Mysterious Yanmar Vibration - Only in forward gear

Apr 12, 2021
8
Hunter 34 n/a
Hey,
I have a Yanmar 3gm in my Hughes 36. At idle, I can rev the engine right up to 3000rpm with no abnormal vibration. The same can be said for when the boat is in reverse. However, when I shift into forward I can only get up to around 1100rpm before I notice significant vibration and engine rocking.

I attempted to align the engine - I don't think its perfect yet, but I also haven't noticed a significant improvement.
Also, yesterday after putting it into reverse for a few minutes and then putting it into forward at slightly higher than usual rpm, the engine performed well in forward. This was until I decreased the rpms again, and then increased them, at which point the rocking vibration returned.

I'm wondering if you guys think it's just an alignment issue, cutlas bearing, transmission issue, spacer issue (there is a red, plastic looking spacer in between the prop shaft and engine), or if it could be something else entirely.

I should also note I only bought this boat recently, so I don't have an overly detailed history on it.

Thanks,
Nik
 
Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
How "not perfect" is the alignment? A popular maximum is .003" spacing between the shaft and tans couplers. I believe this is a little excessive but it gives you a idea of how aligned it should be. Picture of the spacer you are talking about it needed as that doesn't sound right. Have you checked your prop? Growth can do this. A fixed blade prop will dramatically perform different between forward and reverse so comparing the two is almost useless.
 
Feb 21, 2013
4,638
Hunter 46 Point Richmond, CA
I think you have listed the obvious areas to look. I had similar issues on previous boats......one with a damaged prop, which was resolved by a new prop and another with misaligned shaft, which was resolved by rebalancing the engine and shaft.
 
Apr 12, 2021
8
Hunter 34 n/a
How "not perfect" is the alignment? A popular maximum is .003" spacing between the shaft and tans couplers. I believe this is a little excessive but it gives you a idea of how aligned it should be. Picture of the spacer you are talking about it needed as that doesn't sound right. Have you checked your prop? Growth can do this. A fixed blade prop will dramatically perform different between forward and reverse so comparing the two is almost useless.
The alignment was done to 0.025 to start. So not perfect, but I would think I'd see some improvement at the bare minimum.
As for prop function in forward vs reverse, the odd part isn't so much the propulsion, but the fact that I can rev up to 3000rpm in reverse with no hint of vibration issue at all, and only 1100rpm in forward - so I think that eliminates prop shaft warp, and maybe alignment to some degree since I'd think it would cause issues in both directions? (I could be wrong though). That being said I would like to know what you think of the spacer because it seems odd to me as well - and maybe the issue is there is compression on it in forward vs. pulling in reverse?
The boat was trucked after purchasing it, and I didn't notice any damage to the prop while it was out of the water. It's had the vibration issue since I've had it. I can't recall if it had the issue before.
prop spacer.jpg

The yellow tape was to help with the alignment.

Thanks again
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
The spacer is likely a flexible shaft coupling. These are often designed to shear if the prop hits something or adjust for minor misalignments.

Vibration and limited RPMs sounds like a prop issue, which may be combined with a worn cutlass bearing and/or slight misalignment.

The prop issue could be simply too much growth on the prop or it could be a damaged prop. The usual test for a cutlass bearing is visual inspection and shaking the prop to see if there is lateral movement.

The distance from the end of the prop strut/cutlass bearing should be equal to the diameter of the prop shaft, a 1 inch shaft should have a 1 inch gap. There is a little wiggle room in this, a ¼ or ⅜" as the distance gets larger there is more unsupported shaft which can cause issues. Adding a spacer without shortening the shaft would increase the gap.

In reverse the prop is much less efficient, so it would be easier for the engine to get to a higher RPM than in forward. For example on my transmission, forward has a ratio of 2.63:1 while reverse is 1.95:1. The prop spins faster in reverse to compensate for its inefficiency.

Hauling the boat will allow for a thorough inspection of all the parts. As a starter, however, an underwater inspection may be far less expensive. Either dive or have a diver inspect the prop and shaft and take pictures. Look for the items I mentioned, growth, shaft exposure, damaged prop, and if possible play in the cutlass bearing. With that information you'll be better able to come up with a plan of action.

Since there are multiple possible causes try to have good data before taking action. Guessing and trying different solutions may complicate matters. A lesson I learned the hard and expensive way.
 

Tom J

.
Sep 30, 2008
2,301
Catalina 310 Quincy, MA
That looks like some sort of flexible coupling. I think, in order to align it, the flexible core would have to be removed, and the distance between the coupling faces would be measured at four points, equidistant from each other. The .025 measurement would be too great for a fixed coupling.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
The alignment was done to 0.025 to start. So not perfect, but I would think I'd see some improvement at the bare minimum.
I would suggest seat belts on the boat with that amount of misalignment :yikes:.

Take a look here for the process I use. I go lightly on a few of the steps. Should get you down to 0.001 to 0.002 if all parts are true. Lengthy, but does a good job with out too much guess work :
 

Attachments

Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
Consider the possibility that with the spacer installed, the shaft may not have been shortened
 
Nov 26, 2012
1,653
Hunter 34 Berkeley
Been there. Done that. It’s the transmission. Specifically, the clutch. The kanzaki transmission has a cone clutch that hammers when it slips. typically only happens when in forward gear as this is what wears out first.
 
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Dec 14, 2003
1,393
Hunter 34 Lake of Two Mountains, QC, Can
Been there. Done that. It’s the transmission. Specifically, the clutch. The kanzaki transmission has a cone clutch that hammers when it slips. typically only happens when in forward gear as this is what wears out first.
Ditto for myself. Was doing it at about 1400 RPM. Rebuilt the tranny and for good measure replaced the cutlass bearing and issue was resolved.
 
Apr 12, 2021
8
Hunter 34 n/a
Hey, just to give you guys an update. A guy at my marina has also had this issue and helped me diagnose the problem. It is the cone clutch. When manually lifting the gear selector (on the engine side) the vibration goes away.

Thanks for the help!
 
Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
Hey, just to give you guys an update. A guy at my marina has also had this issue and helped me diagnose the problem. It is the cone clutch. When manually lifting the gear selector (on the engine side) the vibration goes away.

Thanks for the help!
How does shifting it manually make the transmission shutter go away compared to using the shifter? Thanks for circling back with a resolution.
 

NYSail

.
Jan 6, 2006
3,048
Beneteau 423 Mt. Sinai, NY
The “spacer” is a shaft saver. When installed it is machined by a prop shop to fit perfect. Sheers if you hit anything that would damage your engine. It is not flexible. Have you looked at the engine while running and vibrating? Checked the engine mounts? Have a friend drive while you stay below with a beer and examine things. A
Missing or sheered bolt on a mount could cause vibration in forward and not in reverse based on direction and force of turning prop. Good luck.

greg
 
Jul 30, 2018
49
Beneteau 321 Tawas MI
Any chance it could be a loose rudder? Prop wash in forward over the rudder could be a cause.
 
Apr 12, 2021
8
Hunter 34 n/a
"How does shifting it manually make the transmission shutter go away compared to using the shifter? Thanks for circling back with a resolution."


Applying pressure to the shifter applies pressure to the cone clutch itself. Normally it's not under much tension, however with it slipping, it requires that extra little bit to stick. Like someone said in the comments above, apparently when the clutch slips and then catches it causes that vibration. After testing the switch on the transmission itself we tested the same method with the shifter lever as well, and pulling up and holding (into forward) achieves the same result - so it's got a temporary fix while I wait to replace the cone clutch.
 
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Dec 28, 2015
1,837
Laser, Hunter H30 Cherubini Tacoma
Sounds like the vibration is from a continuous slip and manually shifting it at the trans applies more force which stops the slip switch stops the vibration. Thanks!