2 cracked exhaust elbows - MTU

[markc]

So, I noticed during the summer that there was an exhaust leak from both engines and now after removing the heat protection blankets it has turned out that one of the exhaust elbows on each engine (outside bank on each engine) has crack in it, one worse than the other, rather than the loose joints I was hoping for. These are in a dry part of the exhaust so I'm wondering why they have cracked - is it usual due to the cycling of high temperatures?

Secondly, I have asked MTU for a replacement cost, but should I be looking elsewhere for an aftermarket fabrication, or even look at getting these ones welded?

Here is a picture of one that is OK on the stbd engine

Here is a picture of the cracked one on the stbd engine. Failure on the weld on the right and also a crack on the flange

 

[jrudge]

I had mine welded ( albeit in the wet section )

 

[[2068]]

I’d start by removing and inspecting them.
The failure around that small block looks repairable, the crack around the flange depends on the state of the metal around and behind it.

 

[MapisM]

I have asked MTU for a replacement cost, but should I be looking elsewhere for an aftermarket fabrication, or even look at getting these ones welded?

I tend to agree with FP that the only crack visible in your pic looks repairable.
But in your boots, I'd definitely want that checked (and welded if feasible) by someone who isn't at his first rodeo.
It the crack would suddenly get much worse while cruising, it wouldn't take much time to get a total mess in the e/r.

As far as replacement goes, are you sure it's an MTU part?
Usually, the exhaust downstream of the turbos is custom made by boatbuilders, because there are different possible ways to skin that cat.
And if it was made by Ferretti, it's likely to have been actually built by this Company.

PS: I would also go around the "OK" elbow in your first pic with a thin piece of paper, to look for even the smallest blow of air while the engine is running.
I might be wrong, but it seems to me that there's some soot also around that flange...

 

[Deleted User YDKXO]

I tend to agree with FP that the only crack visible in your pic looks repairable.
But in your boots, I'd definitely want that checked (and welded if feasible) by someone who isn't at his first rodeo.
It the crack would suddenly get much worse while cruising, it wouldn't take much time to get a total mess in the e/r.
.

Agreed. I once had an exhaust elbow failure in a Fairline I used to own and the mess in the engine room was unbelievable as well as the exhaust smell being in the boat for weeks. Yes you are right; the elbows are not usually supplied by the engine manufacturer

I was very fortunate in CF too when Andrea spotted a tell take exhaust soot leak from the elbow on the port engine in my current Ferretti which turned out to be a failed seal which could have lead on to an elbow failure eventually. Exhaust elbows need to be checked regularly

 

[MapisM]

I was very fortunate in CF too when Andrea spotted a tell take exhaust soot leak

No prize for guessing where I learned to go around exhausts with a thin piece of paper to sense even the smallest air blows... :encouragement:

 

[Assassin]

That crack appears to be from poor welding or welding practices as most welds crack immediately next to the weld through heat fatigue and internal stresses. This can be welded and ensure you use someone who has the correct equipment to pre heat and post cool the weld and ensure they drill a small hole at the ends of the crack to relieve stresses and prevent it spreading, they will need the correct composition of welding rod and bevel it out correctly to get full penetration, and if you can get the metal composition then this will make it easier to weld correctly.

 

[Portofino]

What I,am thinking about is the aetiology.

Those expansion joints look new and I,am wondering if they are the correct spec ? Too stiff ??
Putting too much strain on the elbows as there’s not enough give in the short system.

The fact that it’s happened on both outside banks of the V8 ,s is no coincidence the pipeage is shorter and crucially potentially stiffer and has insufficient give in the L to accommodate the thermal dimensional changes .

MapishM is correct about is point about drawing the line of responsibility re risers to the builder .Most large engine suppliers basically send an engine that ends at the turbo flange . So the engines arrive at the yard crated sans exhaust paraphernalia.Builder fits that ,
Having said that on Hull #1 maybe #2 they collaborate , there are back pressure issues regarding tuning to get the required boost pressures.
Get that wrong could have a knock on effect re power output at certain points in the rev range and impact on excessive thermal stress .
What you see here is the effect of the thermal stress having nowhere to go .

So that brings me round to asking are the exhaust s OEM in every detail ?
Thinking there’s been a mod that’s strayed away from OEM
Doesn’t need much change .

Are there any other expansion joints anywhere else ,like a flexible pipe @ the last / furthest away end where they presume fit an expansion box / silencer ,or if underwater exhaust fit the box inside the ER at the WL ?

Are there any fart pipes at the transom as well as underwater exhaust?
As well as low rpm tick over function whereby the pressure is insufficient to force gas underwater theses additional above WL pipes act as an additional route out at full chat .All part of managing the gas flow ( heat build up ) and back pressures
Sometimes at full chat depending on the underwater exhaust positioning and design they actually start to suck out the gas as -ve pressure is developing in that region of the hull due to the hull speed aiding heat removal when it’s needed .

Which begs the Q ( assuming it’s all OEM ish ? - near enough not to effect normal running ) how have you been using the boat ?

Reason I enquire is there’s a x over zone between low rpm pressure and boat speed .
At the design speed planing the system should get full flow and sure max heat but it’s flowing out as intended .
At tick over the flow and pressure and heat is considerably lower obviously, but it can cope as the thermal stress is with the elbows tolerance.
However there may be a speed / rpm whereby the temps rise greater than the gas flow can get rid quickly enough thus overheating along with inadequate expansion joints = leading to thermal stress and cracking .
Say fast D speed, before the hump kinda speed .

At full chat in the zone planing then the gas flow increases heating all the parts more uniform and along with that the strain on the elbow is reduced .

So for me a Its could be

Wrong speed too low , back pressure issue can’t flow properly heat builds up .
Expansion joint can,t cope .

Not sure exactly which exhaust your late 90 ,s Ferretti has but Amarti pioneered the underwater a decade earlier and others followed.
Underwater exhaust ( if applicable ) just exaggerates the back pressure problem .
Straight through exhaust masks the problem more .

If it’s any help the expansion joints on my Itama are at the wet / cool far end where the pipe fits the Underwater exit box .

 

[Bandit]

It looks to be a cast part? If so its probably OEM. How old is it?

Are the bellows steel or rubber.

Bends like this don't last for ever and when you disassemble it you will probably find its in poor condition on the inside.

If its OEM it probably lasted 10+ Years get a new one.

 

[markc]

Thanks for all the input so far

Thanks P for the link to Cofeme - I have sent them a message. Hopefully they'll be OK with my total lack of Italian!

Portofino - the boat does have underwater exhausts, and supplementary ones. However, it turns out that the Port one has failed in the same way, although more spectacularly, along the weld for some sort of blanked off connection. The inside elbows don't have this weld, so that will explain the problem on both sides.

Here is a pic of the Port elbow. Much worse than the stbd...not sure if this looks repairable!

First job will be to try and get the bolts out to remove the elbows...that should be fun

 

[MapisM]

Are the bellows steel or rubber.
...
If its OEM it probably lasted 10+ Years get a new one.

I understand your doubt based on the pics alone, but since the bellows are still in the first (dry) part of the exhaust, any rubber would melt PDQ, I reckon!

I agree that the elbow is probably cast and OEM built, but still that doesn't necessarily mean it's an MTU part. I'd be surprised if it were, in fact.

 

[MapisM]

Here is a pic of the Port elbow. Much worse than the stbd...not sure if this looks repairable!

First job will be to try and get the bolts out to remove the elbows...that should be fun

TBH, even if you should find anyone swearing to be able to repair THAT crack, I don't think I'd trust such promise.
That definitely looks beyond repair, and if they are still the original ones, they did their job for longer than it's reasonable to expect, so replacing all 4 of them ain't overkill, imho.

I would expect Cofeme to get by with EN, but if you would like me to check anything with them to avoid any risk of misunderstanding, feel free to ask.
Btw, they are based not far from my home, and it wouldn't be a problem for me also to go there - unless I'm in Sardinia, as it happens atm.

As a last suggestion, be very careful with the removal of those bolts, which obviously sustained many thermal stresses.
You don't want them to become stripped, let alone break their head!
I wouldn't be surprised if it would be better to "prepare" them for the removal somehow - with localized heating, or whatever...

 

[MapisM]

Straight through exhaust masks the problem more.

I'd rather say that straight exhaust eliminate the problem completely.
It's a funny coincidence that I've just been with BartW in the e/r of BA, and I told him that I very much like its straight exhausts, which are imho the most reliable and longlasting system (leaving dry stacks aside, which don't make sense on P boats for more reasons than one).
Compared to u/w exhaust, there's no need for low rpm bypass and no backpressure differences depending on speed, due to the variable scavenging effect. What's there not to like?
Ok, they are a bit more noisy. But for those who like cruising in true silence, sailboats are the only proper answer anyway...

 

[Portofino]

I'd rather say that straight exhaust eliminate the problem completely.
It's a funny coincidence that I've just been with BartW in the e/r of BA, and I told him that I very much like its straight exhausts, which are imho the most reliable and longlasting system (leaving dry stacks aside, which don't make sense on P boats for more reasons than one).
Compared to u/w exhaust, there's no need for low rpm bypass and no backpressure differences depending on speed, due to the variable scavenging effect. What's there not to like?
Ok, they are a bit more noisy. But for those who like cruising in true silence, sailboats are the only proper answer anyway...

I suspect you misunderstood what the “ problem “ I was refer to ?
Its differential exhaust pressure at a certain low ish speeds where the UW element is not optimised and the secondary cannot really cope for prolonged periods .Hence the out of spec heat build up in that transition phase .

As you say not an issue with straight trough .


You see if the existing elbows are not ages old ,I seem to remember Marc’s boat had some engine out work not too long a go by the PO -3-4 seasons back , then one really needs to understand the aetiology ( cause ) .
Other wise the new elbows just bolted back will go the same way in the same time frame .

As mentioned earlier I would look at the metal bellows specs as well as agreed replace the elbows ,welding is a stop gap imho .


WD 40 soaked for 24 hrs , keep popping down and soaking every hr or so and leave overnight for the bolts .

or an ice pack , use a bag of frozen peas wrapped round for 10-15 mins before applying the force - shrinks and cracks the rusted seal

 

[[2068]]

Wow, I don't think the elbow in that second picture is repairable.
:ambivalence:

 

[markc]

I would expect Cofeme to get by with EN, but if you would like me to check anything with them to avoid any risk of misunderstanding, feel free to ask.
Btw, they are based not far from my home, and it wouldn't be a problem for me also to go there - unless I'm in Sardinia, as it happens atm.

As a last suggestion, be very careful with the removal of those bolts, which obviously sustained many thermal stresses.
You don't want them to become stripped, let alone break their head!
I wouldn't be surprised if it would be better to "prepare" them for the removal somehow - with localized heating, or whatever...

Many thanks for the offer P, I've not yet heard from them, so will prod them in a couple of days.

My plan is to use WD40 Penetrating Rust Release - treat all bolts a few times over 1 day then try and remove them the following day. I have resigned myself to not getting out on the boat this week, so I can plan fitting the new ones when I am next down here.

 

[MapisM]

I suspect you misunderstood what the “ problem “ I was refer to ?
Its differential exhaust pressure at a certain low ish speeds where the UW element is not optimised and the secondary cannot really cope for prolonged periods .Hence the out of spec heat build up in that transition phase .

As you say not an issue with straight trough.

Nope, I understood what you were referring to.
I was just arguing that straight exhaust pipes eliminate the problem, rather than "mask" it - which to my ears sounds like the problem still exists, but it's somewhat "hidden".
Otoh, it might be just a matter of semantic: if mask in this context actually means solve (or anything along these lines), then I fully agree of course. :encouragement:

 

[MapisM]

My plan is to use WD40 Penetrating Rust Release - treat all bolts a few times over 1 day then try and remove them the following day.

That could be fine, I'm not sure. When I envisaged localized heating, it was just meant as an example.
For all I know, also the cooling suggested by PF could work.
One thing I would NOT do if it were my boat is insist too much, applying a strong force with a long lever.
Before risking to break a bolt head, I'd rather ask some expert!

 

[Assassin]

From what I can see from the picture it appears to be one of two things, poor quality castings or not enough material to dissipate the thermal expansion and contractions, and I suspect a combination of the two, as already stated the engine supplier stops at the turbo output flange and the boat supplier begins from there and as this appears to be the case it is very unlikely they are OE equipment, possibly the cheaper castings found in Asian factories doing low volume casting work.

Mapis, this could be successfully welded with no problems, but as it appears to be a casting of a poor quality and a clearly defined stress line, you are right to be concerned as to the durability and reliability of any welded repair as while you can weld any cast material, any poor quality materials or components with an inherent flaw would be so compromised to the point of "why take the risk" in this instance.

In this instance I would agree with Portofino, ice pack it after covering it in penetrating oil instead of heating it, having it contract as opposed to expanding seems a much safer option as any expansion would open the crack and it may explode in your face if heated slightly too much.

 

[vptech]

Common with overtightening, torque spec here is normally 20-25nm or less just clip it in lightly and wind until the leak stops