Continuing Saga of Massive Vibration with AE38

[Chuck Hawley]

Hi Everyone,

I have been having a serious vibration problem in forward gear with my Alerion Express 38 Yawl, Yanmar 3GM30 engine with 2.62:1 reduction gear and 1" shaft, for about a year. Here are the facts:

1. Only a problem in forward at low RPMs. Cannot be "accelerated out of it" by increasing the engine speed.
2. Propeller was a properly sized Gori two-blade folder. Around 16" dia by 11" pitch. Don't quote me; it was around there.
3. Engine appears to be aligned correctly.
4. Cutless bearing is not worn.
5. In desperation, boat was hauled and prop appeared to be "over hinging" so that the blades hinge forward of 90 degrees. Gori and Bay Propeller said "can't be fixed", so I ordered a new three-blade Gori prop.
6. After installation, problem is better but not resolved. Certain RPMs are just brutally shaky. Biggest issue is right after engaging forward gear.
7. Disconnected the coupling with boat in the water and ran the engine through full RPM range. Silky smooth. I never knew a diesel could be so smooth. Vibration only happens when shaft is attached and in forward.
8. Since our engine mounts are rusty and the rubber has probably deteriorated, I called Phil at PYI and asked about R&D replacements. He said "have you thought that it might be the damper?"

Gadzooks, I am not even sure that I knew the little Yanmar HAD a damper, but it makes a ton of sense. What if a spring or other shock-absorbing component is broken or damaged?

I am thinking of getting the damper (in for a penny, in for a pound!) and four engine mounts, which will pretty-up the engine compartment a ton.

What do you think of Phil's diagnosis?

Chuck Hawley
Alerion Express 38 Yawl "Surprise"

[Jim Cate]

G'Day Chuck,

Not an expert,, but I thought the damper was mainly expected to absorb some of the shock loads when engaging the gearbox. Broken springs in my limited experience made gear engagement harsh, but didn't contribute to vibration. Not to say this explanation isn't correct... just a thought.

I can't remember from your initial posts on this subject, but is the prop tip clearance with the hull adequate? That can be a source of noise and vibration if too small.

But meanwhile, if the mounts are deteriorated, replacement seems warranted, and in the engines that I've owned, the damper is not terribly expensive nor difficult to replace. I think I'd be tempted to follow your plan of replacement.

Jim

[redsky49]

Was the propeller shaft measured for proper alignment/runout?

[leboyd]

If the mounts/damper are in bad shape, you need to replace them anyway.

Now, will this fix the problem? Hopefully, I'm wrong, but I don't think the mounts will make much if any difference. If you run the motor up to high RPMs with it in neutral and it doesn't shake apart, it seems the mounts aren't the problem (in my experience anyway, but as I said, hopefully I'm wrong).

It sounds like you said it does it pretty much immediately when going into forward gear and not reverse - even at very low RPMs. If that's true, it seems it must be in the "drive train" somewhere - out of balance drive shaft, gear box, prop, bearings. You're working on the prop, but it seems to me that if it only happens in forward and not reverse, it must be in either the gear box or a bad bearing some where.

That's not a lot of "help", I know. But maybe a little.

[Captsteve53]

Have you had the propeller shaft checked for being bent (or not)? as it sounds like either alignment or bent shaft,

Saying that if your mounts are in bad shape then nothing could be done in the way of alignment until replacement occurs, if the mounts are bad it could cause all sorts of vibrations under load,

Cheers Steve

[Captsteve53]

Forgot to comment on the vibration damper/flex coupling , i have seen them cause vibration when under load but reasonably stable with no load-depends on the style of the damper,

Cheers Steve

[svHyLyte]

We had a similar problem on our normally smooth running drive gear following a haul out. It began as a modest vibration that worsened over time. Long-story short, the issue proved to be a zinc that, for whatever reason, a yard worker had fastened to the shaft near the mid-point. The zinc wasn't symmetrical to begin with and became less so as it wasted, which caused the shaft to "whip". Our diver discovered and then corrected the problem by removing the wasted zinc and affixing a new replacement about three inches from the shaft strut. Problem solved.

FWIW...

[gjordan]

You state "only a problem at low RPM" and then say "cant be accelerated out of". If it is truly only a low RPM/idle speed vibration, I have had an episode of that before. My previous boat would sound like a garbage grinder when first put in gear. It was a steel boat so you could hear it all too well. Raise the RPM a little and it would smooth right out. I did the alignment check and cutlass bearing check and all seemed OK. Dock experts said it must be the damper plate so I ordered one and installed it. No change. I was considering having the transmission rebuilt when an old timer stopped by and said "just raise the idle speed a little". It smoothed right out and never gave me trouble again. The previous owner had it set too low. Buy or borrow a hand held tachometer and see if your engine idles at the speed that your manual states. It might be just that simple. Good Luck. _____Grant.

[sailorsdittybag]

Hi Chuck
I had a similar problem with my old boat. After numeros haulouts and checks it turned out to be the prop. I somehow at the yard the someone mess with the pitch of the blades. I pestered Gori until I got the right pitch then it was fine. From there on I never let anyone touch the prop unless they were told too.
Hope it helps

EJ Torres
Sailorsdittybag

[Barney21]

Now let me say you will not like this suggestion ,but is a suggestion made in good faith.
Can you move the crankshaft at either the front or the drive end?
If you can mark the end of the crankshaft with an alignment mark you cannot access easily.
Go to the other end of to the crankshaft and see if you can rotate the crank , you only need move it a fraction, then check to see if the crank has moved the same distance . If it hasnt ,you have broken the crankshaft at the boss which holds the drive plate/ fly wheel. It is not unusual, happens when prop hits something really hard and there is nowhere the shock to go.
Hope it isnt but sounds like it could be your problem.

[boden36]

Is the engine firing on all cylinders when under load?

Regards,
Richard.

[Arthur Garfield]

Lever and fulcrum to check mounts are broken.Tranny mount?Tranny bearing?
Test without prop attached in forward.May indicate if prop.Though without load not entirely indicative.I think I would try different props just to try eliminate possibilities
Damper more difficult diagnosis.
Has this vibration always existed? Possible shaft bent via collision.

[mtarcher]

Hi Chuck,
I had a similar issue with my 3GM, with the Kanzaki transmission. I found that the transmission thrust washers that pushed against the cone clutch were weakened which created a significant vibration when engaging the forward gear. Replacing the thrust washers fixed the vibration problem.
Sandy

[Chuck Hawley]

Originally Posted by Jim Cate

I can't remember from your initial posts on this subject, but is the prop tip clearance with the hull adequate? That can be a source of noise and vibration if too small.

But meanwhile, if the mounts are deteriorated, replacement seems warranted, and in the engines that I've owned, the damper is not terribly expensive nor difficult to replace. I think I'd be tempted to follow your plan of replacement.

Jim

Hi Jim,

The prop does appear to have sufficient clearance from the hull. I didn't measure it, but it has to be 4-5 inches, I would guess.

I have ordered both new mounts from PYI and a new damper plate, and my fingers are crossed that one or the other will solve the issue.

Chuck

[Chuck Hawley]

Originally Posted by redsky49

Was the propeller shaft measured for proper alignment/runout?

No, it wasn't. However, when the engine is not jumping all over creation, the shaft turns very smoothly and does not seem to be bent. I probably should have pulled it during my haulout... but it did not seem to be bent.

Chuck