New unwelcome vibration after transmission replacement

Hi,

I would appreciate any thoughts and advice on this new problem. We replaced the Hurth transmission on our 1984 E30+ with a Twin Disc transmission, rated at the same 2:1 reduction ratio (whatever that means) because it seemed the Hurth was slipping occasionally when underway. The diesel mechanic who installed it came highly recommended by several reputable people, and seemed to work very carefully, but when using the engine with the new transmission, we are getting a new vibration that concerns me.

The new transmission is about 5/8" longer than the old one, so the 2 blade folding prop is a bit closer to the rudder than before, but still clears it by a couple of inches. The mechanic aligned the engine/shaft/coupling to 4/1,000 of an inch, but the way the engine mounts are located on this boat, couldn't get it any closer. But he said this was closer than before and "pretty good". Although there was some vibration before, it is more now, at all speeds. When I change the speed a bit, the vibration seems to decrease for a short while, and then gradually increase again, until I change to a new speed where the pattern repeats. I have not been able to find a consistent speed without vibration. When I run it in neutral at the dock it seems smooth until about half-throttle, at which point I get vibration, but if I increase further, it settled down again. The mechanic said that vibration was due to something like the "harmonics" (I think that's what he said).

The prop shaft, cutlass bearing and prop are only about 4 years old and looked good at haul-out a couple months ago, before the transmission change. Prop shaft zincs have been replaced recently by the same diver who has been doing this for me before, and they look fine; steering feels tight; rudder bearings seem fine; underwater hull has been cleaned.

Both me and the mechanic are puzzled as to why the vibration is occuring when everything looks normal and has been installed correctly. Does anyone have any ideas of what could be causing this vibration?

Thanks for any help.

Frank

motor mount trivia

Not sure if or how much it might help, but you might check on the condition of the cast rubber in the old motor mounts, too.
When we had ours replaced a couple seasons ago, we found that the rear ones had collapsed to a metal-on-metal point. This could not be easily observed from looking at them because access is visually limited.
We were trying to lessen vibration, and also concerned that they might be getting too old and the rubber too hard. At least we were right to replace them, although I cannot say that there is really any less vibration now.

Loren

Hi Loren,

Thanks for your suggestion. The mechanic did check the motor mounts and said that Ericson had used good quality neoprene and they were in good shape. And as they were there both before and after the transmission change, I'm thinking it must be something else (ie. new) that is causing the vibration. The engine doesn't seem to vibrate in neutral except for at the one RPM mentioned in my initial post, so I'm actually wondering if it might be related to the drive shaft/prop being 5/8" closer to the rudder and causing some disruption there--everything else seems to be in good shape, so I'm really puzzled.

But any thoughts are welcome!

Frank

Process of Elimination

Vibration out of Gear Points to Engine Mounts.
Do you have a Flexible Coupling ? Sorry..25 year old motor mounts should ALWAYS be replaced. A lot of this is Trial and Error / Process of Elimination...
just like when the Doctor removes something and says "Well that wasn't it"
At rest JERK the motor around VIOLENTLY in every direction possible and check for looseness / clunky sounds / irregularities. All Diesels have a natural vibration point with the harmonics of the boat/shaft/prop. Most of us just avoid that RPM and stay under or over to avoid vibration.

Good Luck