Aggressive engine wobble out of nowhere

[tommycook]

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]

Cracked engine mount?

[MV Wanderlust]

Following. I'm interested in what you find was the root cause.

[a64pilot]

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]

As stated, engine mount or drive line damage....or something wrapped around the shaft.

[tommycook]

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]

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]

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]

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.]

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]

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]

Originally Posted by michaeld

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]

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]

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]

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.