Modification to Nanni N2.14 Exhaust Elbow

[JumbleDuck]

The last time we tried to take the boat out, a change of exhaust note after five minutes suggested that something was wrong. Something was wrong: the spigot through which water is injected into the exhaust elbow had snapped off. Further investigation showed that it was made of brass with a ludicrously thin wall between screwed section and hose tail. I could have replaced it, but ASAP only do 3/8" BSP 19mm hose tails in brass, which I would rather avoid, Furthermore, the clearance between the elbow and the side of the engine compartment is rather tight, forcing a fierce bend into the hose.

I have therefore modified the system. Luckily there is a core plug on top of the elbow, the diameter of which is very close to the tapping diameter for 3/4" BSP. I had a local engineering company remove the core plug and tap the hole - much cheaper than buying the taps needed. I bought a 3/4" BSP 19mm hose tail in bronze from ASAP and reduced the threaded section by about 5mm on a lathe. The old injection point is now filled with a 316 stainless 3/8" BSP blanking plug. Both hose tail and blanking plug will be slathered with duralac before final fitting and I hope that, with the Nanni anode in the elbow, will keep corrosion at bay. However, I'll be doing regular inspection.

Costs: Hose tail £11.32, blanking plug £2.73, machining £10, packs of ten 3/4" and 3/8" copper washers £4.83 and £4.11, Duralac yet to be bought.

I thought I'd write it up here in case it's of use to anyone else - I think other Nannis use similar elbows. Here's the finished product. New hose tail at top, blanking plug on side. The small hex head on top blanks off a passageway which goes through to the inner exhaust tube - my best guess is that it's for emissions testing.

Many thanks to Stu for inspiration in his PBO articles about making exhaust fittings and to Vyv for metallurgical advice.

 

[vyv_cox]

Looks like a most useful modification.

 

[earlybird]

I'm intrigued by the elbow. It looks like a one-piece aluminium casting. How does the water pass from the injection point to the exhaust gas side? Is there some sort of internal cast-in passage-way near the outlet spigot?

 

[Wing Mark]

What is the elbow made of?

 

[vyv_cox]

What is the elbow made of?

It is aluminium although I do not know the grade. The bronze fittings are the best we could come up with, hence the mention of Duralac. The position of the new nozzle at the top should be beneficial.

 

[Wing Mark]

You can buy aluminium hose tails.
I would have thought that the hottest bit of the seawater circuit was no place for dissimilar metals.
The duralac will not isolate the tail from the elbow electrically.
Is Duralac specified for these temperatures?

If anyone else is doing this job, maybe consider tapping with a metric thread and making an ali hose tail if you can't buy one?
Or use an anodised fitting from the likes of Goodridge?

 

[JumbleDuck]

I'm intrigued by the elbow. It looks like a one-piece aluminium casting. How does the water pass from the injection point to the exhaust gas side? Is there some sort of internal cast-in passage-way near the outlet spigot?

Yes. It's very like my old 1GM10 elbow but cast rather than fabricated. The exhaust goes into an inner pipe which is about half the length of the outer one. Casting seems like an odd way to make it and aluminium seems like an odd material. I believe they are about £350 each.

You can buy aluminium hose tails.

An interesting idea. Bronze is my best-so-far option, but I'm game to try anything else. That's another reason for putting it on the top - much, much easier access. However, the only aluminium tails I can find online are BSPT .. do you have a source for parallel ones?

I would have thought that the hottest bit of the seawater circuit was no place for dissimilar metals.

Me too, which makes Nanni's decision to put brass in there even odder.

The duralac will not isolate the tail from the elbow electrically.
Is Duralac specified for these temperatures?

Dunno. This is a work in progress, and I'll be monitoring things carefully. On the bright sides (a) it can't make things worse (b) there is a lot more aluminium than bronze or stainless and (c) there is a Nanni anode it there, which I guess must be magnesium based if it is intended to protect the aluminium. From the look of the remains of the original spigot, it was standard brass which dezincified.

If anyone else is doing this job, maybe consider tapping with a metric thread and making an ali hose tail if you can't buy one?
Or use an anodised fitting from the likes of Goodridge?

All possible, but if ten quid to ASAP and ten quid to Tom McKean will do it's an economical solution.

 

[earlybird]

Dunno if stainless is any better than yellow metal in this situation but 316 hosetails are available eg here .
Cheaper than ASAP it seems.

 

[vyv_cox]

Bronze (and 316 stainless steel) are cathodic to aluminium but the elbow has considerably greater surface area than does the hose tail. The likelihood is that the aluminium will corrode, but at an acceptably slower rate. The composition of the aluminium is not known, but if it is a suitable corrosion resistant alloy such as 5xx or 6 xx the rate should be very low indeed.

The fact that the original brass fitting failed by dezincification illustrates the point I have made many times. Dezincification cannot be prevented by the fitting of an anode, it is an internal reaction known as selective corrosion.

I take the point about Duralac and high temperature and do not know the answer, although I suspect it will do some good. It will not do any harm. I doubt that anything, other than a plastic sleeve, could isolate the two metals from each other.

 

[JumbleDuck]

Dunno if stainless is any better than yellow metal in this situation but 316 hosetails are available eg here .
Cheaper than ASAP it seems.

I'm a bit of a bronze fetishist, and there doesn't seem to be much in it, corrosion-wise.

The fact that the original brass fitting failed by dezincification illustrates the point I have made many times. Dezincification cannot be prevented by the fitting of an anode, it is an internal reaction known as selective corrosion.

This page on the WOA website says that

Initially Nanni used an aluminium casting with no provision for an anode. More recently an anode has been fitted but the inlet spigot still corroded very quickly. In at least one case, in less than two years. The latest design (2012 model see picture) has a brass(?) inlet spigot which hopefully will last longer.​

If the 'orrible thin brass thing was the upgrade, goodness knows what the original was. It seems unlikely that the anode was to protect the brass spigot, since the whole casting would do that.

I take the point about Duralac and high temperature and do not know the answer, although I suspect it will do some good. It will not do any harm. I doubt that anything, other than a plastic sleeve, could isolate the two metals from each other.

When it's all in place I'll check the operating temperature. It's possible that a plastic hosetail would suffice, though the very idea makes me rather nervous.

 

[vas]

When it's all in place I'll check the operating temperature. It's possible that a plastic hosetail would suffice, though the very idea makes me rather nervous.

it will work until the time the impeller gives up the ghost and exhaust gasses heat up and melt the thing...
My EGT probe on a yanmar 2GMF is circa 100mm from the block (I've modified heavily the elbow) and at 3k rpm and moderate load I can see 220-240C

 

[Wing Mark]

Will anodes even work?
Anodes work by controlling the ions in a solution, making a body of water have a certain potential.
There is no 'body of water in an exhaust elbow, water is chucked in a dribs and drabs, it drains down and gets shaken around when the engine is running.
It's a horrible problem!

Anodising?
Ceramic coating?
PTFE coating?
Some kind of plating/chromate metal finish?

 

[JumbleDuck]

Will anodes even work?

Nanni seem to think so, but I agree with you. On the other hand, if anodes won't work then, by definition, electrolytic corrosion won't be a problem ...

 

[Tranona]

From memory the anode is immersed in the water in the heat exchanger body as it is in the Beta version so protecting the housing and the tubes which are copper. I have just looked at a Beta where the owner forgot to replace the anode and the housing corroded to the extent of needing a new complete HE. They use a stainless fabricated elbow.

 

[Wing Mark]

Nanni seem to think so, but I agree with you. On the other hand, if anodes won't work then, by definition, electrolytic corrosion won't be a problem ...

Electrolytic corrosion only need a drop of water to bridge the two metals some of the time?
If the anode is not in that drop of water, it's not a factor.
I expect Nanni thought their previous version would work too.
Perhaps it's better with Italian seawater.

I wonder if the old engines with mixing elbows cobbled together from bits of generic iron pipe have the right idea?
It's a lifed item, it needs to be easily replaced.

Simplicate and add cheapness!

 

[Tranona]

Perhaps it's better with Italian seawater.

Not sure what Italian seawater has got to do with things as although Nanni is owned by Italians the Japanese Kubota engines are marinised in France at Arcachon on the Atlantic coast - just about as far away from Italy as you can get in France!

 

[JumbleDuck]

From memory the anode is immersed in the water in the heat exchanger body ...

This is the one I mean:

Perhaps there is some internal barrier which keeps it continually immersed, but I haven't seen one, so it looks as if it doesn't get more than a splashing.

 

[JohnGC]

Is it possible that the anode works in the damp environment of the elbow when the engine isn't running? Which is probably 99% of the time.
Or does the elbow dry out completely when the engine isn't running?
Is that different when the engine has been at its operating temperature verses a 10 minute idle?

 

[doug748]

This is the one I mean:

Perhaps there is some internal barrier which keeps it continually immersed, but I haven't seen one, so it looks as if it doesn't get more than a splashing.

Is it in good order internally? How long did it go before the failure? No reason, just curious to compare how ally compares to the life of my stainless Beta effort.




.

 

[Tranona]

This is the one I mean:

Perhaps there is some internal barrier which keeps it continually immersed, but I haven't seen one, so it looks as if it doesn't get more than a splashing.

My memory at fault. However, I do recall that the heat exchanger body is not aluminium like the Beta, which was one of the selling points of the Nanni. I don't think that anode does anything. The failure of your original spigot seems to have been dezincification which an anode won't prevent.

Seems to me the folks who design heat exchangers and elbows don't have a grasp of the issues involved. Volvos use aluminium for the exchanger bodies, but seem to be able to isolate the tube stack so don't have an anode. Then they spoil it all by using cast iron for the elbow with an integral spigot for the water injection. The iron inevitably corrodes and blocks with deposits from poor combustion and saltwater and the next thing is the mating face of the aluminium to iron corrodes. Sometimes can be repaired by welding, but otherwise £3k for a new HE!. Stainless elbow and an isolating gasket is the current fix, although not seen any reports on long term success.