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Hi world wanderers,
I have a custom wet exhaust attached to a yanmar 2qm20. There is a section of rigid stainless piping ~2ft which connects the manifold to the riser and point of raw water injection. It fractured 3 times after being remade in 6 months assumedly because of rust and vibration...
I thought i solved this by switching a section of rigid piping for a flexible stainless bellows. Which i just found to be rusted and cracked after a year.
I feel like the only option is to keep replacing the flexible section every year... Say it isn't so!!!
Any help greatly appreciated :)
 

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A mixing elbow made of iron plumbing bits is easy to make and replace. Should be good for 3 or 4 years and shows distress long before it fails. If you can go down off the manifold (injecting into) a piece of rubber hose onto a SS can, you got it made. Lots of talk on 'I hate boats' about this. You can buy big buck parts or make your own with a bit of ingenuity. Every body here has an opinion about this. Wagging a 2 'pipe doesn't sound like good engineering to me!
 

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The problem is possibly two failure modes going on simultaneously.

1. Stainless Steels (18-8) are very prone to fatigue failure --- engine generated vibration that exceeds 1 million load cycles at greater than 30% ultimate tensile strength of the component - the fatigue endurance limit of common stainless steels.
Such exhaust "dry stacks" are usually solely attached to the engine through a pipe adapter flange at the outlet of the exhaust manifold; the dry stack 'sticks up' from the engine similar to a 'flag pole'. This causes the structure to behave as a 'cantilever' whose SHAPE in complex beam structure formulary calculations reduces the inherent strength by approximately 1/4th at the 'root' of the cantilever --- where the dry stack connects to the engine exhaust manifold --- it's the typical failure point for large fatigue cracks. This failure is typical of cantilevers in dynamic situations.
The Rx for this is to support that dry stack at near its top with 'triangular bracing' to the engine ... could be just simple bent pipe, flattened and bent at its ends, attached to near midway to the top of the dry stack with simple hose/muffler clamps and attached onto the one of the cylinder head bots or any available large bolt on the aft vertical face of the 'head' (but NOT on the transmission). The 'triangular' shape of the drystack/engine/bracing is important, the wider the base of that 'triangle, the better. This arrangement will tend to make the top of the dry stack vibrate at the same frequency as the 'root' connection to the engine for less 'flex' at the stack to engine connection. Without the bracing, that 'contraption' sticking up and vibrating independently is a force multiplying LEVER.

2. Stainless is prone to stress corrosion ... in this case, from the sulphur oxides produced by the combustion of diesel fuel. Such combustion generates water and sulphur oxides which then form various sulphur and nitrogen derived acids: sulphur dioxide + water, HOT sulphurous and sulphuric acids, nitrous/nitric acids etc. If there are micro-cracks in the surface caused by the generation of fatigue failure these potent acids then penetrate progressively deeper into the metal via the micro-cracks causing deeper and deeper erosion and which in turn produce 'faster' fatigue, etc. which produces deeper penetration via chemical attack, etc. etc. etc.

A two phase failure!

Suggest you:
1. stop the top of the dry stack from vibrating out of phase with the engine by 'tying' it to the engine with a 'robust' triangular support. 3/4" to 1" thin walled stainless pipe 'will do'.
2. replace the stainless steel components (as they fail) with simple threaded moderately thick walled black iron piping. Black iron is the standard for such exhaust applications:
A. better fatigue resistance; and better high temperature service life due to the formation of 'protective' iron oxides on the internal surface due to the 'heat'.
B. better chemical resistance to exhaust gases due to less propensity to form surface micro-cracks due to better 'fatigue limit' of the material.
C. quite 'cheap' as is made from 'plumbing supply' parts, as well as easily replaceable.

Note - threaded cast iron exhaust manifold to 'dry stack' adapters are readily available at most Yanmar dealers, etc.

I realize that this response is 'quite technical''; so, if you need a simpler explanation, etc. just PM me.

;-)
 

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Sea Sprite 23 #110 (20)
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I would add to Rich's synopsis and solution to add that most sailboat diesels are not run long enough. To get all the condensation and such from the dry stack, the engine needs to get nice and hot and stay that way for a while. If you are not running it like that (and diesels tend to run cold at idle) all you are doing is adding moisture to the stack that is getting into the microfractures and forming more rust
 

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A mixing elbow made of iron plumbing bits is easy to make and replace. Should be good for 3 or 4 years and shows distress long before it fails. If you can go down off the manifold (injecting into) a piece of rubber hose onto a SS can, you got it made. Lots of talk on 'I hate boats' about this. You can buy big buck parts or make your own with a bit of ingenuity. Every body here has an opinion about this. Wagging a 2 'pipe doesn't sound like good engineering to me!
exactly stainless is brittle and a poor choice for exhaust systems in this environment, especially if it has been welded badly and or a lot.

black iron would be my choice and or simple plumbing pipe that can be replaced easily and cheaply

you can also wrap it to keep heat away and to a minimum inside, it will also improve flow a bit to exit gasses faster:)

edit I see all points have been made jajaja

however

this thread might be helpful as a visual or general reference
http://www.sailnet.com/forums/diese...elbow-plumbing-universal-m12.html#post2360650
 

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Christian makes a very important point. If you for some reason you 'must' use stainless, any welding 'must' be a FULL penetration weld and there must be NO welding 'inconsistencies' such as gas holes, teeny 'blow outs', dirt, slag, laps, porosity, undercuts, etc. in the weld or its 'heat affected zone' communicating with the inside surfaces. In other words, any welding must be PERFECT, or it will be vastly more subject to chemical attack than a 'perfect weld'. Such welding that comes in contact with hot exhaust gases 'should' also be ground smooth and polished. Such welding results are usually only obtained from an experienced and certified 'code' welder or a welder who uses automatic 'lathe-type' equipment to do the continuous welding.
 

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if using stainless go broke or just stick to tried and true

on a boat I wouldnt even consider stainless exhaust systems, money down the drain really
 

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I think iron pipe is the way to go, but use lots of pipe dope so you get a good seal, and even more importantly be able to get it apart. I like the stick that you rub onto the threads, but the paste works well also. I think it would be worth going to a good plumbing supply house and get good US made pipe (assuming you are in the US or Canada). The good stuff lasts much longer in aggressive corrosion situations. You won't get much more than a year or two out of the Home Depot stuff. Keep in mind the good stuff is significantly more expensive but I would expect it to last two to three times as long.

Good thing about iron pipe is it can be replaced quickly, cheaply and anywhere.
 

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Over Hill Sailing Club
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From your description, it sounds like a WET exhaust. I would second everyone on the idea of standard black iron or even galvanized plumbing parts. You likely have only 2" parts to buy, readily available once the manifold flange is somehow adapted to std. pipe thread.
 

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Discussion Starter · #13 ·
Thanks so sooo much for all the advice!

RichH especially, i love a good bit of technical dialogue :)

So the initial exhaust that broke wasn't stainless, and not sure how long it lasted engine hours wise... Then the guy who remade it in brisbane made the new one from 316 which i did find out was a rookie mistake after the fact :(

I think all the advice rings true, i'm going to resolve to get an entirely new rigid exhaust made of black steel in the future when the injection elbow in the riser clogs up with corrosion as they do... Eventually
For the moment, i'm getting a guy to weld some flanges to each end of the section with the cracked bellows and supply a new bellows which can be easily bolted onto the flange and replaced with a spare for when it does fail again. Leaving that as the weakest link and inspect regularly because i know that will be the place of failure... Better the devil you know right?

And i will endeavour to run the engine until at working temperature, which i've definitely been guilty of not doing in the past!!!

Thanks again this forum is invaluable A+++

Heres to trouble free sailing ;)
 

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that guy could of done less worse jajaja if using another grade of stainless that is less brittle especially when welding

even 304 or 306 would of faired better if you insisted on s.s

all points being made next time youll find saving money initially will last you longer

a rare case in the boating world! jajaja
 
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