Exhaust Mixing Elbow

[Mac G]

Since cannot find off-the-shelf replacement exhaust mixing elbows for my Ammarine 6404 diesels am going to have custom ones made.

Are there any rules-of-thumb on the height above waterline that are applicable?
Any other rules or guidelines I should be aware of?

Existing ones extend higher than the exhaust manifolds so if an internal leak ever developed raw water could get back down into the engine.

Instead of rising up above the engine, am thinking of exiting exhaust maniford and then immediatley dipping down below its height so any future leak would be impossible to get back up into the engine.

Any thoughts or advice from folks fluent and expert on this topic will be appreciated.

Thank you

 

[Bmarler]

A lot depends on elevation of the exhaust manifold in relation to the waterline. There’s some fairly simple guidelines to follow, I think Peter (weebles) has posted them a few times, maybe he’ll see this and chime in.
But the thing is, your elbows are common, and the water injection point is past the riser and water can only drain away from the engine.
There are some water jacketed risers, and they incorporate a risk of water backflow in case of failure.
If you decide to go immediately down after the manifold, you may need to add a water lift muffler, depending on where the waterline is.

 

[Bmarler]

I was thinking 12 inches above waterline at the injection point, but can’t recall exactly.

 

[mvweebles]

Sounds like your old risers had a large #10 can sized drum over them. I don't think there's a reason you have to recreate that exactly. I have to seen ones like that in years.

Here's the diagram BMarler mentioned. It's from a Northern Lights article "Please don't drown me." You should be able to find a fairly common mixing elbow though frankly, even those are usually made to order so semi custom just the same.

I've used Marine Exhaust Systems of Alabama (MESA Marine) for exhaust fabrication. They do a decent job, though there are others that specialize in exhaust systems too.

Good luck



Peter

 

[Mac G]

Perfect. Thank you!

 

[firehoser75]

Hi Mac,
Check out sbmar.com (Seaboard Marine in California"s website) for great free info. Look under Tony's Tips or in the forums. Under the Tips, see "Doomed to fail" for some eye opening info regarding how exhausts are designed and installed poorly even from the factory!
Yes, Seaboard does make custom exhaust and sells parts, but that is not required to be active on the forum or use the free info provided.
I used National Marine Exhaust in Marysville, Wash. and was very happy with their knowledge, product, professionalism, etc. but I am sure there must be good providers on the east coast as well? My NME elbow is dry with the only wet part being well below the turbo/exhaust manifold where the cooling water is injected, so no chance of water running uphill into the turbo. I also suggest installing an exhaust hose overheat alarm (Borel and Aqualarm are 2 providers that I know of), as this is the location where any overheat will first show up and it will provider an early warning to prevent possible engine damage. Easy install and inexpensive "insurance".

Good luck

 

[Nick F]

What a coincidence - I am working on replacing the elbows on my AmMarine 6404's as we speak. My old elbows were custom made in steel and have lasted at least 25 years (I don't know the date exactly).

I will post more info shortly. Also sending PM.

Nick

 

[Nick F]

Here are photos of one of my existing elbows which I have just removed. The left hand photo is as it looks when when viewed from above. The other photo is looking forward at the back of the (port) engine. (Note that the small tube was longer - I cut a piece off)

I also include a sketch of the engine room layout which shows how the exhaust has to do an immediate 90 degree turn towards outboard in order to avoid the fuel tank.

As you can see from the photos, my elbows are badly corroded. They are made of carbon steel (not stainless). However, I know that they were made prior to 1998, so they have lasted 25 years!

I am going to remake them in carbon steel, although I might use 316SS for the small saltwater inlet tube.

There is no baffle etc where the small tube sticks into the main pipe. I was surprised at this, but it has obviously worked for the last 25 years and, based on this, I intend to stay with the same arrangement.

I include a sketch showing the individual components of this elbow design. (Note that I have shown the small tube in the wrong rotation, for clarity.)

In my case the flange and the reducer (items 1 and 2) are both in good shape and I intend to reuse them.

I am buying the elbow (a standard pipe component).

For the straight part I have found a pipe with exactly 3" OD to match the exhaust hose. (The standard pipe size of 2-1/2" (nominal) has an OD of 2.875" which is close to the hose ID (3"), but I prefer to have a good match to the hose. This 3"OD pipe is not a standard pressure piping type material, but I found it available at our local metal supermarket. It is called "DOM" pipe ("Drawn Over Mandrel"). It is available in various wall thicknesses and I chose a heavy one (0.25").

The old 3" pipe section was 6" long and I am thinking to increase this by some 2" to increase the vertical separation between the engine exhaust manifold and the point of injection of seawater.

I am going to pick up the elbows tomorrow.

 

[Steve DAntonio]

There are indeed rules for the design of exhaust risers. This article includes a few diagrams from different manufacturers, including Northern Lights, which was shared above, as well as general advice on exhaust system design and fabrication. https://stevedmarineconsulting.com/wp-content/uploads/2014/03/ExhaustSystems170-FINAL.pdf

 

[Nick F]

There are indeed rules for the design of exhaust risers. This article includes a few diagrams from different manufacturers, including Northern Lights, which was shared above, as well as general advice on exhaust system design and fabrication. https://stevedmarineconsulting.com/wp-content/uploads/2014/03/ExhaustSystems170-FINAL.pdf

Good article - thanks for this, Steve.

 

[Maerin]

A significant function of the riser involves the water injection, the intent being to cool the exhaust gases as they leave the manifold and enter the hose. Without cooling, the exhaust gases subject the hose to temps high enough to burn the hose.


Most exhaust risers are designed to force the exhaust gases to mix with water, causing the water to boil, or flash into steam. That change of state from liquid to vapor absorbs significantly more heat than simply lowering the temperature. (Latent heat of vaporization.) A good mixer or riser will be designed to cause the water to spray a curtain from the circumference of the mixer toward the center of the pipe, not just along the surface of the hose. The finer the spray, the more the gases will flash the water to steam, more effectively cooling the exhaust and more effectively protecting the hose & related system.



I fought high exhaust temps- burned hose, damaged FG lift muffler, and hot spots until a new mixer with a better spray was installed. Just that change dramatically reduced the exhaust temps, resolving the problem for good.
So while you're making changes, pay close attention to the spray pattern of your mixer. Your whole system will benefit!