Insights on engine mount deterioration

I am looking for some insights on engine mount deterioration from the forum members. First the background: I have a trawler with twin Cummins 555’s, so big old heavy beasts. I am looking to cut down noise and vibration – of which there is quite a bit! The recommended process is to address vibration before investing in upgraded sound barriers etc. I am therefore wondering about the engine mounts. The ones installed are twin mount DF 242’s as per DF 242 Mounts | Marine Engine | Rubber Vibration Isolators, four twin mounts per engine. They are not obviously damaged or broken, but they are likely 25 years old. It would be difficult / expensive to change to a different engine mounting system, so replacement DF 242’s are the most practical option to consider.

My question for the forum is: do the elastomer elements harden over time such that replacement engine mounts - like for like - would decrease the vibration / improve things?

I look forward to thoughts and views!

I don't know if they get harder over time or compress. Might be worth finding someone to give a professional opinion with each engine running. Certainly motor mounts have improved significantly over the last 25 years. Think I would seek out some comparative information based on what you have and what's available.

Ted

I suggest that you search for vibration isolator catalogs that contain engineering information. Vibration isolaters do require some research or you risk not getting the results you hope for.

Isolators are rated for both vertical and horizontal loading and frequency. They also have another quirk, they can resonate which actually will increase the vibration you want attenuated. And remember, unless your boat has thrust bearings on your shafts, all the forces needed to propel the boat will be applied against the isolators. Just my humble contribution

I'd recomend Poly Flex mounts.

They make mounts for engines much larger than yours. Made in Australia and many are made of some kind of plastic. Just the part that absorbes the vibration. Mine are and they absorb the vibration very well w/o a lot of engine movement like soft rubber mounts. After 10+ years there is some rust on the metal parts. But they do absorb vibration.
I bought them thinking there must be something better than rubber. Consider a rubber ball. When you drop a ball on a hard surface it bounces back up almost as far as ot fell down. The ball repells or returns the most all of the energy. It does not absorb the energy. If it did absorb most all the energy the ball would bounce back a very small amount like an inch. A lot of damper plates use plastic absorbers instead of springs. Cars have springs but you know what the ride would be w/o the shock absorbers. Rubber "alows" the vibration. It does not absorb it. I would think though that if a mount did absorb engine movement it would get warm or even hot. Acting kind of like a brake. Can't explane that though.

We replaced our mounts last November on a FL SP135 and it was bit of a pain. The mounting holes were the same on the steel engine bed but the actual height of the mounts is what caused the grief. A day and a boat buck later they fit.

Have we noticed a difference? Not that we can tell but we were able to get the alignment better by a couple of thousands which can only help the running gear. I have no regrets about doing it. The mounts were 28 years old.

Nmuir
If those Df242 are what you have then we have the same mounts.
Yes, the rubber will age losing resilience.
Whether replacement will help I can,t say as I still have mine.

Before replacement ensure mount fastenings are secure, alignment is good, shafts are straight, couplings are trued, props are not damaged.

I've adjusted over the years and any misadjusted will make noise worse. Usually especially noticeable afterwords when a bunch of noise disappears.

An eight cylinder four stroke is fairly smooth at anything above dead idle. Mounts do not need to be as soft as they need to be on a six. Pretty much any brand properly spec'd should work.

Mounts do age and collapse, causing a sound short. Often there is a measurement to check for collapse, hopefully available in tech lit.

If vibes are an issue above idle, then the cause is likely something other than mounts.

I do'nt think there were rubber mounts in the 50's. Most engines were bolted directly to the wood engine beds. But I'm sure the flat head 6's were smoother than modern V8's. The flywheel was on the front of the engine to allow mounting the engine lower in the hull and reduce prop shaft angle.

Thanks all for the comments – perfect and what I was hoping for!


Eric: yes I spotted PolyFlex and they sound good however with the limited clearances I am hoping to stick to the same low-profile mounts. Otherwise I’ll have to be totally reconstructing the mounting system and I expect it will end up a bigger job than I can pull off now. Those budgeted boat-bucks are in great demand!

FoggySail: Thanks for the reminder on shaft and forces. Based on my readings it has to be addressed in conjunction with flexible mounts to address vibration-based noise – or indeed more flexible engine mounts without flexible shaft couplings can make things worse. I have some flexible couplings that I plan to put in place that apparently work well to help isolate engine vibration from the shaft (if I go ahead).

Larry: Thanks for the comments – great experience to share. Much appreciated.

C Lectric: Looking to isolate things I took a lot of formal Decibel readings and acecdotal vibration estimates whilst at the dock without the boat in gear, with little difference to when underway, so I think I can rule out prop and shaft misalignment for now.

Keep those comments coming!

Below is what a mount manufacturer says about natural rubber mounts. They are referring to performance here, and even after 10 years a rubber mount will isolate, just not as well as when it was younger. Rubber mounts are also susceptible to contamination from fuel, especially the one under the injection pump.

I will also add that "soft" is not necessarily "better." What is best is something specifically for your installation. Weight, weight distribution, thrust, space between gear flange and 1st bearing, engine and boat type all play a part in proper mount selection.

"The life expectancy of the rubber elements will be approx. 20 years in ideal circumstances. Unfortunately ideal circumstances are not feasible, therefore the (working) life expectancy will be approx. 10 years. The life expectancy of the rubber elements is dependent on the environmental circumstances (weather influences, contaminants, etc)."​

​

A bit off topic but what have you for insulation in the E.R.?. If insulation is poor a huge amount of noise will transfer to point spaces.
I used Soundown ,1" decoupled foam and it made a BIG difference. Access hatches were worst as tiny gaps bled noise. It,s not cheap but it is effective.
My boat had/has lead sheet under the sole but it needed the foam also.
Pay attention to any gaps, vent or access hole to e.r. as air transmitted noise loves those gaps.
Structure borne noise is much harder and less effective to deal with after fact.
Check the insulation and sealing carefully as maybe more noise than you realize is poor insulation and hole transmitted.

Engine mounts

Hi All
On The subject of engine mounts
Attached is a photo of one of the engine mounts on one of the Cummins V504m engines on my boat at the time of purchase. Apart from being cleaner nothing much has changed.
These engines where shipped from Indiana in 1969,Marinised in England by Parsons and fitted to my VanLent in Holland in 1971 so tracing the type of engine mount shown is proving to be a little tricky. The engines have clearly sat down a little over the last 47 years.

A "fitter", who had a look mentioned the best solution is just to jack up the engine a fraction and replace the rubber.I cant really see where the rubber goes.
It all seemed like a great plan until 30 minutes later when I just managed to prevent him from holing the boat on a separate and very minor job. At which point he was promptly escorted to his truck, paid in full ,and wished all the best.

However is this possible (not with the Fitter)
1 Can I just replace the rubber and if so with what type?
2 I presume hoisting the engine from above is preferable to jacking from below to avoid a jack going through the Hull
3 as we are talking mm is there a need to slacken off the drive shaft

Kind regards to all
Liam

Hi Liam.

I will almost guarantee those mounts are as per every other engine mount I have seen - where the rubber is bonded to the metal. A common failure on engine mounts is for the rubber/metal bond to fail. You cannot just replace the rubber.

So what you have to do is replace the entire mount - on yours attached on the bottom with the 4 bolts and the top single bolt to the engine mount bracket.

How to support the engine will be a trick. On my boat I did not have the clearance to build a support frame over, so I could not use an engine hoist. I ended up getting 4 small bottle jacks (one for each corner of the engine/xmission), having them on the engine stringers and jacking slowly. It is difficult to find things strong enough to jack against. Jacking against the hull would have holed it for sure!

I expect you will have to disconnect the shaft and perhaps other things too if they do not have the clearance and/or non-flexible lines. I also had to remove the transmission mount brackets to get the rear mounts in place.

It was a tedious job for me. Two days including the shaft re-alignment.

Your mounts look pretty low profile. If you can find matching ones it will be easier as you can avoid having to re-engineer everything, though finding matching ones may be a trick. I managed to find matching ones for mine at Home | Bushings Inc

EngineMounts...ahhhhhhh !

yea,
it sound like something I will approach with a lot of caution and a lot of research
That's good advice o the transmission mounts
Many thanks for your Time

Liam

"I used Soundown ,1" decoupled foam and it made a BIG difference."

Soundown also sells engine mounts , and has good info on the rest if the instalation, plumbing, piping, shifting , exhaust connection etc.

Mounts

Thanks for that
will certainly have a look