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Old 01-24-2021, 03:02 PM   #1
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Exhaust Mixing Elbow question

I have a 2008 Mainship 34T. Engine is Yanmar 6LYSTP. So age is 13 years and hours is 1630. in 2018, I noticed rust spots on top of the exhaust elbow. (Pic 1) This suggested that the elbow had corroded and could be due for replacement. So I opened the elbow and the inside looked really good. (Pic 2) At that time, I posted on Mainship Yahoo forum and the feedback was that if the elbow was stainless steel it should have a much longer life than a cast iron elbow. So I painted over the spots and decided to wait...

Fast forward 2 1/2 years and the rust spots are back ( but not that bad - Pic 4). So this time I sanded the spots to see if the rust is superficial or not. To me it looks like the rust is superficial as the sanding seemed to remove everything. (Pic 5) I have painted over and plan to wait another year or two.

So I am wondering if others have seen similar behavior or superficial vs through rusting? If I talk to the local dealer I am sure he would convince me to replace the elbow (and they could do the work). If I leave it, I have the impression that any failure would be gradual not catastrophic? I have seem mention of 2000 hours as a reasonable life for an elbow, so 1600 should be good for another 3-5 years?

Attached pics a) April 2018, b) Inside April 2018, c) reference cross section d) Jan 21 and e) Jan 21 after sanding. Any feedback or thoughts welcome.
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Apr2018_elbow.jpg   Inside_elbow.jpg   Elbow_section.jpg   IMG_0543.jpg   IMG_0549.jpg  

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Old 01-24-2021, 03:20 PM   #2
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Those rust spots are a result of residues from the cast iron mold that was used to cast the stainless steel exhaust elbow. To be sure, check the elbow with a magnet. If it doesn't stick it is stainless steel.

You can probably grind down the rust blemish with a Dremel tool and it won't ever come back.

I had the same engine, same rust spots and the elbow was SS. A pic of mine is attached with a minor rust spot on it.

BTW the inside of yours looks great.

David
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Old 01-24-2021, 04:04 PM   #3
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Sadly, the magnet test for stainless is specious. 304 and 316, common marine alloys, are somewhat magnetic, 304 only after it has been "worked" or formed somehow.

Some steel handling places use relative strength magnetism to determine stainless alloys and for sorting.
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Old 01-24-2021, 05:28 PM   #4
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Sadly, the magnet test for stainless is specious. 304 and 316, common marine alloys, are somewhat magnetic, 304 only after it has been "worked" or formed somehow.
Other way around - 304 usually slightly magnetic, 316 not unless welded or worked.

But the magnet test will surely tell you if it is SS of any alloy vs. iron.
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Old 01-24-2021, 06:02 PM   #5
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Just a word of caution. I am not an expert, and I am not familiar with your exact engine and exhaust setup. However, you may want to check out Tony Athens' website sbmar.com and look up under Tony's Tips or in the various forums info specific to your situation. There are a fair number of exhaust setups that are (using Tony's words) "doomed to fail" and when they do (they all eventually fail but these can cause real problems) they can allow exhaust water (usually saltwater) to get back into your turbo or even into the engine itself. Big problems if this happens!

You can join his forum and post good pictures from all angles showing your setup and ask for his and other forum member advise pertaining to your setup.

I am not trying to create a scare, just wanting to ensure that you don't get bit by "not knowing".

My Cummins engine had a SS exhaust elbow of the "doomed to fail" type. Luckily I caught the fact that the inside had corroded through, with my exhaust hose overheat alarm going off. The first time I removed the elbow for inspection, the inside looked very good. On the second removal, it was sent to the radiator shop for cleaning and then the internal corrosion showed up. I did not have any turbo damage so all was well. I replaced my corroded elbow with a new custom made "dry" riser with the water injected well below my turbo eliminating the "doomed to fail" aspect. I feel lucky that I caught it in time. My engine had just over 2000 hours and was 15 years old at the time.
I hope this helps.
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Old 01-24-2021, 06:26 PM   #6
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"AISI 304 stainless steel (UNS S30400) is the most widely used stainless steel, containing 18-20% Cr and 8-10.5% Ni, and also known as 18-8 stainless steel. Type 304 is non-magnetic under annealing conditions, but after cold working (such as stamping, stretching, bending, rolling), part of the austenite structure may be converted into martensite and therefore weakly magnetic."
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Old 01-25-2021, 11:16 AM   #7
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DavidM: Thanks for the info and pic. Your explanation makes a lot of sense.

Firehoser75: The site sbmar.com provided valuable insights re elbow failure issues. I believe my engine exhaust design would have a low probability for water getting back into the turbo charger. (Maybe other Mainship owners with Yanmar 6LYSTP engines could comment?)

Previously I had thought that failure would result in a lot of water leaking into the bilge, but it seems that a failure would be more likely to occur in the wall between the exhaust (dry) and cooling water (Wet) zones of the mixer then on the outer layer. That means that nothing would be visible even after the failure.
So that raises the question: If you have a poorly designed exhaust system and the elbow dies, then you engine can die as well, but if the design is good and water does not run back, how would you know if there was an internal failure within the elbow?
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Old 01-25-2021, 11:49 AM   #8
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Sadly, the magnet test for stainless is specious. 304 and 316, common marine alloys, are somewhat magnetic, 304 only after it has been "worked" or formed somehow.

Some steel handling places use relative strength magnetism to determine stainless alloys and for sorting.
Nothing scientific here, but my 2-cents is that SS is barely magnetic. Cast iron is about as magnetic as it can get (it is "Ferrous" after all). Should not be hard to tell the difference. Compare magnetic resistance to a SS handrail or something vs the engine block (cast iron). Given the apparent advanced age of the elbow, I'd guess it's SS.

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Old 01-25-2021, 12:04 PM   #9
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Quote:
Originally Posted by DavidM View Post
Those rust spots are a result of residues from the cast iron mold that was used to cast the stainless steel exhaust elbow. To be sure, check the elbow with a magnet. If it doesn't stick it is stainless steel.

You can probably grind down the rust blemish with a Dremel tool and it won't ever come back.

I had the same engine, same rust spots and the elbow was SS. A pic of mine is attached with a minor rust spot on it.

BTW the inside of yours looks great.

David
David
With the raw water addition in your picture on the uphill side from the turbo it would seem any failure of the elbow and resultant leakage would go back towards turbo and number 6 cylinder.

BTW, my 304 SS elbows made in England failed after cooling system treated with muriatic acid per Tony Athen's protocols, so said the yard. Never will use that stuff again. The upside was a good deal on specialty made 316 elbows.
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Old 01-25-2021, 12:28 PM   #10
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Hi PocketAces,

If the pictures of your mixing elbow are OEM and now 13 years old, you are (in my opinion, of course), WAY WAY past end of service life for that elbow. And, contrary to someone's observation that "...the inside of yours looks great", I'd have to vigorously beg to differ.

In the second picture, presumably taken after the elbow was removed and cleaned some 2 years ago, visible erosion of the inner wall of the elbow can be seen. And, if the elbow looks anything like the UTE mixing elbow shown in your original post, it is IMPOSSIBLE to determine the depth and/or severity of those erosion pits. And the external visual condition of the elbow means little. Rust inclusions are common on cast stainless parts, irregardless of the casting's chemistry. And again, meaningless as an indicator of an elbow's health.

A very wise and competent diesel mechanic, after surveying the failed exhaust risers on a boat I was purchasing, told me that he considers exhaust risers a routine maintenance item on ALL marine diesel engines, to be removed, cleaned and inspected every five years, and routinely replaced every ten years. That advice has saved my bacon for many, many years.

IMHO, you are on borrowed time with your riser. Loss of a riser underway can be VERY expensive, as you risk backflow of hot raw water, which can cause havoc with turbochargers and possibly endanger the main engine itself. Ditto the likelihood of backflow and failure, even with a brand new riser, on an engine with a poor exhaust installation. I second reading Tony Athen's work at Seaboard Marine. He's pretty well versed in exhaust stuff.

Regards,

Pete
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Old 01-25-2021, 12:45 PM   #11
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Pete
Some years ago we were in love with a Tolly 48. It had Cat 3208s. Feeling as you do about elbows being a routine maintenance item, we chose to pursue 6 cylinder T48s that had half as many elbows.
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Old 01-25-2021, 01:12 PM   #12
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"AISI 304 stainless steel (UNS S30400) is the most widely used stainless steel, containing 18-20% Cr and 8-10.5% Ni, and also known as 18-8 stainless steel. Type 304 is non-magnetic under annealing conditions, but after cold working (such as stamping, stretching, bending, rolling), part of the austenite structure may be converted into martensite and therefore weakly magnetic."
In theory, maybe. I work with a lot of stainless steel, rarely 304 is non-magnetic, and rarely 316 is magnetic. You can buy annealed plate if you specifically order it, but tube, most plate, fasteners, etc are all cold worked. Almost all 304 fasteners are slightly magnetic, while 316 are not. I've only seen 316 become slightly magnetic from welding, usually with too much heat.

304 would be a very poor choice for an exhaust elbow in salt water.
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Old 01-25-2021, 01:28 PM   #13
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Is that water stains on the elbow? If so where is it coming from?
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Old 01-25-2021, 01:37 PM   #14
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Why not engineer an exhaust pipe that moves the exhaust "elbow" a couple of feet from the engine, wrap that plain exhaust pipe for heat then put in a device to introduce salt water. Why have it right by the engine? If it wasn't installed on an angle it might not attract so much heat. Gases hate bends...if it does fail it won't bake your engine.
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Old 01-25-2021, 04:44 PM   #15
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David
With the raw water addition in your picture on the uphill side from the turbo it would seem any failure of the elbow and resultant leakage would go back towards turbo and number 6 cylinder.

BTW, my 304 SS elbows made in England failed after cooling system treated with muriatic acid per Tony Athen's protocols, so said the yard. Never will use that stuff again. The upside was a good deal on specialty made 316 elbows.
Good eye. I subsequently modified the exhaust by welding on an additional 8" of riser height that made the overall height above the waterline about 16" as well as turned the elbow down to make it self draining. I am very aware of the hazards of both issues.

David
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Old 01-25-2021, 04:52 PM   #16
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Pokcketaces:

I owned a Mainship 34T for several years before I moved from SoCal to the east coast and bought the MS Pilot 34 that the picture I posted above comes from. So I am very well aware of the 34T's exhaust system layout, dimensions and performance.

The 34T has a very different layout from the Pilot 34 in that it has a water lift muffler. The exhaust drops down from the exhaust elbow to the water lift muffler where it goes up and then down and out the side exhaust. The water lift muffler has an equalization hose, about 3/4" from near the bottom of the muffler to a thruhull just at the water line. The hydraulics are such that the water cannot back up from the lift muffler into the engine exhaust. The equalization line will drain it before it gets that high. So your exhaust is pretty safe.

I can't specifically remember if it also has a self draining injection elbow, but it think it does.

David
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Old 01-25-2021, 04:54 PM   #17
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Good eye. I subsequently modified the exhaust by welding on an additional 8" of riser height that made the overall height above the waterline about 16" as well as turned the elbow down to make it self draining. I am very aware of the hazards of both issues.

David
I do remember your expertise on this area and modifying that setup. Given your familiarity with Mainship elbows maybe the OP could post a picture of his exhaust elbow area setup for your comments.
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Old 01-25-2021, 05:43 PM   #18
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PocketAces,
Besides just following a timeline for elbow replacement, I recommend that you install an exhaust hose overtemp alarm. Both Aqualarm and Borel make these. They look like a large "zip tie" and it installs on the exhaust hose as close to the water injection point as possible. This is the first point in your "cooling system" where an overheat is likely to show up, and the only point where an elbow leak (internal) will be detected. That alarm is what warned me of my exhaust elbow's internal failure. Nothing was visible on the outside.

Personally, I installed the Borel as it alarms at a slightly lower temp than the Aqualarm, giving potentially an earlier warning. Engine overheats can do damage before the "normal" coolant alarm responds.
This alarm (exhaust hose alarm) is relatively inexpensive insurance. Total cost for parts is under $200 US.
My new elbow is dry and insulated for the first 3 (+) feet, and the path from the turbo is upward first, and the water is only injected at the bottom well below the "height" of the turbo, so it is "self draining" and water cannot run "uphill" to the turbo.
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Old 01-25-2021, 11:00 PM   #19
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Thanks folks for the varied replies! A lot to digest. I can get more pics of the exhaust set up when at the boat on Thursday. However, it seems that the prudent course of action would be to get a new elbow soon. Am thinking that the Yanmar replacement would be best as long as it is SS.

Based on the comments in this thread and magnetism etc, I am sure the current one is SS, but have no idea about 304 or 316 SS, but if the next elbow can last another 13 years, that will likely last longer than me!

If nothing else, a new elbow will provide a lot more peace of mind which would be a big plus.
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Old 02-01-2021, 10:51 PM   #20
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Is there by any chance something above the elbow is dripping on it creating rust spots?
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