engine vibration

[brad minda]

85 c30 with the universal m25 engine. I feel i have excessive noise and vibration. Motor mounts look OK but may not be. anyone have experience with any of the shaft coupling vibration dampeners out on the market? Any thoughts in and ideas would be helpful. The young fellow in my picture is one of my grandsons and is now 18 and on his way to becoming a diesel mechanic....wish he'd hurry...

 

[dmax]

First question: does the vibration occur when you are in neutral at the dock or only when in forward gear and underway?
Diesels are noisy but it shouldn't be excessive.

 

[sail sfbay]

My experience with engine vibration was caused by misalignment of a new shaft. Had to add shims to the rear motor mounts. This will be a troubleshooting seriatam How to Diagnose and Fix Inboard Shaft Vibration | Boating Mag.

 

[brad minda]

in neutral running at dock

 

[dmax]

While you still may want to re-align the prop shaft as sail sfbaya says afterwards, you first need to get rid of the vibration in neutral. Most likely culprit here is the motor mounts - they can look ok and be worn out. If they are the originals, they are worn out. See what make of motor mount is in there now - the originals were likely univerals /westerbeke mounts, which are unfortunately expensive to replace. There are less expensive options. Lots of information on this site on motor mounts plus here's an excellent article by the incomparable MaineSail: Replacing Motor Mounts - The Snowball Effect - Marine How To

 

[Hayden Watson]

Three cylinder diesels are notoriously rough running because they are not well balanced as it relates to the two of the three elements of engine vibration. Here is a good video explainging engine vibration.

• Even firing interval - 20º 0-power intervals. = not good
• Primary balance - imbalance about the front to back centerline = not good
• Secondary balance - all pistons are moving at a different point in the stroke at all times. = good

Given that you are feeling vibration when the transmission is out of gear, I would look to your motor mounts. You said that your motor mounts are good. Most people believe that if an engine mount is not broken so that the engine is connected to the engine bed that it is "good". That is not the case. Most often a bad motor mount is one on which the rubber mounting has become rock hard so that all engine vibration goes straight to the hull.

A good motor mount will be flexible so that it absorbs the engine vibration rather than transferring it to the hull.

I installed new motor mounts on my boat a couple of years ago. There are two Vetus motor mounts that most people use on their C30, the K50 or the K75. I have seen many use the K75 under the assumption that stronger is better but in motor mounts that is not the case. I want the lightest mount that will handle the weight and loads of the engine and used the lighter K50 and have been very happy with with it. Now, I see the enging vibrating but the boat does not.

 

[Stu Jackson]

in neutral running at dock

What rpm? Many folks have their idle speed set too low.

Separate info bank for you:
Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too This includes a link to the Critical Upgrades topic which has more engine information please read it too.

Diesel Engine - c34.org

 

[Dave Groshong]

Other than motor mounts, it could be: a compromised (electrolysis/corrosion) stern tube, strut, or failed cutless bearing, or even a loose shaft zinc

 

[capta]

I brought in a Yanmar specialist to survey the engine of a boat I'm looking at, today. He found that they had installed the engine on the shipping motor mounts during the repower, instead of buying the correct ones for the operation of the boat. Perhaps this is your problem. If all else fails, I'd call a specialist in your engine and have him check that.
I'd bet dollars to donuts that there are a lot of repowers out there like that, perhaps breaking motor mounts more frequently than they should, Anyway, another case of spending a few hundred bucks to save a lot more, in this case, $ 7500.00! I certainly wouldn't have caught that one, even though I've been messing about with boats for more than 6 decades. He even offered me a job, as he only does Yanmar engines and I made the mistake of mentioning that GM diesels were more my specialty. lol

 

[Ralph Johnstone]

What rpm? Many folks have their idle speed set too low.

Carrying that sage remark one step further, does the vibration remain the same from idle to maximum while in neutral.

#1 Step In Engine Analysis check all of the no cost test items first.

 

[kloudie1]

After verifying the above items, if the vibration is still there, you may have one "lazy" cylinder that is not doing as much work as the other two, resulting in a shake. A prelim looksee would be to loosen one HP injector line while the engine is running. note the change in engine feel and shake and sound. Tighten it and go to the next.. if one does not cause as much shake, that is the one that isn't pulling its load. from there, could be a bunch of things related to that cylinder, like valves not set correctly, a bad injector, bad piston rings.. and other expensive things..

 

[JoeWhite]

On my c36 M35B it was a cracked alternator bracket which totally broke off finally. After replacing it is smooth.