and a bolt/nut that was sheared in half. These were all down in the bottom of the engine bilge. Looking closer these were all missing from the coupling from the drive shaft to the transmission. There was one (1 of 4) loose bolt left in the coupler.

I reinstalled one and a lot of the wobble in neutral while being towed went away. I haven't been able to crank her up since we're still being towed.

I also found that the top nuts on all 4 engine mounts were loose. I tightened them, but the shaft does not seem to be aligned properly now. I checked the engine mounts and they do not appear to be cracked, but they are extremely hard to see.

a64pilot 09-12-2019 19:39

Aggressive engine wobble out of nowhere
 
Be sure to replace the bolts with at least grade 5 bolts, ideally there should not be threads where it goes though the coupling.
Do not use hardware store bolts.
You tell bolt grading by head markings.
Do not use bolts with no head markings
https://www.boltdepot.com/fastener-i...ade-chart.aspx

Align or have the engine re-aligned before you try to go anywhere.

jamhass 10-12-2019 09:11

Re: Aggressive engine wobble out of nowhere
 
While you are at it, don't forget to check the shaft pinch screws, key and coupling-to-shaft fit. THe vibration may have loosened stuff there too.

michaeld 10-12-2019 09:31

Re: Aggressive engine wobble out of nowhere
 
I had a similar thing happen, after "the yard" realigned my engine, only four of the eight bolts were installed in the coupler, and the "experts" used SS bolts with locknuts on top of that. Eventually all either fell out or sheared off. The resulting damage was a leaking V-drive. After buying a new V-drive (it was already way too old to rebuild) and transmission rebuild (it needed it anyway), I replaced all with grade 8 bolts. Engine alignment was an exercise in frustration, but after a few hours with a bottle jack and various levers, I was successful getting it perfectly aligned. Good luck.

Larry T. 10-12-2019 11:04

Re: Aggressive engine wobble out of nowhere
 
Can't believe how common this seems to be. Few years ago my buddy and I were motoring along with me steering while sitting on the box. Felt a small vibration and asked him if he felt anything. Nope, so like a bozo I motored on. Pulled into Port Townsend for a pit stop. He was at the helm backing out of the slip and had just we got some way on when all at once the engine spun up and all drive controls stopped!! Tell me God doesn't watch out for Bozos!! There was an empty slip straight behind us and we backed in just like we knew what we were doing. Found that in reverse the shaft had pulled back away from the coupeler. Two bolts sheared and one in the bilge. The fourth bolt was hanging in the shaft portion. And as luck would have it there was a West Marine at the top of the ramp. Lesson learned, never ignore a new vibration.

sanibel sailor 10-12-2019 11:17

Re: Aggressive engine wobble out of nowhere
 
Grade 5 (3 marks) is INADEQUATE. Those are just one step up from household bolts. Use grade 8 (6 marks) at least. Yield strength is about 50% greater.

Cadence 10-12-2019 11:37

Re: Aggressive engine wobble out of nowhere
 
Quote:

Originally Posted by michaeld (Post 3033180)
I had a similar thing happen, after "the yard" realigned my engine, only four of the eight bolts were installed in the coupler, and the "experts" used SS bolts with locknuts on top of that. Eventually all either fell out or sheared off. The resulting damage was a leaking V-drive. After buying a new V-drive (it was already way too old to rebuild) and transmission rebuild (it needed it anyway), I replaced all with grade 8 bolts. Engine alignment was an exercise in frustration, but after a few hours with a bottle jack and various levers, I was successful getting it perfectly aligned. Good luck.

They better requalify as experts using SS.

Cheechako 10-12-2019 11:40

Re: Aggressive engine wobble out of nowhere
 
I thought you had a plastic bag, or line caught on your prop.
While derusting and cleaning up my coupling once I replaced my coupling bolts and nuts with SS bolts and SS Nylok nuts.
DO NOT do that! They came loose within several hours of motoring.

tkeithlu 10-12-2019 12:26

Re: Aggressive engine wobble out of nowhere
 
You're going to pull the boat and check for a mangled propeller and/or a bent shaft, right?

Good. If you are lucky, those two will be OK and you are back at the engine mounts and the shear bolts. You are capable of doing those jobs yourself, and from the tenor of the posts above, your doing it would be a better idea than having a yard do it.

So, the engine is in new or inspected mounts. Aligning it to the shaft is not rocket science. You raise front and back on the engine simultaneously to shift upward, you raise one or the other end to correct vertical tip, pry the engine on the mounts to correct lateral misalignment. Tipping the engine to the side actually shifts the shaft left and right, since the mounts are not concentric with the shaft. If it seems confusing, start with some big changes in your mounts just to see what loosening and tightening various mounts does to the alignment. You can do it.

If your boat is more flexible than it should be, you may have to do a final alignment in the water.

Bolts: Grade 8 gives you about 25% more shear strength than Grade 5. Stainless steel gives you less shear strength than Grade 5, but how much is hard to say because there are lots of things called stainless steel. In general, it is gummy steel compared to real bolts. Just because it usually doesn't rust does not mean it's strong.

CassidyNZ 10-12-2019 12:36

Re: Aggressive engine wobble out of nowhere
 
Threads on the bolts also important. Often standard 1.5 or 1.75 thread pitches are used. These threads are too coarse to provide good clamping pressure and are prone coming loose in vibration-prone conditions.

A finer thread (1.0, 1.25, UNF) are better for this application. It’s no coincidence that fine thread bolts are used in places like big-end bearing caps and automotive differential gears, for example. They allow a more regulated, relatively slower but much better clamping rate. Contrary to common opinion, they are not more prone to being stripped.

For the record, my shaft coupler is bolted using 10mm 1.25 thread, 8.8 grade stainless bolts with metal (not Nyloc) locking nuts. Never had a problem. Important consideration: never use metal locking nuts (or Nylocs, for that matter) twice if locking is paramount.

Also, note the difference between bolts and set screws. Set screws are threaded all the way, bolts have an unthreaded portion. Always use bolts for this application, not set screws.

Chuck Hawley 10-12-2019 13:28

Re: Aggressive engine wobble out of nowhere
 
Quote:

Originally Posted by a64pilot (Post 3032856)
Be sure to replace the bolts with at least grade 5 bolts, ideally there should not be threads where it goes though the coupling.
Do not use hardware store bolts.
You tell bolt grading by head markings.
Do not use bolts with no head markings

Yes! Higher strength bolts do not cost THAT much more considering the critical nature of this application. I foolishly replaced drive shaft bolts with SS because of the salty wet environment in which they were located, and they broke very quickly. I would soak your new bolts in Boeshield or similar protective spray (I am sure that CRC makes something similar) and then remember to check them annually. This would also be a good fastener to torque correctly.

One other thought: could it be the vibration damper in your engine? Or could this be implicated, if not the sole cause? When I had vibration, I wondered if I should have replaced it. Not too expensive ($280 for my Yanmar) and not THAT difficult to replace.

Chuck

hamburking 10-12-2019 13:54

Re: Aggressive engine wobble out of nowhere
 
5 Attachment(s)
Quote:

Originally Posted by tommycook (Post 3032698)
....I checked the engine mounts and they do not appear to be cracked, but they are extremely hard to see.

For hard to see areas, like behind the engine, down in the bilge looking up, etc...stick your camera/phone in there and take a picture.

Sometimes, I would just sit quietly staring at the engine. Then touch every bolt, screw, and hose. My family thought I was crazy. But its surprising how you can find loose hoses, nuts, and tiny leaks when its all quiet and you have time to look. Although not glamorous, I also take lots of engine pics to review at home. Its amazing what you can see in a picture on a big computer screen.

Chuck Hawley 10-12-2019 14:04

Re: Aggressive engine wobble out of nowhere
 
I agree with the general theme that you cannot tell if engine mounts are broken when there is the weight of the engine on them. Three of my four Yanmar engine mounts were in terrible shape, relying on gravity to keep the engine in place. Extremely hard to determine, even with the engine shaking. If in doubt, undo the flange, measure the engine mounting bolts so you can attempt to replace the engine in the same place, and jack up the engine so you can see if the mounts are broken. Very satisfying when you determine that they were, in fact, torn or broken.

Cadence 10-12-2019 15:17

Re: Aggressive engine wobble out of nowhere
 
Quote:

Originally Posted by tommycook (Post 3032637)
My wife and I are on our way down the ICW in Jupiter Florida. As we slowed to wait for a bridge opening we heard a clunk and the engine started shaking aggressively. It runs perfectly in neutral at all RPMs, but as soon as I put it into gear (forward and reverse) the engine starts shaking very hard.

We are now being towed (all systems off) and I can see in the engine bay that the engine is still shaking as the drive shaft turns while we're being towed. I tried to get a look at the prop off the back of the boat, and it did not appear to be fouled.

Any ideas for troubleshooting would be greatly appreciated!

Cheers!
Tom Cook

I don't think you have more than picking something up on your prop.. Take a swim

charliehows 10-12-2019 15:36

Re: Aggressive engine wobble out of nowhere
 
Theres a lot to be said for learning the basics of general engine maintenance - aside from reducing costs, things like regular oilchanges, anodes, impellers, hose checks, tappets, checking for internal accumulation of residues, etc, etc, almost every time I start off doing something routine I stumble on something else that needs attention - case in point - Last time I changed the transmission oil I noticed that one of the driveshaft nuts was loose - on closer inspection 2 out of 4 were loose, probably only a matter of time before the other 2 worked their way loose. Locktited and tightened, will keep an eye on them but no problem caused.

CassidyNZ 10-12-2019 16:48

Re: Aggressive engine wobble out of nowhere
 
1 Attachment(s)
Quote:

Originally Posted by tkeithlu (Post 3033328)
Bolts: Grade 8 gives you about 25% more shear strength than Grade 5. Stainless steel gives you less shear strength than Grade 5, but how much is hard to say because there are lots of things called stainless steel. In general, it is gummy steel compared to real bolts. Just because it usually doesn't rust does not mean it's strong.

No reason to assume that stainless steel bolts are inferior. As you said, there are lots of things called “stainless”. Amongst them 8.8 graded or better SS bolts. They are “real” bolts and are way more preferential than zinc plated steel or worse, just ordinary steel.

How do ratings compare? The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Grade 8.8 bolts have an ultimate strength of 800 N/mm2 and a ratio of yield/proof stress to ultimate strength of 80% (640N/mm2). So a grade 5.6 bolt will have 500N/mm2 and a yield/proof ratio of 60% (300N/mm2), nearly half.

But just because it’s stainless does not mean it’s weak.

RLaird 10-12-2019 18:26

Re: Aggressive engine wobble out of nowhere
 
I believe it’s the rubber of mount separating from the flanges. Very hard to see without applying some lift. May need an assistant!
Shouldn’t be ant washers on the coupling bolts in my ‘limited ‘ experience?..

dfelsent 10-12-2019 19:07

Re: Aggressive engine wobble out of nowhere
 
I had vibration. One loose nut on the stainless fully threaded stud on the flex coupler. It was whacking the transmission flange. Ouch but not terminal. Very fortunate.
Bought proper grade 8 steel studs. Used loctite. Torqued to spec. Ran an hour. Re-torqued. Ran ten hours. Checked torque again.
That plus realignment made a much smoother engine.
Check those studs and bolts.


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