What is inside a mixing elbow?

[Simi 60]

What is inside these mixing elbows?
I have never cut one down so genuine question.

Is it simply a 2 inch tube with an angled 3/4 inch tube stuck in as a water injection port?
(Made up sizes)

Reason I ask is mine is rotting out on a Kubota based Genset as can be seen.
Cast aluminium elbow bolted to cast aluminium with well frozen in steel bolts.
Water injection port is brass so adjacent to it the ally is failing.
Made to fail.

I was going to wait until next out of the water to get it done but I doubt it'll hold up that long.
It will be a time consuming and expensive job to remove, manufacture and replace if sending it off and as we are living aboard cruising full time we can't simply plug into shore power and get it fixed, well we could but marina berth alone would be $500 for 3 days plus cost of repair.

There is a healthy couple of inches sticking out and I wonder if I can simply cut off the dead bit at the line and mig on a new bit of ally pipe with a smaller ally tube for water injection welded in, all metals are then the same.
For this job I can anchor out, remove cut and prep and run into a waterfront shop and have it migged up in under an hour.

Pic below shows the elbow, epoxy glue with chopped glass fibres is holding up ok, but I ain't holding my breath it's going to last another 6 mths until we come out if the water.

 

[Simi 60]

Some cutaways I see online look like this but I find it hard to believe mine would be like that as I take off the water injection hose and can see clearly down the pipe

 

[BruceK]

They probably vary, engine to engine. I think the ones on my Lehman 120s were simply constructed just a stubby tube to take a hose for water to flow into the pipe, but it`s 10 years since I replaced them, though I had a good look, I was painting them red with heat resistant paint. They were rusty and weeping, pending new ones winging their way from USA I used epoxy putty to "repair" them externally, worked a treat.
Recently I replaced the elbows on the Cummins 210s. By comparison, a complex structure,a work of art in stainless steel covered by a ceramic coating, a tube within a tube, with spray holes around the full circumference at the far end where the salt water gets introduced to the exhaust.

Unless someone knows, I hope they do, you might need to remove it to see how it`s made and what can be done to preserve it.

 

[kchace]

Looking inside my Lehman Stainless Steel mixer it looks like this: A slightly larger hole at the top and a series of small holes around the perimeter.

 

[Keysdisease]

Basically what has been described is the two types of exhaust elbow mixers.

The simpler unit is often referred to as a "dump elbow" It just has an inlet that carries raw water and "dumps" into the larger exhaust pipe.

A true "mixer" or "spray ring" is more like what kchace posted, and it does a better job of distributing water and reduces "hot spots."

On a gasoline engine with cast iron elbows (and some diesels) the holes are replaced with slots on the 4 sides of the squarish exhaust elbow.

I have seen simple dump elbows on small engines made of steel pipe fittings just threaded together with a simple Tee for the water inlet. I have also seen similar arrangements on larger yachts where the exhaust is hot all the way to the hull side and water is "dumped" into the pipe just before the hull side exit. This is to keep the hull paint on and reduce soot on the hull.

Basically at this stage of the exhaust run the idea is to cool the exhaust gas enough so the plumbing can handle the temps until it all goes overboard. The engine really doesn't care except if your design creates excessive back pressure, which on a small Kubota would be difficult as they are very forgiving about back pressure.

Your plan to remove and repair sounds very viable for the short run, and maybe consider having a stainless mixer fabricated at a marine exhaust place. Any good exhaust shop should all have the flange dimensions for your engine as it is a common generator unit. That way you can just remove and replace. A few pictures and measurements and a tracing of the flange from when you have it off for welding should ensure a correct fit.