Low RPM vibration--Hunter 280

[matts997]

I have a Hunter 280 w/ the Yanmar 2GM20F. It’s my first sailboat, which I’ve owned for a little over a year. We ran into something submerged in the marina last October and I thought I noticed a very slight vibration afterwards between ~ 10 and 20 RPMs (e.g., the life lines, dodger windows visibly shook a little). But it was subtle and I eventually dismissed it, thinking it may have been there all along and was “normal” boat vibration. Two weeks ago I went for a test ride with my mechanic (for another issue) and he brought up the vibration and said it was serious enough to warrant diagnosis (could damage the transmission). So I hauled the boat out of the water. Prop shaft checked out with a dial indicator (.003”-.004”). The prop was slightly asymmetric, out of balance and was repaired by a prop shop. Thinking the prop was the culprit, I put her back in the water today--but the vibration was still there, no change. My mechanic wants to check the coupler next, but I'm wondering if this is the next best step.

My questions are how much low RPM vibration is normal in a boat this size and engine configuration? And should I continue to track down the vibration—in the absence of other symptoms—and if so, what’s the next best direction to take? Coupler? Engine mounts? Pull the shaft? The dollar signs are flooding my imagination.

 

[Ralph Johnstone]

I noticed a very slight vibration afterwards between ~ 10 and 20 RPMs

Does the vibration disappear over 2,000 RPM ?

 

[matts997]

Yes..it attenuates > 1500 and disappears by 2000. Most noticeable when motoring slowly out of the marina at ~ 1000-1200 RPMs. No vibration when idling in the slip, boat needs to be in gear and/or moving fast enough for the prop to spin--guess that rules out engine vibration alone?

 

[kito]

Sounds like my ysm12. At low rpm it about shook the boat off it's stands. It smoothed out nicely as the rpm's increased. I replace the engine mounts and all is good. Your mechanic should have checked those first imo.

 

[Benny17441]

In the war between $$$ and low RPM vibrations I would question how much time I really spend in that RPM range. All these small engines have inherent vibrations at certain RPM which are usually tamed by the motor mounts. I know you said the engine did not vibrate at idle speed but how about in neutral at a range of 1000 to 1500 RPM? How many hours are on the engine? If you can completely rule out engine vibration then you have alignment, motor mounts, coupler, shaft, cutless bearing and prop. With your boat model going on close to 20 years I would not rule out softening of the motor mounts allowing some of the inherent vibrations to show through. I think your mechanic might be overplaying the gravity of the problem and the need for repairs at this point in time. For a vibration to damage the transmission it would have to be violent or ongoing for a long time. You described the vibration as "subtle" and with the usual complaint being described as "bone shaking" I would think yours is still minor. Is it woth it chasing a small vibration? I take it back to my first statement "how much time I really spend in that RPM range"

 

[matts997]

Good point about % of time spent at low RPMs--not much!

Not sure about vibration at ~ 1500 RPMs in neutral--I need to more carefully observe what happens at various RPMs in neutral at the dock.

I reviewed my receipts for the boat (going back to it's original sale) and the engine mounts have never been replaced--or at least no record of it. Is there any need to assess their integrity--or, in a 20 yr old boat just go ahead and replace them?

 

[kito]

I believe my mounts were 35 years old. They actually looked ok......until I pulled them out. On one, the adjusting stud was broke in half. My vibration at low rpm was really bad......but I was on stands.

 

[Ralph Johnstone]

Is there any need to assess their integrity--or, in a 20 yr old boat just go ahead and replace them?

You may want to consider robbing a bank before replacing them ! Maybe even two banks as the invoice is seven years old.

Check here for a few more details on replacing engine mounts.

http://forums.sailboatowners.com/in...ount.172452/&highlight=engine mount clearance

 

[Benny17441]

You cannot tell the condition of a motor mount via a visual inspection. It could be shot and it would likely look good. You could try to rock the engine back and forth by hand to see if there is a broken one (not likely in your case) and the easiest way to identify weak ones is when the vibrations become excessive. Like I said at 20 years they could be starting to weaken but by thirty I would go ahead and replace them. Replacing motor mounts entails loosening and lifting the engine and then the delicate job of re-aligning it. This work plus the cost of new mounts makes it a big repair to be done when it becomes necessary.

 

[matts997]

So that was $300 for a single engine mount! Well--all of this convinces me to take the "watch and wait" approach to the problem. The vibration itself is minor, goes away by the time I leave the marina, and then I put up the sails. I should be worrying more about my sailing skills--as woeful as they are--and less about a minor vibration. My mechanic does want to check the coupler, which I'm going to go ahead and have him do--if that's OK, I'll put the issue to rest.

 

[kito]

I bought my motor mounts from TOAD Marine supply. Their suggested mounts for the Yanmar GM series are these:
http://shop.toadmarinesupply.com/ma...berresults&pn=DF-2205-2&storetype=mdd&x=0&y=0
These are not original Yanmar mounts. They don't carry them anymore. I bought the Yanmars and were $150 each!

 

[Crazy Dave Condon]

Not knowing the specifics as to what exactly happened or what you came in contact, you may want to pull the boat for inspection of both the strut and shaft making sure they were not bent along with the oh heck I forgot the name of the pieces within the strut but check those out if replacement of the engine mounts does not solve your issue.