Hunter 34 '86 Transmission woes Yanmar 3GM30

I

Ilanortho

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Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
After spending every single weekend this winter refitting her I took her out for a first run on an absolutely magnificent day on Green Bay/ Lake Michigan. As I clear the first bridge 10 minutes out of the dock a horrible rattle starts. Only occurs in forward, not neutral or reverse. New prop probable not loose or unbalanced as such. Engine runs like a purring kitten . Took off new shift cable to make sure was fully engaging forward gear manually at the transmission, oil clean and at level. Brought her to shipyard and walked off. Engine was realigned this winter, new stuffing box put in, prop shaft runs smoothly in fwd and reverse.

Sounds like a very nasty loud rattle from the transmission only in fwd gear.

After the tremendous amount of comprehensive work put in all winter I felt like taking her out and opening the sea cocks (State Farm just joking, I'm a bad swimmer and the water is cold and the life raft is not on board yet.)

Anyone been there? Thanks. Ilan
 
I

Ilanortho

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Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
Good thought; will have to check when back at the boat. I know that when the shipyard aligned the engine the mounts were at the end of the slotted holes so they had to drill/ elongate the existing holes. I wish I had thought to check.
 
KD3PC

KD3PC

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Sep 25, 2008
1,069
boatless rainbow Callao, VA
my experience is that the crack is directional and loose in one direction, but snugs up in the other direction...
 
datasailor2

datasailor2

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May 17, 2010
25
Hunter 1983 - 34' Port of Everett
I posted similar issue with my 83 34' Hunter 3MGF20. after much effort investigating all possible causes i.e. prop shaft alignment or engine mounts. I finally took to task removing the transmission and having it torn down on the bench by qualified Mechanic. The mechanic found that the forward Cone clutch was worn down to it's travel limits and it likely was intermittently failing to fully engage the forward drive gear. The slipping and cripping of the clutch is what most likely the cause a very noticeable bucking in the engine and drive train that only happened in forward between 1500 and 2000 RPM and most frustratingly happened very infrequently at first. I hope to reinstall the new rebuilt trans in next couple of days and do a serious sea trial. BTW pulling the trans was a lot of fun. NOT!
 
I

Ilanortho

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Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
Thanks for your reply. Mine seems different; the engine does not buck and runs smoothly throughout. The boat was pulled today and they found that the shaft seems to hit the side of the exit hole as it rotates. The engine was realigned this winter when the packing gland was replaced. No sure however why this only happens in forward. They will try realigning the engine and see if this changes things
 
isaksp00

isaksp00

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Apr 27, 2010
1,279
Hunter 23 Lake Wallenpaupack
Just a random thought... If the shaft is barely touching the exit hole side, is it possible that the thrust from the prop pushing on the shaft causes it to bend slightly and touch, while in reverse the prop is pulling the shaft to be more straight and thus clear the side? I am not sure how much these shafts can deform from prop thrust.
 
I

Ilanortho

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Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
Don't know how I would even check this proposition!. On the other hand the shaft is incredibly rigid.
 
kloudie1

kloudie1

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Nov 6, 2006
10,093
Hunter 34 Mandeville Louisiana
Couple of thoughts.. Yanmar mounts are very soft compared to most mounts and the engine moves a lot.. reverse gear is a lower ratio than forward so the prop does not generate the same load (opposite) in reverse as in forward.. the prop is less efficient in reverse and makes it develop even less load .. My shaft used to hit the side of the stuffing box holder (stern tube?) in forward and not reverse.. until I aligned everything..
The damper plate is always taking load in the same direction regardless of the transmission being in forward or reverse.. it is attached to the flywheel before the transmission. I wouldn't suspect it to be a problem..
 
I

Ilanortho

.
Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
kloudie1 said:
Couple of thoughts.. Yanmar mounts are very soft compared to most mounts and the engine moves a lot.. reverse gear is a lower ratio than forward so the prop does not generate the same load (opposite) in reverse as in forward.. the prop is less efficient in reverse and makes it develop even less load .. My shaft used to hit the side of the stuffing box holder (stern tube?) in forward and not reverse.. until I aligned everything..
The damper plate is always taking load in the same direction regardless of the transmission being in forward or reverse.. it is attached to the flywheel before the transmission. I wouldn't suspect it to be a problem..
Click to expand
It appears you are correct. The realignment appears to have resolved the issue. So far!
Thanks for your advice. Ilan
 
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