Full Ahead to Full Astern . . . Very Quickly

[deltasig]

Due to some very unfortunate circumstances I switched from full ahead to full astern.
That gave me the desired result (avoided danger) but when I shifted back to forward I noticed significant vibration from under 1000 and getting worse at 1500. I did not dare try higher rpm.
Working the control lever back to idle and forward couple of times the vibration disappeared and as long as the engine was running I did not experience any at all possible rpm. The scenario repeats every time I restart the engine and try to move the boat forward and only forward since reverse has no problem at any rpm.
Engine: Yanmar 3JH2TE with KM3A transmission
Flexible coupling: CENTAFLEX-M
3 blade fixed prop on 38mm shaft; I own the boat since new in 1997.
After inspection, I noticed 2 out of 4 nuts holding the flexible coupling to the transmission were loose (No idea if they got loose because of the maneuver) which I tightened wihout resolving the problem.
Checked prop, shaft, strut, engine mounts and they check out fine. No idea how to check the status of the flexible coupling.

Observing the drive train vibrate, I think the excitation comes from the transmission but I cannot think/see of anything that could have gone wrong. The fact that the whole system can be "persuaded" to not vibrate by repeatedly engaging and disengaging the forward gear AND
once it stops vibrating it works fine all the way to 3600 rpm and for extended time, puzzles me.
BTW I have put more than 70 hrs motoring since this happened.

Any thoughts? thank you.

[rigormortis]

I'm not much of a thinker and you probably checked this too, but: Did a shaft zinc (if you have them) come loose or off?

[Bash]

Is that "flexible coupling" a Shaft Saver? If so, you might have destroyed it, but at the same time it might have saved you from blowing the entire transmission. Take the entire thing out and see whether you notice any cracks or deformities. If so, the good news is that they cost less than a couple hundred dollars to replace.

Your story is a good reminder to all of us how important it is pause in neutral, even in a panic situation.

[James S]

On many set-ups there is a Damper plate between the transmission and the engine...this plate is bolted to the engines flywheel.
Some have springs and some have rubber chunks.
You may have damaged this plate or more likely its springs of rubber...uh...chunks.

[senormechanico]

Motor mounts ok?
Try prying up to see if they've separated.

[capn_billl]

I spun a prop off of an outboard that way once. Good luck. You may have to take out drive train peice by peice until you find out what got tweaked.

[Chief Engineer]

I would look at the flixible coupling first.

Curios...what kind of danger were you avoiding that required such a drastic manoevre?

[Mark Zarley]

My flexible coupling has 1 bolt out of the 4 that has a rounded head. The head of this bolt is used to check the alignment of the shaft to the trans (ie engine). Just for snicks check for alignment before start-up or better yet start it up and shut it down while it is vibrating and check your alignment. To check the alignment use a feeler guage to check the clearance at 3,6,9 and the 12 o'clock position. One thousands difference in clearance per inch in diameter. 4 inch in diameter flexible coupling = .004 thousands of difference in all the readings.

[osirissail]

Looking at the Centraflex-M on their website the unit has a large piece of rubber that acts as the dampening material much like the rubber inside an engine mount. It is possible that you deformed/damaged that rubber by the high torque reversal. Your repeated shifting in and out of forward may "reset" the rubber inside the flexible coupling or move it in such a manner as to avoid the deformed/worn/sheared part.
- - I would suggest a very detailed examination of the flexible coupling including - if possible - removing it and connecting the prop shaft flange directly to the transmission flange and see if the vibration still exists.

[deltasig]

To answer any curious minds as to the unfortunate circumstance, here is what happened.
As it always the case, a number of events conspire to create a panic situation, lack of planing and forethought being one and two on the list.
Have a look at Porto Koufo (location 39 58.2N 23 55.13E) on GE.
I'm pined along side the SE side of the pier by a force 6 SE. The pier was build during WWII to be used by navy boats; to reach the top we climb up 4+ ft from our deck. I'm tending the fenders trying to keep the side of the boat from scraping against the rough sides of the pier when a Greek Coast Guard official comes and tells me I have to move on 'cause an excursion boat is coming to refuel.

After trying to explain in vain that it's impossible to move the way the wind is blowing, I decided to move just in case the excursion boat comes along side mine and that would have completed the picture ... totally!

I planned as calmly as I could the "escape" springing the bow out and keeping enough clearance for the stern so that my solar panels don't scrape against the high pier. I then steam ahead at full throttle keeping an eye to the stern and the panels. I'm about to clear the head of the pier when I find myself just about to broad side a 55ft cruiser who was tied at the head of the pier and now he's executing a similar maneuver. You see the GCG official asked him to leave, also, after talking to me.
Well, that's when the throttle did a 180. Fortunately, I slowed down and turn to starboard enough for the cruiser to pass and the stern to pull away from the pier. I started moving forward again and noticed the vibration.

On the technical side, I thing the flexible coupling may be the culprit/savior. Unfortunately, there's no way to attach the shaft directly to the transmission for testing. Also, I get very little in terms of support from the manufacturer, CENTA. I plan to have the boat hauled out for a bit since I don't feel comfortable doing shaft work on the water. I'll report back on findings.

Thank you, all, for your consideration.

[osirissail]

The flexible couplings/drive savers are fabulous on any boat if you can fit them between your transmission flange and the prop shaft flange. I have the Glove Drivesaver and after it saved my boat's butt in the Bahamas I went out and bought a spare to have onboard.
- - They are good to remove very minor misalignment problems that might cause vibration, and in my opinion are excellent at absorbing high torque overloads that might cause transmission problems. Cheap insurance against a multi-thousand dollar transmission rebuild or replacement.

- - Do you have a split flange situation where the transmission and prop shaft come together? If so and you are not satisfied with the Centraflex-M you might consider the Globe Drivesaver. Here is a website about their product:
Globe Composite Solutions > Application Design Center > Products > Marine

- - However, if your system is using the Centraflex-M to enable the engine and prop shaft to be more than just a little bit out of alignment, then you are pretty much stuck with continuing with Centraflex-M.

[Mark Zarley]

When you mentioned that a "few of the bolts were loose " that indicated to me that some surface wear had occured . After dismantling check that your bolt holes on the flanges are not out of spec "wallered out".

[redhead78]

One of the things the drivesaver is for, is just that....Any kind of twisting force, on the prop or trans, it should shear or maybe crack, it takes the hit and saves your drive system. I believe outboards have similar equipt called a shear pin....Red

[deltasig]

I'd like to post an update how the vibration was fixed.
It turns out that the adjusting screw that sets the length of the shift cable was loose. So, in forward gear the cone would not engage completely and it would set off a vibration. The important piece of info that someone did clue in was that in reverse everything worked perfectly. He started investigating what's different between operating in forward and reverse and the first thing he came up with was the the shift cable.
It is trivially simple once you know the answer!

[skipmac]

Thanks for posting the solution you found. I hate it when someone posts questions about some odd problem, generates a lot of replies and then never finding out what happened.

Like reading a detective book with the last chapter torn out.