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Old 06-28-2016, 08:51 AM   #1
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Turbo failure and exhaust water backup solutions

I am starting this thread to hopefully attract some Mainship Yahoo forum members and others who have had turbo freezing problems, probably caused by sea water backing up into the exhaust. I have several solutions to offer and will discuss these in this thread once a few Mainship guys have joined this discussion from the Yahoo forum.

I will start the ball rolling by discussing turbo failures and the generally accepted criteria for a fail safe exhaust system design. Please refer to the attached drawing and pics.

Turbos usually don't fail by themselves. Over the road turbos on diesels last for hundreds of thousands of miles. When a marine turbo fails after a a thousand hours, it is almost always caused by something other than the turbo just crapping out by itself.

The most common cause is sea water backing up into the exhaust turbo throat. This occurs after the engine is shut down and waves hit the transom, forcing a slug of water up into the exhaust, over the riser and into the turbo.

A second cause is mixer failure due to poor design which lets water sit in the mixer until it corrodes through the interior metal and then down into the turbo.

Turbo symptoms of this effect are poor performance, usually low power and lots of black smoke. This often means that the turbo is stuck and won't turn. Sometimes the turbo will spin freely but won't perform well because of the blade tip to housing dimension being corroded out.

Take the exhaust riser off of your turbo and look inside. If it has heavy flaky corrosion and if the blade tip to housing dimension is greater than 0.030" then you have had water getting in. If your mixer doesn't self drain like the pic, below then you will soon have a problem. Also take a look at the turbo exaust throat and the extensive corrosion.

As you can see from the next pic, a good marine exhaust system should have the riser go up at least 12" above the water line. It should also have the mixer pointed downward so it will self drain after the engine shuts down. Otherwise a pool of water will sit in the mixer, corrode it and then flow back into the turbo.


The pictures are courtesy of Tony Athens, the moderator of boatdiesel.com and a crusader for good exhaust system design.

More to come.

David
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Old 06-28-2016, 08:58 AM   #2
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David

NAs can suffer as well from improper exhaust design. I've been on several vessels where this is the case when engines are installed near or below the waterline.
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Old 06-28-2016, 09:44 AM   #3
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You don't hear much about NA engines getting sea water back into them, particularly on boatdiesel (but boatdiesel is mostly a high power forum), but I know it happens.


I had it happen on my genset in a sailboat. The exhaust system looked fine: a water lift muffler with a high loop afterwards before it exited the transom near the waterline. But we sailed for a day or so in high winds and heavy stern seas and water pushed up over the loop, filled the water lift muffler and eventually flooded the genset engine. The solution was a side exhaust exit.


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Old 06-28-2016, 10:01 AM   #4
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What is crazy with installations like that Monk, Cummins has to sign off on the design for warranty purposes. I guess they figure it will at least make it past the warranty period and that is all they care about.
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Old 06-28-2016, 11:21 AM   #5
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Difficult in a small boat, but if there is a way to get a silencer well above the waterline, is to install a draining (or waterdrop / or separator) silencer. With these units any water that backs up the exhaust run goes into the silencer and out the drain, next to impossible (unless something fails) to get water past the silencer with these units. They are common on larger yachts because those engine rooms have the space above the waterline to use them.

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Old 06-28-2016, 12:03 PM   #6
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Difficult in a small boat, but if there is a way to get a silencer well above the waterline, is to install a draining (or waterdrop / or separator) silencer. With these units any water that backs up the exhaust run goes into the silencer and out the drain, next to impossible (unless something fails) to get water past the silencer with these units. They are common on larger yachts because those engine rooms have the space above the waterline to use them.

I saw something similar on a Mainship 34T I used to own. It had a lift muffler like on a sailboat but did not have enough room for a high exhaust loop. So they installed an equalization hose from the bottom of the lift muffler then out the hull right at the water line. With the equalization line the water level couldn't rise in the lift muffler beyond the waterline no matter how much water backed up and the turbo exit was 6-8" above that, so it was almost impossible for water to get up into the turbo.

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Old 06-28-2016, 12:10 PM   #7
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I have followed those discussions on boatdiesel for years now. I started in 1999 when Mainship switched to the CAT 3126 and had several turbochargers fail and also several engines became hydrolocked, some replaced.
Everything Mainship built as a 390 with those engines were poor designs and subject to ingesting water. And the factory "fix" was not to properly replace the elbows, but to change the location of where the exhaust went into the waterlift muffler.
Clearly they did not get it.
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Old 06-28-2016, 12:58 PM   #8
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Similar but not exactly, but under the circumstances the best they probably could do.

What people don't always realize, it that after any proper spray ring / mixer the water isn't necessary for cooling any more. Often it is used for additional silencing as in a lift, but it could be discarded right after the water injection as far as cooling goes.

There was a Power Catamaran that had the engine deep in the hulls, and there was no vertical space for a tall riser at the turbo. The water / gas mixed at the factory dump elbow, went down to a lift, and then up above the waterline where there was space to a separator. The water was separated and went to a below the waterline drain and the gas exited at the transom dry and quiet. Any water backing up the exhaust would just drain out the separator.


The only caveat is that the lift silencer needs to have adequate volume to contain any water that drains back into it after shutdown.

You need some vertical space to make these type systems work, but they are pretty reliable for keeping water away from your turbo




Quote:
Originally Posted by djmarchand View Post
I saw something similar on a Mainship 34T I used to own. It had a lift muffler like on a sailboat but did not have enough room for a high exhaust loop. So they installed an equalization hose from the bottom of the lift muffler then out the hull right at the water line. With the equalization line the water level couldn't rise in the lift muffler beyond the waterline no matter how much water backed up and the turbo exit was 6-8" above that, so it was almost impossible for water to get up into the turbo.

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Old 06-29-2016, 10:26 AM   #9
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Well, no one has posted a specific problem with turbo failure. Turbo failure due to exhaust water intrusion is a real problem. I suspect that many if not most downeast style boat have this potential problem due to low floor height. But even the Monk 36 is plagued with it. Also I will bet that a majority of mixer installations are not self draining and are "doomed to fail" to use Tony's words. I see a new case of water intrusion discussed every month over on boatdiesel.


So I am going to summarize the possible solutions to exhaust water intrusion and leave this thread at that:


1. Rework the exhaust elbow and mixer- This is what I did on my Mainship Pilot 34. I added 8" of height to bring it up to 16" above the water line and pointed the mixer down so it would drain. It was a fairly inexpensive fix- $850 for parts and welding, but everything came together to make it easy. A more realistic estimate is about $3,000 for parts which includes a new mixer.


2. Put in a surge tube- This isn't a real fix but sometimes works. You install a dead end fiberglass exhaust tube so when a slug of water comes in from the transom, the surge tube will absorb it and won't push up over the riser. This and the following "fix" is all you can do sometimes with limited head room over the engine.


3. Install an exhaust flapper at the transom- These certainly work but run the risk of sticking open and leaving you with no protection.


4. Reroute your exhaust out the side- This will protect from most stern seas, but if you are tied to a dock, you can still get waves hitting the side although they probably won't climb up far inside the exhaust.


5. Install a water separator- If it is installed high enough over the water line this can be effective. Works particularly well for gensets where it also eliminates the chug chug sound of water and exhaust exiting.


6. Put an equalization tube on a lift muffler- I described this fix in a previous post on this thread. It can work well where the water lift muffler's bottom is near the water line and you don't have enough room for a high exhaust loop.


7. Use a high mount turbo- Cummins 6BTA's have this capability which adds a lot of water line clearance. Needs to be installed with a proper draining mixer.


David
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Old 06-29-2016, 12:08 PM   #10
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Sometimes I swear there is a little gremlin that crawls up the exhaust pipe at 3am and gets all the way to the turbo and pee's in it, causing it to corrode.

Point is that I have seen 1. Systems that appear well designed yet have clear signs of wet turbos. And 2. Systems clearly flunking common sense design criteria that have been in service for 20yrs with zero issue.

But most wet turbos can be traced to bad design.
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Old 06-29-2016, 12:19 PM   #11
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Quote:
Originally Posted by djmarchand View Post
Well, no one has posted a specific problem with turbo failure. Turbo failure due to exhaust water intrusion is a real problem. I suspect that many if not most downeast style boat have this potential problem due to low floor height. But even the Monk 36 is plagued with it. Also I will bet that a majority of mixer installations are not self draining and are "doomed to fail" to use Tony's words. I see a new case of water intrusion discussed every month over on boatdiesel.

So I am going to summarize the possible solutions to exhaust water intrusion and leave this thread at that:

...

3. Install an exhaust flapper at the transom- These certainly work but run the risk of sticking open and leaving you with no protection.

...

David
I did this after a careful examination of my exhaust system on my old Mainship 34. Cummins motor has a high exhaust riser and a water lift down low. It was a 20 minute/$20 effort for a bit more peace of mind. I can't see why the flaps wouldn't be standard equipment - not a complete solution but easy to maintain or retrofit.
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Old 06-29-2016, 04:11 PM   #12
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Well, no one has posted a specific problem with turbo failure. Turbo failure due to exhaust water intrusion is a real problem.
It was for me due to a poor design similar to the your description of the Main Ship.

Cummins certified the installation of my 6BT 330 M-3's on my Carver 355 with the stock Cummins exhaust riser. Can't really call them risers because they didn't rise much at all. After 10 years the starboard engine turbo on the exhaust side was so corroded I can't believe it pushed any air. The port engine riser was changed by the OP at some point but the installer didn't design it properly either.

During the installation of the port engine reman (failure due to several issues including over propped) I had the mechanic (one recommended by Tony Athens and knows him well) install real risers on both engines. I really didn't think he could do much as the floor leaves little room. With some ingenuity he was able to find two risers than went between the floor supports. I installed boost gauges and in the 2.5 years since, the boost on both engines is the same as when the reman was certified by Cummins.
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Old 06-29-2016, 04:19 PM   #13
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Nicely said Ski, I agree with your observations. If its a boat, it can happen...



Quote:
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Sometimes I swear there is a little gremlin that crawls up the exhaust pipe at 3am and gets all the way to the turbo and pee's in it, causing it to corrode.

Point is that I have seen 1. Systems that appear well designed yet have clear signs of wet turbos. And 2. Systems clearly flunking common sense design criteria that have been in service for 20yrs with zero issue.

But most wet turbos can be traced to bad design.
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