Engine Mounts

[Hawkshaw1]

I have a 2001 390 that I am enjoying. It's our first "big" boat.
I am slowly sorting it out and understanding how all the systems work.

I have a question about the main engine mounts. All four of them are equipped with a washer under the large lock nut and all four washer will spin under the nut. There isn't really play to speak of but the nut doesn't touch the washer.

I don't have any records from the previous owner but I suspect they're original. I don't know if I have a lot of vibration because I don't have a reference point. It seems smooth up to about 2,000 rpms after that the boat seems a little rough. I don't like the way it feels or handles past 2,000 anyway.

I certainly think I'll tighten each nut but does this automatically mean they're bad?

Thanks,
Hawk

 

[Lepke]

You're screwing with the engine/shaft alignment. Learn or get someone to check alignment. Bad alignment can wear bearings and shaft, sometimes quickly.
When I was young, 60 years ago, I ruined a rear main shaft on a transmission that was no longer made. So I had to have one made. That's when I learned everything I need to make a accurate alignment.

 

[Keysdisease]

No. There are other indicators that might mean your mounts are bad, but not what you describe.
Are you hauled out for the winter? I see you're in TN. As Lepke mentioned mounts are responsible for alignment, and an indication of misalignment can be excessive wear of your cutlass bearing. A close look at that might tell you something, if you're on the hard.
Another thing to look at is a close inspection of the mount itself. You don't indicate if your are in a CAT/Cummins or Yanmar 390, but there are things about most mounts that are pretty universal. Start by taking a close look at the mount under the injection pump. Fuel will ruin a rubber mount, and bleeding or leaks can lead to mount failure by contamination. If the rubber is "gummy" its bad.
Also mounts are typically 2 parts, a top and a bottom with a flexible medium, usually rubber, between. Is the "top" touching the bottom? Does the mount look like its loaded "square," that is the space between the top and bottom uniform at all corners?
Another test is at slow idle in neutral. Engines vibrate more at slower RPM, put your hand on the engine bracket above the mount and feel the vibration, then move it to the structure below the mount, you should feel a significant reduction in vibration. This simple "test" also leaves out running gear issues like alignment, shaft straightness and prop condition and focuses on the mount.
Lastly, 20 years is retirement age for a rubber mount. Rubber "settles" with age and after 20 years its pretty well settled. They may be "servicable" for many more years, but they will not reduce vibration as they did 20 years ago. Also, unless the stud fails, the mount is not doing to "break" and let the engine ram the forward bulkhead. All marine mounts intended to take thrust will "capture" the mount even if he rubber has completely failed.

If you replace, do all 4. Let them settle a few days before alignment, and final alignments should always be done in the water. The absolute easiest replacement is direct replacement. Different mounts almost always have different footprints, different mounting hole patterns and most important, height differences. Messing with the foundation to accommodate a different mount than a direct replacement just isn't worth the trouble or expense.

Good luck

I certainly think I'll tighten each nut but does this automatically mean they're bad?

 

[TStiles]

There is a science to choosing the correct mounts for your engine(s). I'd suggest starting with a write-up such as what you'll find here: https://www.avproductsinc.com/mobile-marine/fail-safe-mounts.html and give serious consideration to engaging an expert. Alignment is critical and can be the root cause of a number of issues including component wear (cutlass bearing, esp) vibration, noise, and more. I suggest not putting it off - have it looked into.

 

[Comodave]

I think that you are saying that the mounts are not bad but just loose. So I would tighten them up and then check engine alignment. Loosen the bolts on the prop shaft coupler slightly. Slide a feeler gauge in between the transmission side of the coupler and the prop shaft side. Measure 12 o'clock and 6 o'clock positions. Say you have a 6” coupler the difference should be no more than .006”. For each inch of coupler you get .001” difference. Then measure between 3 o’clock and 9 o’clock. Same thing, .001” per inch of coupler. If the difference is more than .001” per inch of coupler you need to do an alignment of the engine. You have to move the motor mount height so the couplers come essentially parallel within the allowance. That is the hard part, checking it is pretty easy but moving the motor mounts up and down to move the engine into alignment takes some practice. But if it is in alignment then checking it is all you will need to do. Good luck.

 

[Easternshore]

Not a reply but a question regarding the correct installation of motor mounts. I had mounts “serviced” and I’m not sure which mount is correctly installed. Per the two photos should the one nut be tight against the bottom as in photo 1 or against the other nut as in photo 2? Photo 1 seems correct to me.

 

[Comodave]

I think photo 1 is wrong. The bottom nut is a lock nut IMO. It will stop the top nut from working loose. I would snug it up to the top nut under the mount. But then again maybe not...

 

[Easternshore]

I think photo 1 is wrong. The bottom nut is a lock nut IMO. It will stop the top nut from working loose. I would snug it up to the top nut under the mount. But then again maybe not...

Thanks, you’re probably correct as I did find a stock photo of my engine showing both nuts snugged together.

 

[Nomad Willy]

Looks to me like photo #2 has twice as many threads holding up the engine.

The lock-nut is doing the locking function by the lock-nut on top of the engine “foot”.

 

[Gordon J]

These are the same engine mounts that I have on my Cummins 330s. They are made by Barry controls. The idea is that there should be an eighth of an inch clearance both above and below the rubber bumper. The eighth of an inch clearance is minimum. If the rubber bumper is bottoming out or snug against the top of the metal then it is not doing it’s job. The second not in the top picture is incorrect. The purpose of that nut is to lock the top nut into position so it doesn’t move. From what I can see in the pictures both engine mounts you seem to be properly positioned.

 

[Easternshore]

These are the same engine mounts that I have on my Cummins 330s. They are made by Barry controls. The idea is that there should be an eighth of an inch clearance both above and below the rubber bumper. The eighth of an inch clearance is minimum. If the rubber bumper is bottoming out or snug against the top of the metal then it is not doing it’s job. The second not in the top picture is incorrect. The purpose of that nut is to lock the top nut into position so it doesn’t move. From what I can see in the pictures both engine mounts you seem to be properly positioned.

So I should have both nuts locked together. Thanks to all for your help.

 

[Keysdisease]

There is a science to choosing the correct mounts for your engine(s).

This is correct, but for our boats I don't believe it's cost effective to do the engineering and often foundation modifications to get those last few percent's of isolation performance over a direct replacement. If it was a new build, or first vessel in a series, absolutely, for a 10 or 20+ yr old boat with no previous history of vibration issues, direct replacement is very adequate and by far the least expensive way to go.

I have 2 observations regarding Eastern Shores Barry Mount picture. First is that the mount appears to not be loaded "square." It may be the photo but the engine bracket should be parallel with the top of the mount so the mount is loaded in compression evenly. This is typically done with shims or washers depending on the mount configuration.

The second observation, and there may be some disagreement, is that as pictured with the stud being used as a height adjuster is not per manufacturer recommendation or best practice. On most boats like ours, the engine mounts take the thrust that pushes the boat through the water. The prop pushes the shaft which pushes the engine and through the mounts to the stringers pushing the boat. By "pushing" the boat up on the stud the cantilever effect does no favors to mount performance and increases the chance of stud failure.
The proper way for a mount to be installed is to do the coarse adjustment with shims and foundation design and use the factory adjuster for fine adjustment and to adjust for creep over time.
Having said that many if not most boats use the studs as height adjusters. This practice also can exaggerate vibration into the vessel by not transmitting vibration to the mount body directly but instead through the stud at a place above the mount body. Far from ideal for vibration isolation.

$0.02

 

[boathealer]

This is correct, but for our boats I don't believe it's cost effective to do the engineering and often foundation modifications to get those last few percent's of isolation performance over a direct replacement. If it was a new build, or first vessel in a series, absolutely, for a 10 or 20+ yr old boat with no previous history of vibration issues, direct replacement is very adequate and by far the least expensive way to go.

I have 2 observations regarding Eastern Shores Barry Mount picture. First is that the mount appears to not be loaded "square." It may be the photo but the engine bracket should be parallel with the top of the mount so the mount is loaded in compression evenly. This is typically done with shims or washers depending on the mount configuration.

The second observation, and there may be some disagreement, is that as pictured with the stud being used as a height adjuster is not per manufacturer recommendation or best practice. On most boats like ours, the engine mounts take the thrust that pushes the boat through the water. The prop pushes the shaft which pushes the engine and through the mounts to the stringers pushing the boat. By "pushing" the boat up on the stud the cantilever effect does no favors to mount performance and increases the chance of stud failure.
The proper way for a mount to be installed is to do the coarse adjustment with shims and foundation design and use the factory adjuster for fine adjustment and to adjust for creep over time.
Having said that many if not most boats use the studs as height adjusters. This practice also can exaggerate vibration into the vessel by not transmitting vibration to the mount body directly but instead through the stud at a place above the mount body. Far from ideal for vibration isolation.

$0.02

That's some great stuff -- thanks!