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Before I knew any better (4 years ago, before SailNet fever) I had new mounts installed on my 3GMD, along with new shaft and cutlass (for excess play), etc. The main reason for the mounts was to reduce the fairly significant engine vibration at certain speeds. All work was done well, and has run well for the past 4 years and around 200 hours. However, the engine vibrations did not particularly change (not nearly as much as I hoped for). There is little to any visible movement of the engine at any speed, while vibrations are particularly bothersome at a couple of interim speed ranges that i would otherwise use a lot. I now usually avoid the 1600-2400 range for the most part, since it runs so much smoother below and above that, all the way to the absolute redline of 3600. As background, the motor has around 500 hours total time, and is in excellent condition including valve adjustments, etc. to be sure it is as smooth as possible.

I assume that part of the issue is that I don't have a lot of boat mass to absorb normal vibrations, being a 22 foot and 5600 pounds with motor right under foot under cockpit sole. However,I am still seeking ways to reduce the vibrations, and am thinking of another set of mounts, this time using Yanmar originals. The shop I used is very good, but not particularly a Yanmar shop, and I don't have the name of the actual mounts they used, but don't believe they were Yanmar parts. Anyway, a long way to get to my basic question: Are there really significant differences between thte usual after market mounts and the specific Yanmar items designed for the 3GMD? In other words, am I likely to see any noticeable change in vibrations felt through the boat. Also, is the fact that the motor seems to move/shake very little in current mounts a tell-tale of hard mounts? I have seen various YouTube videos of little diesels that seem to shake around quite a bit, and that is probably due to softer mounts that may or may not actually reduce the vibrations felt by passengers. My ignorance is showing here, but I would realy like to improve current levels, as that is the only single thing I don't particularly like in my boat, and I have to run several miles each way to reach open water, with a 4+ knot current to fight if I don't get the times right.

All thoughts greatly appreciated.
 

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I have the same engine Yanmar 3GMD (20 HP).

In the next few months I will be replacing my original Yanmar mounts with new Yanmar mounts. I looked at some after market mounts, and they do not seem as robust as the Genuine Yanmar. So I will go with Yanmar.

Here is another thing to look for. The Genunine Yanmar mounts are different for the forward and rear mounts. The have a pound rating (moulded number in the rubber part of the mount). The non Yanmar mounts you have may not be the correct size. I would install Yanmar mounts and make sure they are correct size. Front (where belts are) should be 100 lb and rear 75lb.

Most boats will probably have a resonant vibration range where the boat vibrates a lot and engine rpm should be avoided. Your range seems wide, maybe it is the mounts.
 

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A friend of ours had a mount failure on his 20 Yanmar decided to go with after market mounts (price) mounts were suposed to be exact replacement not. Work involved to make them fit outweighed the cost .Went with OEM and replaced all 4 mounts in an afternoon with boat in the water no issues with alignment and no vibrations.Actually vibration was less than before failure.Suspectedthe mount was failing mode for some time.(was a rear mount)
 
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