Vibration, Vibration, Vibration?

[Marlin Hunt]

Mechanic's can't help can you,  I have a 31' Hunter with a GM2 diesel motor 1984, and have replaced everything from the trany back and everything has been balanced and alligned twice, and still getting an entermiten violent shaking or vibration. Sometimes it will do it in neuteral and sometimes in gear and not always at the same rpm. can this be a fule problem? Can someone help i'm about to go nuts and broke! Thanks, Marlin

 

[Doug]

Difficult to diagnose

Marlin - As an automotive technician I can tell you an intermittent problem is the most difficult to repair. A good tech can repair a problem that is present, since it will be gone when the repair is complete. With an intermittent problem you can't be sure.Here are my thoughts - is it a shaking or vibration? - those are two different symptoms. A shaft is either aligned or not - it won't change on its own unless an engine mount is broken and "cocks" it one position then shifts. Having the problem in gear or neutral also leads my thought away from shaft alignment. That would leaves me to think the engine has an occasional "miss". Poor fuel condition/quality? Clogging injector? Air leak in fuel system?DougS/V Brenda Gail

 

[Alex]

Try this for neutral..

Sail in neutral with engine off ,'draging' the propeller ,about 4 kts and up speed -not folding propeller, this one will stay closed. If the engine shake ( assuming cutless bearing and engine mounts OK and tight)you may have a slightly bent shaft!!If you have it out , check also if the unsuported lenght of shaft is not above the advise max lenght for that shaft diameter( there is a formula) , it might cause vibrations-and bent !.If you have premature cutless bearing failure ,check -along with prop condition and balance- if the lenght of shaft -from back end of cutless bearing to the end of shaft is not above 1.5-2 times it's diameter.Check if the engine is not mounted too high on it's bolts( there is an advised formula to that too ),this might cause excessive vibration , especially while heeling or rolling , and less if not...All this assuming other related variants to this problem were checked and found OK.Alternatelly ,if vibration change according to angle of heel , you might have problem with some engine mounts- or cutless bearing.These are just some options among others..Hope it helps. Alex.

 

[Clyde Lichtenwalner]

Engine mounts

Check the archives, but I'm pretty sure that the engine mounts on Yanmars are precisely tuned to minimize vibration -front and rear are different. The problem is they wear out. I think there is actually a recommended change interval. I'd look there next.

 

[Jack]

shaking

Check not only engine alignment, but also check strut and stern tube alignment. This will require the boat to be out of the water, carefully blocked with about six stands. Try turning the prop with engine in neutral, it should turn readily. If it turns with difficulty, sight the way the shaft exits the stern tube, and whether it is aligned equally within the cutlass bearing. If it appears to be either hard turning or misaligned, pull the shaft and run a string from the center of the coupling through the stern tube and cutlass bearing. You then can judge the alignment of all three. As a Marine Surveyor I dscovered the problem in a 1984 31. Both the stern tube and the strut had to be moved to align the shaft. This solved the problem. Just resurveyed the boat last month (15 years later) and the owner reports it has little vibration.Jack Mackinnon, AMS/SMS