SS Vs Black Iron Exhaust Elbows

[kmacdonald]

I have heard arguments for both and not sure which is better or all the factors to consider when choosing. Opinions wanted.

[Sailmonkey]

I’ve switched from a cast aluminum mixer that was at least 10 years old and going fine to a stainless riser with bronze pipe mixer. 2 years in and no problems so far.

[Wotname]

Originally Posted by kmacdonald

I have heard arguments for both and not sure which is better or all the factors to consider when choosing. Opinions wanted.

Perhaps it depends more on the design.

Are you considering just the a "mixer" elbow where the raw water is injected directly into the exhaust stream or where the elbow has a "water jacket" around the elbow to keep it "cool" before the the raw water finally enters the exhaust stream?

Is the elbow to be part of a riser?

Is the elbow bolted directly to the head [exhaust port(s)] or to an existing manifold (water cooled?)?

I have looked at a variety of small Yanmar designs over the years and there seems to quite of variety of design.

I think it mainly depends on what is principally in contact with the elbow i.e. exhaust gas mainly, raw water mainly or a exhaust gas mixed with raw water.

I hope you get some definitive answers as I need to make an exhaust pipe/riser/mixer soon (for a small single cylinder). Perfect thread timing

[JPA Cate]

Hi, kmacdonald,

On our first Insatiable, the mixing elbow was bolted directly to the exhaust output, and it did have both gas and hot salt water. The first one was some kind of iron, and rusted out. Jim made up an s/s elbow, and welded it to the original's base. That latter elbow lasted from the time of the repair till we sold the boat, some 15 yrs. later.

Ann

[kmacdonald]

I don't like the elbow designs that will leak salt water into the engine exhaust ports when the elbow fails. All the manufacturers seem to favor a design that destroys the engine in the event of elbow failure. I like a dry exhaust manifold, a riser to the highest point in the engine box, a loop, a y-fitting and water injection on the downward side of the loop. I wrap it with lagging. It's a fail safe design, less than $75, and easy to make and install. The price difference between SS and black iron isn't much but if I use SS there will be a joint of SS and black iron somewhere and that causes me concern. 15+ years out of SS sound like a winner though.

[Cheechako]

Properly designed a failed elbow does not have to leak water into the engine.

[kmacdonald]

Originally Posted by Cheechako

Properly designed a failed elbow does not have to leak water into the engine.

True, but that's not how they do it.

[Sailmonkey]

Originally Posted by kmacdonald

True, but that's not how they do it.

Who’s this they? Mine can rot through and there is no salt water in the engine.

[a64pilot]

SS will last longer than Cast Iron, definitely.
Neither will last as long as a bronze or cupro nickel elbow.
A fail safe design is to inject the water downstream so that in the event of a rust out, it won’t put water into the engine, however this leaves s significant portion dry, and therefore hot.
It can be insulated with silicone high temp cover meant to be fire shield for aircraft, the Nexgen 3.5 generator is like that as an example.
You can see it here, the orange covering is the heat shield, the exhaust is dry there, past the heat shield water is injected and of course then the exhaust is cooled and no need for a heat shield.

[kmacdonald]

Yanmar and others.

[kmacdonald]

Originally Posted by a64pilot

SS will last longer than Cast Iron, definitely.
Neither will last as long as a bronze or cupro nickel elbow.
A fail safe design is to inject the water downstream so that in the event of a rust out, it won’t put water into the engine, however this leaves s significant portion dry, and therefore hot.
It can be insulated with silicone high temp cover meant to be fire shield for aircraft, the Nexgen 3.5 generator is like that as an example.
You can see it here, the orange covering is the heat shield, the exhaust is dry there, past the heat shield water is injected and of course then the exhaust is cooled and no need for a heat shield.Attachment 164247

There are companies that will insulate exhaust manifolds, risers, and elbows if you send them in. The car racing crowd seems to do that. The lagging wrap works well also and then there is exhaust blankets. People will tell you the engine compartment will get real hot but that isn't true on the installations I've see. A small exhaust fan helps though.

[Lepke]

If you plan on having the boat a long time, get stainless.

[Jim Cate]

FWIW:

The elbow that I fabbed for our previous boat was dirt simple. I kept the original mild steel flange which had a rectangular hole in it, small radii in the corners, and found some s/s tubing whose circumference was about equal to the edges of the hole. Used a vice, mallets and other high tech instruments to shape the end of the tube into a roughly matching rectangle. TIG welded it in place. about 3 inches out from the flange I made an angled cut, rotated the tube 180 degrees to form a ~ 45 degree downturn and welded that up. About another 4 inches down from the bend I welded in a pipe nipple at about 45 degrees to the axis of the tube and hooked that to the cooling water hose. Another few inches down I cut it off and attached the exhaust hose.

All simple fabrication, simple design and no obvious way for water go get back up to the engine. Didn't last quite as well as Ann implied, for after some 12 or 13 years and some 5000+ engine hours, a couple of pinholes developed a bit downstream from the injection point, and (no longer having access to a TiG welder) I had to get a local welder to put a patch on it. Was still working when we sold the boat, then with more than 6000 hours running time. BTW, this was on a BMW D35 engine.

I haven't done the same on the Nanni (Kubota) engine in our current boat because the previous owner built a s/s elbow very similar to the one I had done, and it is still working well at getting on 5000 hours.

I've never understood why Yanmar uses those loop type exhausts... they seem to be a frequent cause of trouble, and Yanmar views them as consumables (profit motive?)

Jim

[kmacdonald]

Thanks Jim for the info. Big profit motive if it ruins engines and they can claim lack of maintenance caused it. It's a Kubota I'm marinizing and have the black iron pipe and fittings to make what you describe. It's a good fail safe and inexpensive way of doing it. I might go ahead and get SS pipe and fittings instead or just use the black iron for a few years first. The exhaust manifold is cast iron and I worry about mating that with the SS but since that part will be dry, it may be OK.

[Wotname]

I do wonder what the noise ramifications are if using just single wall pipe for the riser etc. e.g. would a water jacket around the riser / elbow decrease the exhaust noise from the engine; or even using a goodly amount of lagging to reduce noise???