engine vibration

Status
Not open for further replies.
Oct 14, 2012
3
Hunter 34 Portland
Hi all,
My sailing partner and I (both quite experienced sailors, he owned a new Hunter 34 the first year they came out and i sailed my Sabre 28 for ten years) just purchased a beautiful, well maintained, 1984 Hunter 34. Yanmar diesel runs like a top in neutral and under power. On our first 4 sails using the engine to both motor and motor sail things went peachy. Boat has a Yanmar 3GMF with a fixed 15 x 15 prop.
Then on about our fifth sail as we prepared to drop the main, we rather suddenly noticed a severe vibration at about 1500 rpm as we powered up to get to the dock. Didn't sound like either a loose zinc which in my Sabre 28 experience is a more hammer like bang bang bang until it threw itself off, but rather more like the hard thump thump of picking up a lobster buoy, along with the motor really almost bouncing in it's bed at that rpm, only to go immediately away as we throttled down..
Thing is we don't believe we hit a buoy, except that the tide was flowing out very had and they were in some cases pushed slightly below the surface by the tide so it might be possible we clipped a submerged one (but we both doubt it). Neither of us heard any thump (one of us happened to be at the helm and one lying on the bow.
After docking (we still had power and had vibrationless motoring under 1500) I dove on the prop, and to my dismay did not find the hoped for loose zinc, lobster buoy, or visibly bent shaft or prop that was expected.
After tying up, we powered up again to 1500 and again the vibration and sound came back, but only in forward, not in neutral (we took the engine to almost 2800 rpm in neutral) or reverse (didn't rev reverse quite that high).
Fortunately we are having the boat hauled on Thursday as it is the end of the season. so we can get a good look while it's on the "hard" for the winter. Idea? Yanmar engine trouble shooting guide talks about engine misfires, motor mount looseness (not checked yet), prop imbalance and shaft alignment in that order. (it does have an apparently plastic shaft saver in the drive chain and I observed both it and the stuffing box and coupling at 1500 during the vibration and couldn't see anything obviously wrong) and I also held on to the two bolts ends which protrude into the hull which mount the prop shaft and they didn't seem to be vibrating, but the vibration did seem to originate just below that area).
Seems the vibration shouldn't shouldn't be engine misfires as it is still smooth as silk to 2800 rpm in neutral (we didn't try to push it beyond the vibration at 1500 to see if it would settle down above 1500). The cutlass bearing seemed to be good with no play two weeks ago when we had it hauled to do the marine survey (it has been replaced at least twice (maybe more) by its two previous owners in its 20 something years), there is one old removed shaft saver some where on the boat I saw during the survey, so it has been replaced at least once in it's life)....so ideas?
Can cutlass bearings fail so suddenly, can lobster pots be hit without one knowing hard enough to bend a shaft or prop, or can motor mounts fail suddenly? ideas!! please Help!!! Chuck
 
Jun 3, 2004
890
Hunter 34 Toronto, Ontario Canada
I would check the motor mounts- both for looseness and for clearance between the /\ and the top plate should be at least 3/16". Sounds more like a loose mount because of the sudden onset. I fought a similar vibration at about 2100 for 2+ seasons. Would only show up after running at cruising speed for an hour or so and only on the throttle down. It would go away if the revs were increased then reduced again. Never did figure it out. Changed to a CS 3 blade, re-aligned and it is gone.
 
Oct 14, 2012
3
Hunter 34 Portland
Richard, Me being a brand new member, I hope I haven't sent this twice. I left the text page to scroll up and check what you said and my draft reply disappeared. So if it got sent accidentally and you, therefore, got it twice, my apologies. Again, thanks for your quick reply. Not knowing a lot, I was thinking the same way as you, given the sudden onset. Again, I assume your /\ is the metal top plate of the motor mount that sandwiches the rubber? pad between it and the angle iron engine bed. As for the 3/16th clearance, I assume this is to allow the pad to dampen the normal vibration without the /\ actually striking the angle iron engine mount. Is there a both a bolt and a nut at the other side so that I need to hold the nut while I turn the bolt, or is it threaded into the plate? Also, I assume i'll need some sort of pry bar to see if the engine is loose (prying when it is not running of course), or will tightening the hold down bolts make it all apparent? I'll try tomorrow, and thanks so much.
These threads are great. There is an urban legend you may have heard about between the designers of a turkey cannon (seriously) that test fired turkeys against airplane windshields to make sure they didn't fail if the plane hit a flock of geese.
Anyway, another aviation company wanted to borrow the turkey cannon to test their new windows, but after using it replied with horror to the original turkey cannon designers that their turkey not only smashed the window, but took out the pilot's head rest in the mockup, and the bulkhead behind it, to which the original turkey cannon owners replied...."ugh...you're supposed to thaw out the turkey first!"
I hope I don't turn out to be the turkey here.......Again thanks, Chuck in Maine (and Toronto is a very nice place!)
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Those three cylinder 4 cycle engines are a bit lumpy even when well ties down. My yanmar had issues at 1500 also. i suspect one or more motor mounts need replacing. the easyset way to check them is with a big pry bar and stout biece of 2x4 as a fulcrum. Just lift the engine near the mount to see if it is attached to the hull. It should not seperate from the mount at the rubber.
 
Jun 3, 2004
890
Hunter 34 Toronto, Ontario Canada
Good turkey story!!
The mounts have two bolts each, not threaded thru the angle iron but with a nut on the end. You have to be a bit of a contortionist like all things boat, to hold and tighten. Check all 8 bolts, if you find looseness, you will need to re-do the alignment and tighten down all of them. The /\ is the v shaped piece of iron that separates the two rubber chunks. There needs to be 3/16 clearance between the top of the /\ and the plate at the top of the mount. Some one- maybe Kloudie - posted some pics of Yanmar mounts not long ago
 
Oct 14, 2012
3
Hunter 34 Portland
will try your suggestions. thanks so much....chuck.....
gobble....gobble.
humm, let's see, fixed by Thanksgiving...let's hope
 
May 24, 2004
7,174
CC 30 South Florida
If the engine runs smooth as silk in neutral and vibrates badly when the transmission is engaged the problem likely lies in the running gear. I would start by checking the prop. Check for plastic bags or sea grass that may have become tangled. Check the blades for integrity . Check the transmission coupler and shaft alingment. You indicate everything was peachy the first four outings and then the problem. I think that would rule out motor mounts as they usually fail in a gradual manner. Check for loose or broken bolts.
 
Jun 25, 2012
942
hunter 356 Kemah,the Republic of Texas
Benny17441 said:
If the engine runs smooth as silk in neutral and vibrates badly when the transmission is engaged the problem likely lies in the running gear. I would start by checking the prop. Check for plastic bags or sea grass that may have become tangled. Check the blades for integrity . Check the transmission coupler and shaft alingment. You indicate everything was peachy the first four outings and then the problem. I think that would rule out motor mounts as they usually fail in a gradual manner. Check for loose or broken bolts.
Also check the strut when you haulout.
 
Status
Not open for further replies.