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Old 08-17-2014, 10:14 PM   #1
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Best way to quiet my Lehman 120 Exhaust

Hi All:

So I have a pair of 120 Ford Lehmans which currently have a very "droning" exhaust note at most any RPM.

From my engines there is about 6ft of 3.5" wet exhaust hose going to Stainless lift type mufflers which then slope very gradually downhill via more 3.5" hose to straight outlets at the stern.

I bought a Vetus DMP90 muffler and installed it inline on my stbd engine (to see what if any difference it made) and (despite Vetus assurances) it made absolutely no difference at all. The "muffler" would need significantly more water flow (at least 2-3 times IMHO) to make any real difference.

I have ordered and am planning on trying a Vetus LT90 water lift right at the stern on the port engine in a few days in hopes that the mixing right at the transom may make a difference.

Wondering what (if anything) others have done to quiet the exhaust note.
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Old 08-17-2014, 10:32 PM   #2
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There used to be a company, who's name escapes me at the moment, that made transom mounted exhaust outlet diverters out of heavy rubber that directed your exhaust down into the water. If they are still around you could try those.

Or make one yourself out of hose and a fiberglass 90 deg. fitting.
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Old 08-17-2014, 10:54 PM   #3
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Are you sure the noise is coming from the exhaust? Non turbo diesels have a lot of noise coming from the air intake and that is fixable.
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Old 08-18-2014, 12:14 AM   #4
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That sound is pretty common to Lehman's. If it changes to a hollow sound it means you've lost or are loosing raw water flow.
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Old 08-18-2014, 01:15 AM   #5
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I'm having trouble visualising( what's the audio description of visualising) a droning exhaust note over the shaking and rattling the old girl makes when I start her up.
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Old 08-18-2014, 02:48 AM   #6
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Quote:
Originally Posted by Rduval View Post
...I have a pair of 120 Ford Lehmans which currently have a very "droning" exhaust note at most any RPM...
Wondering what (if anything) others have done to quiet the exhaust note.
Only thing I can think of is shutting them down. They`re Lehmans.
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Old 08-18-2014, 02:57 AM   #7
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Our two FL120s make a fair amount of mechanical noise inside the boat. But there is no exhaust noise at all unless one stands on the aft deck right at the transom. We have a custom exhaust system (see photo) with fiberglass lift mufflers feeding the stock exhaust hoses. The mufflers, which have drain fittings at the bottom, and the fiberglass top elbows, were made specifically for our engines and boat by an exhaust shop in Bellingham. The diesel shop we use made the platforms the mufflers sit on and installed the systems using water-cooled manifold elbows from American Diesel. The exhausts themselves are pretty quiet at the transom.

A nice feature of this system is everything but the upper elbows are below the rear of the manifold. So no water can make its way back through the system and into the manifold. This was not the case when we bought the boat. The systems on the boat when we acquired it had the mufflers suspended from the engine room overhead.
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Old 08-18-2014, 03:27 AM   #8
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The systems on the boat when we acquired it had the mufflers suspended from the engine room overhead.
Yikes! That is not good.

A couple of years ago I had the unfortunate experience of the muffler box being raised higher than the manifold, when our boat was slipped. That cost about $24,500 and six months off the water. Thank god for insurance.

By the way Marin good to see you around again.
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Old 08-18-2014, 07:27 AM   #9
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We have a 3.5" exhaust on our stbd engine and a 3" exhaust on our port engine, both 2715e Lees Marine (same engine as Lehman, different marinization). The Lehman manual calls for a 3" exhaust. The 3" exhaust on our port engine has a mellow, fairly quiet sound. The 3.5" exhaust has a throaty sound, much louder than the 3" exhaust. Perhaps your problem is the oversize exhaust and not the muffler.
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Old 08-18-2014, 07:43 AM   #10
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Originally Posted by Capt.Bill11 View Post
That sound is pretty common to Lehman's. If it changes to a hollow sound it means you've lost or are loosing raw water flow.
Funny you should say that Capt Bill, I'm also seeing a lot of steam in the exhaust which I'm told shouldn't be there if there is enough cooling water flow.

Impellers are new 3 months ago, raw water strainers are clear... How do I know if I'm getting enough.
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Old 08-18-2014, 07:48 AM   #11
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Originally Posted by Ski in NC View Post
Are you sure the noise is coming from the exhaust? Non turbo diesels have a lot of noise coming from the air intake and that is fixable.
That's something I've thought about as well but in this case it's definitely exhaust that's the problem but I'm open to suggestions on quieting the intakes as well.

What's the solution to that? I've just got the standard coffee can type screens with foam around them at the moment.
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Old 08-18-2014, 07:53 AM   #12
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Originally Posted by Tomas View Post
We have a 3.5" exhaust on our stbd engine and a 3" exhaust on our port engine, both 2715e Lees Marine (same engine as Lehman, different marinization). The Lehman manual calls for a 3" exhaust. The 3" exhaust on our port engine has a mellow, fairly quiet sound. The 3.5" exhaust has a throaty sound, much louder than the 3" exhaust. Perhaps your problem is the oversize exhaust and not the muffler.
That's something I've considered as well. It sure doesn't seem like it would need a 3.5"ID exhaust system but that's what the Lehman mixing elbow is so it's been continued all the way through.

I noticed Marin's photo of his custom exhaust that the outlet looks about half the size of the inlet. Is that that case Marin?
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Old 08-18-2014, 08:20 AM   #13
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Could it be bad or dried out hardened engine mounts?I know that bad engine mounts can cause a drone thru the hull, that can sound like exhaust noise.
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Old 08-18-2014, 08:39 AM   #14
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Could it be bad or dried out hardened engine mounts?I know that bad engine mounts can cause a drone thru the hull, that can sound like exhaust noise.
Nope, brand new mounts that made the boat MUCH quieter overall. This is definitely the exhaust note.
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Old 08-18-2014, 09:43 AM   #15
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A 400 cu in old diesel engine is bound to be noisy. I'll bet if you had the engine suspended by magic and not touching the boat the "droning" noise would be gone. You're probably not getting the "rumrumrumrum" noise directly from the engine.

The engine shakes the boat and lots of parts of the boat shake at various frequencies and those parts in turn shake other parts at frequencies not the same as the original exciter frequency. A bulkhead may be resonant to 1100 rpm and your reduction gear provides that frequency through the propeller shaft and prop.

Whenever you have two frequencies vibrating at close to the same frequency another vibration and frequency emerge called the "beat frequency" in electronics. It's frequently a low frequency like the drumming of a twin engined boat or airplane w the engines not synced. It's usually called a "harmonic" frequency.

Look for something like a bulkhead or other flat panel that "drum". Feel all around for things other than the engine that are vibrating. It will probably feel like the vibration is coming and going. A fix is to attach something solid to that thing vibrating and not in the center is best.

Another thing to consider is the exhaust hose shaking things. Exhaust hose is frequently very stiff and will pass shaking to another boat part ... like a plywood platform that a lift muffler is attached to.
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Old 08-18-2014, 09:51 AM   #16
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That's something I've considered as well. It sure doesn't seem like it would need a 3.5"ID exhaust system but that's what the Lehman mixing elbow is so it's been continued all the way through.

I noticed Marin's photo of his custom exhaust that the outlet looks about half the size of the inlet. Is that that case Marin?
It does look that way. But it shouldn't be. Might just be the way the picture makes it look. That is pretty much the standard exhaust set up on a GB.
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Old 08-18-2014, 09:54 AM   #17
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Funny you should say that Capt Bill, I'm also seeing a lot of steam in the exhaust which I'm told shouldn't be there if there is enough cooling water flow.

Impellers are new 3 months ago, raw water strainers are clear... How do I know if I'm getting enough.
When was the last time you boiled out your coolers? And how old is you mixing elbow? Is it SS or stock steel?
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Old 08-18-2014, 09:56 AM   #18
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Many good suggestions, especially quieting the intakes. I'll add consider sound insulation under the deck. Feeling for vibrating items is a good idea. There is an app for iPhone called Db meter you can point your phone at something to see if sound is coming from there.
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Old 08-18-2014, 09:59 AM   #19
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Steam usually means insufficient water flow. Water quiets exhaust.

Is the Sind more apparent outside at the stern.
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Old 08-18-2014, 10:12 AM   #20
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Call me crazy but I guessing this guy knows the difference from a sound made by vibration and the sound his exhaust is making at the transom.
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