Do new engine mounts equal less noise and vibration?

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Mappy

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Nov 9, 2010
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Hi All

I have a Californian 34 with twin Perkins 4-236 85hp diesels. The boat is a 1981 and I assume the engine mounts are also 30 years old.

My question is whether replacing the engine mounts would result in an appreciable reduction in vibration based noise?

See the attached photo for the type of mount now in place.

Thanks

Roger
 

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Good question Roger. Within the last month I have removed the injectors on my 135 Lehmans and had them tested. I have a noticeable vibration at idle I am trying to solve. My motor mounts were replaced about 6 years ago by the prior owner after being noted on a survey. The injectors were OK, so 2 weekends ago I disconnected both prop shaft couplers and checked alignment (even though it only shows up in neutral at an idle), and other than a minor adjustment on the stbd engine, all was well. I was able to snug up the fasteners that hold the mounts to the stringers a little bit, and this seemed to help. My next step will be to talk with my diesel guy to see if there are any minor fuel adjustments can be made.
I look forward to any input from the forum.
 
Check out the mounts very closely as you may find the resiliant mounting material has deteriorated over time.
I have found my gen set mounts fail after about 5/6 years.
Benn
 
By far the best way to mount an engine in a boat is w an Aqua Drive or Python Drive. That's a thrust bearing hard mounted to the hull and able to take considerably more thrust than the engine can develop. In the 10 to 15" between the thrust bearing and the TX flange is a constant velocity joint much like that of a fwd car. The engine can move in any direction much more than it need to on very soft rubber engine mounts. One of the advantages of diesel electric or hydraulic drive. Yanmar said to me in the late 90s to replace eng mounts every year. Over kill I think but they do break down over time and are subject to lots of abuse. I bought Poly Flex mounts and am very happy w them. They are made out of plastic. I think they actually absorb vibration. www.polyflex.com.au None of the above will cure some vibration problems. The engine vibrates the boat and ther'es so very many things on a boat that can vibrate. Through harmonics and resonance almost anything on a boat can get set off by the engine's vibration. The smaller the engine, the higher the rpm and more cylinders the better but not always. Some materials and boatbuilding methods are more subject to vibration than others.

To answer your question Roger*** ...probably and almost certainly yes. But not guaranteed. First check the mounts relative to what weight they are designed to support. The fwd pair are usually to frequently different. The aft mounts need to support the flywheel and the gearbox as well as half the engine. My aft mounts are attached to the transmission for this reason. One also gets more pitch stability doing this. R&D engine mounts are good low tech products. Then do as Forklift has done and do a very fine job of alignment. Start w the prop shaft. This can be challenging if there is an intermediate bearing. That will locate the fwd end of the prop shaft and it's flange. Then align the engine in it's yaw axis, it's pitch axis and it's height. Ten thousandths is frequently considered OK but I'd settle for less that 5 and work hard for less than 2. Like nut torquing you may need to do it again soon after work has been done. The only way to know you're done is to do it several times and find no change at all. Of course all this is done in the water so the boat is supported evenly.


-- Edited by nomadwilly on Tuesday 26th of April 2011 09:57:06 PM
 
Mappy wrote:
Hi All

I have a Californian 34 with twin Perkins 4-236 85hp diesels. The boat is a 1981 and I assume the engine mounts are also 30 years old.

My question is whether replacing the engine mounts would result in an appreciable reduction in vibration based noise?
On my last boat,*after checking alignment, prop balance, injectors and intermediate bearing, went to softer ME mounts to try to get more a smoother cruise at 1600 rpm.*

The result was that the vibration became smoother at 1600, but more pronounced at 1400 and 1800.

While the solution worked somewhat in this case, sometimes it is not uncommon for different/newer mounts to simply shift the vibration to another rpm range.*

Also, in general, it can be very difficult to smooth out 4 cylinder engines.

*
 
Roger, what RPM are you running at? I have the same boat/engines and have for the most part run at 2350 RPM for 8 Kts. That's what the PO did and I have followed his advice.

Two weeks ago, I spent a week on the boat and had the time to try other power settings. I found that 2100 RPM cost me about 1 kt with MUCH improved sound levels and vibrations. Conversations were much less strained in the salon during cruise. I probably have aging mounts, too, but found this reduced power to be a great improvement.

Keep us posted on your findings and decisions.
 
Hi Flywright

You say you run at 2350 rpm. That is WOT on my boat!! The PO ran at 17 - 1800 and I am finding 15 - 1600 to be quieter and less vibes. There is only about .2 or .3 knots diff from 1700 to 1500.

Maybe I just need to get used to the somewhat buzzy 4 cylinders.

Roger

*
 
Thanks Eric

There is not enough room to do the thrust bearing installation so I have to stick with mounts that will take the thrust force as well.

Thanks for the link to the Polyflex mounts and the info on possible different front and rear mounts.

Roger

*
 
Soundown the noise control folks have a line of mounts that may help, and is built to take the engine thrust.
 
I replaced all 4 mounts on my FL120 because they were obviously collapsed.* Can't say that I really noticed any real difference in vibration.* The racks in the oven still go nuts when the engine is at certain RPM's.

*
 
BaltimoreLurker wrote:


I replaced all 4 mounts on my FL120 because they were obviously collapsed.* Can't say that I really noticed any real difference in vibration.* The racks in the oven still go nuts when the engine is at certain RPM's.

*




That's what I'm trying to solve at an idle!! Maybe Soundown should offer "stove mounts" :)
 
Forkliftt wrote:That's what I'm trying to solve at an idle!! Maybe Soundown should offer "stove mounts" :)
Whenever I'm not on the boat I tell myself to just bungee cord the racks.* When I'm on the boat and they rattle I tell myself I'll get to it back at the dock.* One of these days I'll actually do it!

*
 
Old Stone wrote:
Hmmmm - The racks might vibrate, but does the food taste better?
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*Puts a whole new meaning to "shake and bake".

(mine rattle too)

*
 
Thirty year old* mounts - you indeed need new ones for not only potential smoother operation but maintaining alignment during operation. It is not uncommon to have old mounts fail with less than nice happenings. You may want to pick them up in Seattle to save some $$ vs Canada.
 
Mappy,

I'm thinking you may be very over propped or even under propped. We have a number of Willard 36s w that Perkins engine and they talk about 2000rpm. One had me run a prop calculator on Boat Diesel and he gave me 2000rpm to work with.
 
We store stuff in our oven, so no rack rattles. The top however can make some noise at a certain rpm, dish towel and then cutting board on top of it fixed that. Replaced my FL120 mounts last year and it made a difference, I would recommend it. Is your idle set too low?
 
Daddyo wrote:


We store stuff in our oven, so no rack rattles. The top however can make some noise at a certain rpm, dish towel and then cutting board on top of it fixed that. Replaced my FL120 mounts last year and it made a difference, I would recommend it. Is your idle set too low?



Daddyo,
It's possible it could be too low. I'm showing around 700 RPM's on the lower helm tachs. I don't really trust these old tachometers. I want to get a photo tach and set the idle with them. Does anybody have the idle speed spec for the Lehman 135?
 
Mappy, My WOT is 2800 and that was confirmed recently when replacing the Stbd fuel pump.

I use my new stovetop cutting board to calm the grates and the oven contents quiet the shelves.


-- Edited by FlyWright on Thursday 28th of April 2011 02:02:54 PM
 
Idle is 600-700. You definitely want to get yourself a handheld infrared tach and calibrate on a regular basis your tachs. You can probably pick one up at Harbour Freight. I think mine costs me about $30
 
I get a pretty rough idle below ...* oh ... 800-900 RPM, or around there.* What's the trick to drop the idle RPM and keep it relatively smooth?
 
Daddyo wrote:


Idle is 600-700. You definitely want to get yourself a handheld infrared tach and calibrate on a regular basis your tachs. You can probably pick one up at Harbour Freight. I think mine costs me about $30



Thanks Daddyo!
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Most engines have a sweet spot where they seem to run, sound and feel smoother.* For our 671 its around 1200 to 1400 rpm. Our 671 is hard mounted direct to the structs. I usually sit on the pilot house table that is bolted to the floor when my BUTT starts vibrating I know it not in the sweet spot. So instead of replacing mounts*first try changing the rpm.*
 
Just replaced my old 1982 Lehman 120 engine with a rebuilt long block (incl head and valves). While the old one was out, dropped in new motor mounts. The ones I bought were the standard motor mounts from American Diesel ($35 each).

So far there seems to be a SIGNIFICANT difference. Just ran the engine through the break in hours and will run it for 15 hours this weekend to get to our home dock in Rock Hall MD. I'll be monitoring the feel of vibration on this two day trip.

R.
 
Most boats are run at the sweet spot. Keep in mind though that the sweet spot has nothing to do with what rpm one should run. It's just a comfort zone. I suspect that most engine vibration felt is not so much the engine but hulls, bulkheads, rudders, prop shafts, windows, exhaust pipes and many to any other flexible part of the boat. Much of the time it's easy to find what's vibrating by going around in the boat while underway and feel flat surfaces and other things that can be resonant to the engine. I have a 4cyl boat that is quite smooth so you guys w 6cyl engines should be able to fix most all vibration issues.
 
With my last engine and old motor mounts, the vibration was felt on the fly bridge but not in the main cabin (for the same RPM).

I used to look at my St Croix davits while running. I could see them vibrating and shaking at certain RPM then smooth out at others.

R.
 
Hi Flywright

Can you tell me what prop pitch and diameter you are using. My engines reach max rpm very quickly but they top out at 2350 - 2400. You seem to get to 2800, there must be some diff somewhere. I have the normally asprirated 4.236 which are rated at 85hp each, is that what you are running?

Thanks

Roger

*
 
I suspect that most engine vibration felt is not so much the engine

All engines will vibrate , it is common for the engine mfg to attempt to place the worst vibration below idle , and above peak RPM.

Sadly not all work , so soft mounts built for a boat , not a universal or gen set (must accept thrust) style are required.
 
Mappy, The 4-236 Perkins is rated;

82hp at 2800rpm

194 ft lbs at 1460rpm

So at wide open throttle (WOT) you should run 2800rpm.

500rpm over propped is way overpropped.

Probably need 2" less pitch.
 
The 4-236 Perkins is rated;82hp at 2800rpm,194 ft lbs at 1460rpm.

I would use 2500 as MAX for , and contemplate proping to get down even lower.

These are not really robust engines.

What is the usual fuel burn per hour?
 
ralphyost wrote:


Just replaced my old 1982 Lehman 120 engine with a rebuilt long block (incl head and valves). While the old one was out, dropped in new motor mounts. The ones I bought were the standard motor mounts from American Diesel ($35 each).

So far there seems to be a SIGNIFICANT difference. Just ran the engine through the break in hours and will run it for 15 hours this weekend to get to our home dock in Rock Hall MD. I'll be monitoring the feel of vibration on this two day trip.

R.


Ralph,
I look forward to the follow up. My mounts appear fine but are at least 6 years old. How did you original mounts look?
 
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