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Old 25-07-2021, 09:29   #1
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Yanmar 2gm20 vibration

Hi, newer owner of a 81 sabre 28 it was repowered by PO. Recently we noticed a vibration when in forward at @ 2100 rpm, lower or higher it goes away.
Looking in the lazerette we can see the transmision/engine shaking.... Motor mounts?? Any ideas plz
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Old 25-07-2021, 09:59   #2
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Re: Yanmar 2gm20 vibration

There can be many reasons, and / or combinations of reasons for engine vibration. Start with the simplest possible problem, and proceed to the most complicated.

Is your engine fresh water cooled? The 2GM20 is raw water cooled, a 2GM20F is FW cooled. This does make a difference concerning the correct flexible engine mounts, as the engine is heavier on the starb'd side.

FIRST - how old are the flexible engine mounts? Does the rubber appear collapsed, partially dissolved or liquified, or cracked? Is the rubber still firmly attached to the steel parts of the mount? Are they genuine Yanmar mounts, or aftermarket ?

Are the correct Yanmar mounts installed on the correct side of the engine? As I recall, this engine model uses 2 different mounts. The mounts look identical, but differ in the shore rating [ measure of rubber firmness] The stiffer mounts are to be installed on the exhaust manifold side of the engine.

Are the mounts firmly attached to the engine mount brackets? Are the brackets firmly attached to the engine? Are the flex mounts firmly attached to the engine bed? Joe
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Old 25-07-2021, 10:23   #3
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Re: Yanmar 2gm20 vibration

What you've got is a harmonic vibration. The engine itself or some major part thereof, or for that matter the drive shaft/propeller has a natural frequency of about 2100 Hz. Something is not damping that vibration, and the engine mounts are an obvious choice. It could be something else,such as the shaft/propeller being out of balance, perhaps due to a ding on the prop. Some airplanes have certain RPMs prohibited from use in cruising because of harmonic vibrations that the engineers were not able to damp.
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Old 25-07-2021, 20:33   #4
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Re: Yanmar 2gm20 vibration

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Originally Posted by Key Wester View Post
There can be many reasons, and / or combinations of reasons for engine vibration. Start with the simplest possible problem, and proceed to the most complicated.

Is your engine fresh water cooled? The 2GM20 is raw water cooled, a 2GM20F is FW cooled. This does make a difference concerning the correct flexible engine mounts, as the engine is heavier on the starb'd side.

FIRST - how old are the flexible engine mounts? Does the rubber appear collapsed, partially dissolved or liquified, or cracked? Is the rubber still firmly attached to the steel parts of the mount? Are they genuine Yanmar mounts, or aftermarket ?

Are the correct Yanmar mounts installed on the correct side of the engine? As I recall, this engine model uses 2 different mounts. The mounts look identical, but differ in the shore rating [ measure of rubber firmness] The stiffer mounts are to be installed on the exhaust manifold side of the engine.

Are the mounts firmly attached to the engine mount brackets? Are the brackets firmly attached to the engine? Are the flex mounts firmly attached to the engine bed? Joe
This different weights are front and back. Not side to side. 2gm20f have 75lb mounts in the rear and 100 on the fronts.
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Old 25-07-2021, 20:36   #5
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Re: Yanmar 2gm20 vibration

It could be a number of issue. Was it doing it when you bought it or did it just start? Start with the easy stuff....dive on it and check the shaft, prop and cut less bearing. Rule that out then work on the inside
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Old 25-07-2021, 21:50   #6
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Re: Yanmar 2gm20 vibration

You have received some good advice already upthread.

The question about when did this start is very important. If recently, then something has changed and can most likely be resolved however if it has always been there, it may not be able to be 'fixed'.

Even Yanmar in it's 2GM20 operation manual, advises the engine may have harmonic vibrations in certain configurations - as explained upthread by tkeithlu. If so, the only solution is not to operate at that RPM.

I had a 2GM20 that from new, would vibrate badly at 2,500 but was OK either side of that.
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Old 26-07-2021, 07:02   #7
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Re: Yanmar 2gm20 vibration

Thanks everyone.. It used antifreeze does that mean freshwater cooled? The cutlass bearing and shaft seal were replaced by the previous owner 2 years ago. Since then i maybe 25-30 hours on it? It is a new vibration not noticed before, two weeks ago we went motoring, (no wind) for 5 hours no issues noted.
I'll be checking the motor mount bolds for tightness and check the rubber for collapsing..
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Old 26-07-2021, 07:10   #8
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Re: Yanmar 2gm20 vibration

As previously noted, if your engine data tag reads "2GM20 F", the engine is fresh water cooled. The 2GM20 version of your engine is raw water cooled. Also, all fresh water cooled yacht engines have a heat exchanger, [ work boats usually have a keel cooler].

If your engine has a pressure cap [ radiator cap in automotive parlance ] the engine is fresh water cooled. Joe
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Old 26-07-2021, 21:01   #9
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Re: Yanmar 2gm20 vibration

Quote:
Originally Posted by jlcarreiro View Post
Thanks everyone.. It used antifreeze does that mean freshwater cooled? The cutlass bearing and shaft seal were replaced by the previous owner 2 years ago. Since then i maybe 25-30 hours on it? It is a new vibration not noticed before, two weeks ago we went motoring, (no wind) for 5 hours no issues noted.
I'll be checking the motor mount bolds for tightness and check the rubber for collapsing..

It may pay to check the alignment of the prop shaft, there are youtube vids on that.
But yes could be harmonics as suggested earlier.
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