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Old 07-11-2018, 07:06 PM   #1
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Vibration issue and motor mounts

Posted this on boatdiesel but figured I'd post here as well to see what people thought:

Would love to see if anyone has guidance here. The prior owner of my boat had a grounding event (there was some superficial fiberglass damage and both props got beat up), and I am in the process of fixing some things that werenīt picked up during the survey.

The starboard engine had a vibration that has since been fixed. The shaft was bent about 15 thousandths. The starboard engine now runs smoothly.

When the starboard engine vibration was fixed, the boat yard also re-aligned (even though I didnīt ask them to) the port engine.

When I picked the boat up, I noticed that the port engine was now bucking pretty badly, especially at low rpmīs. Amazing they didnīt see this. Never working with that yard again.

They pulled the boat out, checked the cutlass bearing, shaft, etc., and said everything looked fine. They spent a good deal of time trying to get the alignment better and the engine wasnīt bucking anymore, but it did have a minor movement between 900 - 1,500rpm.

I was done dealing with them because I had little confidence in them. I ran the boat for half the season, and didnīt spend any time at all in the 900 - 1,500 rpm range.

I just brought the boat to a yard I have more confidence in for my annual Volvo service and also asked them to work on eliminating the vibration. They pulled the shaft and determined that it was bent (by 12 thousandths at the center). They had it straightened and aligned the engine, and things improved. There is now a more minor engine movement (they said itīs not really a vibration, but rather an up and down movement) between 980 - 1,050 rpm.

They are turning their attention to the motor mounts at this point, which could possibly have been broken during the grounding. When they sea trialed the boat, they noticed that the inboard mount toward the stern had a lot of movement, and the inboard mount toward the bow had about half the movement.

The outboard mounts were stiffer.

They also notices the mounts were stamped with an "A" on them, and they sent a note to Volvo on the movement and the stamp, and are awaiting a response.

So wondering what people think. The mounts are pricey (around $800 a pop I believe) and there is labor too obviously.

Any thoughts on the movement differences? They checked everything else and didnīt find anything wrong (engine alignment, shaft alignment, cutlass bearing, prop).

Really what I am wondering is this. If the movement is only happening in that tight rpm range where I typically am not operating the boat (I am usually between 1,800 - 2,500), should I even worry about it, or should I just enjoy the boat?

Obviously one can keep going with these things, but I donīt want to throw money at something that isnīt really an issue.

Thanks, and sorry for the long post!

Mike
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Old 07-12-2018, 01:55 AM   #2
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I worked in several yards and owned a yard. Mostly commercial boats. In grounding damage I have repaired where the props are beat up, the shafts are usually bent more than .015 and almost always twisted. Each blade hitting the bottom is a new bend. The shafts I've seen are never just bent in one place. Also the strut alignment goes bad. The struts most often end up with a slight twist or worse. I sent the shafts out to a company whose specialty was marine shafts. Often it was cheaper to replace the shaft than repair it to like new.
With all the torque at play when the prop hits a hard bottom, sometimes the transmission bearings get damaged. I have seen roller bearings get flat spots on the rollers and that can create a vibration.
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Old 07-12-2018, 05:38 AM   #3
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Volvo has used Trellborge Cushyfloat mounts on some engines for years. The 1657 size cost is around $200 ea.

Engine manufacturers do not make engine mounts, they buy them. If you do have these mounts you will need the shore number stamped into the base.

Cushyfloat Product Range - Trelleborg IAVS


The nice thing about these mounts for replacement is that they come disassembled. You can use a sawsall to cut the stud on the old mount between the bracket and mount body, slide the old body out, the new one in, and thread the new stud in from above. This means you only have to lift the engine a few fractions of an inch and don't have to disconnect anything. Reduces replacement time by a bunch.

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Old 07-12-2018, 05:45 AM   #4
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I think $800 isolators/mounts are a bit pricey, unless that is for all 4.

I cant be sure, but unless a mount is damaged and misaligning the engine (just replaced my rears as one rubber had disintegrated) , it in irself should not cause the movement....but it is recomnended that isolators be replaced in pairs.... both rears or fronts or all at once.

Once the props and shafts are truly redone and there are no struts to straighten or realign.... the coupler, tranny flanges become next in line and then the engine and its mounting.....

I would make sure all the alignment issues are fixed before dropping money on parts, especially expensive ones (isolators are not usually that expensive).
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Old 07-15-2018, 02:44 AM   #5
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The prop was tuned, shaft straightened, and everything was aligned and checked. Still have some vibration. Here is a photo of the mount. Click image for larger version

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