Extreme vibration sometimes in forward

[Chuck Hawley]

I am at my wit's end with a problem on Surprise, my Alerion Express 38 Yawl. She has a completely conventional engine/drive system, consisting of a Yanmar 3GM30, 1" shaft, PSS Shaft Seal, bronze strut, and Gori 2-blade folding prop. For those who don't know, the folding blades are geared together, so you cannot have a single blade deployed.

The problem is that about 50% of the time, I experienced extreme vibration when shifting into forward. The aft end of the engine jumps around, the shaft seal starts to spray water, and it sounds like there is a massive clump of kelp on a single blade of the prop. After several R-N-F-N-R shifts, we can generally get it to settle down.

A diver reports that everything is hunky-dory underwater. No excessive wear on the cutlass bearing, nothing stuck on the prop blades.

We believe the transmission lever on the engine is fully engaged (full throw), and so the transmission isn't partially engaged.

This is especially bad at low RPM engagements, when the engine is naturally prone to vibration, but increasing the engine speed doesn't resolve the problem.

What am I missing?

Chuck Hawley
Alerion Express 38 Yawl, Surprise
Santa Cruz, CA

[Paul L]

Do you have a line cutter on the shaft?

[Geoff54]

I had a similar problem, except it was when I went astern. The coupling bolts had worked loose so that the shaft / shaft coupling wasn't always concentric with the gearbox output shaft flange.

[Lepke]

I assume you have checked your motor mounts? The change in direction changes the torque on the engine. In one direction it would try to lift a broken mount and the other would force it back together.
More remote, loose flywheel, torque/clutch plate. Very remote, cracked crank.

[h20man]

I have seen heard of this issue with a transmission problem.

How is the transmission? There is another thread here: regarding a similar issue.

Best of Luck

[TurninTurtle]

Originally Posted by Chuck Hawley

I am at my wit's end with a problem on Surprise, my Alerion Express 38 Yawl. She has a completely conventional engine/drive system, consisting of a Yanmar 3GM30, 1" shaft, PSS Shaft Seal, bronze strut, and Gori 2-blade folding prop. For those who don't know, the folding blades are geared together, so you cannot have a single blade deployed.

The problem is that about 50% of the time, I experienced extreme vibration when shifting into forward. The aft end of the engine jumps around, the shaft seal starts to spray water, and it sounds like there is a massive clump of kelp on a single blade of the prop. After several R-N-F-N-R shifts, we can generally get it to settle down.

A diver reports that everything is hunky-dory underwater. No excessive wear on the cutlass bearing, nothing stuck on the prop blades.

We believe the transmission lever on the engine is fully engaged (full throw), and so the transmission isn't partially engaged.

This is especially bad at low RPM engagements, when the engine is naturally prone to vibration, but increasing the engine speed doesn't resolve the problem.

What am I missing?

Chuck Hawley
Alerion Express 38 Yawl, Surprise
Santa Cruz, CA

Failed engine mount(s)?

Then any slight misalignment of coupling due to engine torquing and the engine wants to take a walk through the bilge.

If this is it... FIX IT NOW! before you bend the shafts. (if you haven't bent them already)

A failing coupler can do it too. (and can bend the shafts and rip out the engine mounts)

[Chuck Hawley]

Originally Posted by Paul L

Do you have a line cutter on the shaft?

Good question! No, despite numerous crab traps and heavy kelp.

Chuck

[Chuck Hawley]

Originally Posted by Geoff54

I had a similar problem, except it was when I went astern. The coupling bolts had worked loose so that the shaft / shaft coupling wasn't always concentric with the gearbox output shaft flange.

I have recently replaced the coupling bolts and nuts, and they are tight. I was surprised that when I "broke" the flanges apart, it was very hard to know if the engine was in alignment. I could move the shaft hither and yon, and could not imagine how one could measure the alignment to within thousandths.

Chuck

[TurninTurtle]

Originally Posted by Chuck Hawley

I have recently replaced the coupling bolts and nuts, and they are tight. I was surprised that when I "broke" the flanges apart, it was very hard to know if the engine was in alignment. I could move the shaft hither and yon, and could not imagine how one could measure the alignment to within thousandths.

Chuck

sounds like a failed bearing or failed coupler.

Were you actually moving the shaft or moving the coupler on the shaft?

Whatever it is... its needing fixing before the engine hops up and goes through the hull.

[Chuck Hawley]

Originally Posted by Lepke

I assume you have checked your motor mounts? The change in direction changes the torque on the engine.

Good thought. While the engine mounts are not entirely accessible, I have lifted the engine from all corners to see if I can detect a shear in the rubber. While they are very flexible, I can't see any breaks.

I used to sail on a Santa Cruz 40 with the same engine, and one of its mounts sheared. Hard to diagnose since the halves fit together perfectly at rest, and when the engine was running smoothly.

Good thought!

Chuck

[senormechanico]

I used to have a Gori 2 blade folder which came with the boat (since replaced with a Varifold).
The Gori was kinda "Loosey Goosey" and had a bit of a rattle at idle.
This condition was caused by lack of zinc protection from previous owner's neglect.
Every once in a while I got the same vibration you mention.
Eventually, I went over the side to check (in PNW salt water of 49 f. !!!)
Sure enough, the gears had jumped one tooth.
I shifted into and out of forward and reverse gear a few times and it regained proper indexing.
That's when I convinced my wife we needed a new prop !

[notiesbob]

We have and continue to have similar problem - 1984 32' C&C, Yanmar 2GM. We recently sheared the shaft key at the coupler, damaged the coupler and ultimately replaced shaft and coupler, inspected strut bearing for wear (mechanic indicated no need to replace) and inspected the 2 blade Flexofold geared prop (again no problem there). All this after spring launch May 13. All work was done by mechanic that services for pre and post launch as well as pre & post haul out.

The prop vibration existed before all of this work and continues. Playing the F-N-R, F-N-R takes it away the problem remains. What else is left besides the engine mounts?

[Clivevon]

Diesel or other oils can damage the rubber in your engine mounts. They can be intact but too soft. How old are they? If you can move your engine by hand that doesnt sound right to me...

How long does your cutless bearing last - poor alignment will wear it much quicker.

You align with a feeler gauge between the mating faces of your coupling - thats how you can measure in thousandths of an inch.

Good luck.

[Calif.Ted]

Originally Posted by Chuck Hawley

I have recently replaced the coupling bolts and nuts, and they are tight. I was surprised that when I "broke" the flanges apart, it was very hard to know if the engine was in alignment. I could move the shaft hither and yon, and could not imagine how one could measure the alignment to within thousandths.

Chuck

Hmmm, The shaft will move "in or out" or fore and aft a bit, but when you butt the flanges you can usually run a feeler gauge around for a "go, no go" check of alignment. There shouldn't be any side to side or up and down movement at the flanges ?