Engine shuddering - Anyone heard of this?

[MV Wanderlust]

I would appreciate opinions and insights on this.

We have a new-to-us 1986 Chris Craft 42' aft cabin with Cummins 6BT engines with about 1000 hours each. They performed perfectly during the sea trial. Afterwards, we had a local mechanic change the impellers. On these engines, the motor mount needs to be loosened so the engine can be jacked up about 1/4" to access the water pump.

During the five-day repositioning trip from Jacksonville, FL to Bradenton, FL the starboard engine would periodically shudder or "spasm". In neutral, it is rocksteady at all RPMs. It only shudders when in gear and gets progressively worse above 2000 RPM.

After we arrived in Bradenton, someone told us the props could be out of balance so we had them removed and sent out to be balanced. However, I just discovered these two articles about engine alignment possibly causing this situation, too.
http://www.yachtsurvey.com/alignment.htm
http://www.yachtsurvey.com/Alignment2.htm

Has anyone heard of this or experienced it before?

Is it possible… or maybe even probable… that the starboard motor mounts are not quite in alignment after the impeller change and this is what's causing the shuddering?

[ontherocks83]

poor engine alignment could absolutely cause a shudder. Other things can too, but where you said he had to loosen the engine mounts I would definitely start there.

[MV Wanderlust]

Originally Posted by ontherocks83

poor engine alignment could absolutely cause a shudder. Other things can too, but where you said he had to loosen the engine mounts I would definitely start there.

Thanks, rocks. I've had fuel problems before that would cause an engine to choke off and die. As it was doing so, it would sort of shudder, too, as all engines will when they're choking off. But that happened in neutral, as well. That's not the case this time. In neutral, the starboard engine can be run up to WOT with no unusual vibrations. This only happens under load and, the higher the load, the greater the vibration. And it's only the starboard engine. The port engine is steady at all speeds under load. Seems to make sense that there is some sort of unexpected strain on the engine/running gear connection.

[MV Wanderlust]

Here's another symptom, though this may or may not be related to the shuddering: about the same time as the vibrations started on the starboard engine, I noticed that the rudders have to be turned about ten degrees to port in order to maintain a straight line at cruising speeds. Idling around in the marina, the rudders must be straightened up, of course. Made me wonder if the port engine was running stronger than the starboard engine even though they're at the same RPM. That would/could make the track to starboard and the rudders would have to be to port to compensate. Is it all related or is that something else? Divers recently cleaned the hull and confirmed the rudders appear correctly aligned.

[techmans]

Your engines may run at same RPM but if your transmission slips your props do not.
Hence I would suspect that there's a problem with the SB transmission which might cause both problems.

[skipmac]

Originally Posted by JohnEasley

Here's another symptom, though this may or may not be related to the shuddering: about the same time as the vibrations started on the starboard engine, I noticed that the rudders have to be turned about ten degrees to port in order to maintain a straight line at cruising speeds. Idling around in the marina, the rudders must be straightened up, of course. Made me wonder if the port engine was running stronger than the starboard engine even though they're at the same RPM. That would/could make the track to starboard and the rudders would have to be to port to compensate. Is it all related or is that something else? Divers recently cleaned the hull and confirmed the rudders appear correctly aligned.

Assume the engines are counter rotating? If not that could be the reason for having to keep the rudders turned at speed.

How do you know the engines are running at the same RPM? Tachs aren't accurate enough. You need to get an engine synch gadget or you can get very close by ear with most boats if you listen for the harmonics.

Back to original question, my first guess would be the alignment since that was just messed with. For standard prop shaft flange connection the tolerance should be +/- 0.005". Loosening the motor mount and jacking up the engine could definitely screw that up.

[MV Wanderlust]

Originally Posted by techmans

Your engines may run at same RPM but if your transmission slips your props do not.
Hence I would suspect that there's a problem with the SB transmission which might cause both problems.

Okay, that makes sense. Is there an easy way to check that to confirm if it is or is not the issue? And what does it take to fix a slipping transmission?

[MV Wanderlust]

Originally Posted by skipmac

Assume the engines are counter rotating? If not that could be the reason for having to keep the rudders turned at speed.

How do you know the engines are running at the same RPM? Tachs aren't accurate enough. You need to get an engine synch gadget or you can get very close by ear with most boats if you listen for the harmonics.

Back to original question, my first guess would be the alignment since that was just messed with. For standard prop shaft flange connection the tolerance should be +/- 0.005". Loosening the motor mount and jacking up the engine could definitely screw that up.

The Glendinning synchronizer stopped synchronizing after the sea trial but before we took possession. I haven't yet had a chance to work on it. But I can hear the harmonics easily enough and when they are harmonized, the tachs are also usually at the same RPMs, too.

Okay. Sounds like the mechanic needs to check the alignment and the transmission for slippage.

[a64pilot]

Originally Posted by JohnEasley

Here's another symptom, though this may or may not be related to the shuddering: about the same time as the vibrations started on the starboard engine, I noticed that the rudders have to be turned about ten degrees to port in order to maintain a straight line at cruising speeds. Idling around in the marina, the rudders must be straightened up, of course. Made me wonder if the port engine was running stronger than the starboard engine even though they're at the same RPM. That would/could make the track to starboard and the rudders would have to be to port to compensate. Is it all related or is that something else? Divers recently cleaned the hull and confirmed the rudders appear correctly aligned.

I'm going with a fuel related issue, and your tach just isn't showing it, a slipping transmission should have an increase in RPM and I've not seen once slip that didn't get worse fast one it started slipping.
You should hear the engines slip out of sync, I sync by ear. I hate the out of sync sound.
I'm assuming your don't have a synchronizer?

[MV Wanderlust]

Originally Posted by a64pilot

I'm going with a fuel related issue, and your tach just isn't showing it, a slipping transmission should have an increase in RPM and I've not seen once slip that didn't get worse fast one it started slipping.
You should hear the engines slip out of sync, I sync by ear. I hate the out of sync sound.
I'm assuming your don't have a synchronizer?

Valid point, pilot. Makes sense that the engine RPMs would rise and fall as the load slips on and off. ...and that isn't happening. According to the tachs and to my ears, the RPMs stay steady, once set.

I do have a Glendinning synchronizer but it currently isn't working. There have been enough other things to do that I haven't gotten around to troubleshooting it yet.

We did have some algae issues with the starboard tank and engine when we first set out. Bad enough that we had to lay over for a couple of days while a mechanic came out and cleaned out the Racors, blew out the fuel lines, etc. But when algae was clogging the system, the RPMs would drop off rapidly and the engine would die in neutral or under load. After he cleaned it out, that engine held a steady RPM for the rest of the trip.

[Cadence]

Engine alignment.

[MV Wanderlust]

Originally Posted by Cadence

Engine alignment.

Looks like it takes a couple of special tools so this is outside of my expertise. Is this something that can be done in the water or will it need to be hauled?

[donradcliffe]

Alignment should be done in the water.

I doubt the Gendinning failed--they are usually pretty reliable. Did you move the slave throttle to max after you turned it on??

[Cadence]

Originally Posted by JohnEasley

Looks like it takes a couple of special tools so this is outside of my expertise. Is this something that can be done in the water or will it need to be hauled?

It should be done in the water and doesn't take much more than feeler gauges and acquiring a few more gray hairs however have someone do it if you haven't. JMHO

[a64pilot]

Engine alignment is easy to check, dead easy. Little more to aligning though, but it's just trial and error from what I can tell, no real Science.
You won't really know if it's right or not if you pay someone. I did, and later found out of course that it wasn't in alignment, after pulling my hair out, cause I knew it was aligned, I just paid to have that done.