water inlet locations on exhaust elbow

[jtsai]

I have been puzzled by the location of the water inlet positions on the various exhaust elbows. Below are three examples of popular configurations. The first one is Beta marine, and the second and third are Universal and Yanmar styles. In the event of an exhaust elbow failure at or near the exhaust flange, won't the water back-flow into the engine block if the water inlet is upstream of the riser injecting water toward the exhaust flange? This is the case except Beta marina exhaust elbow, where the water inlet is downstream of the riser.

This happened to me this summer when the Yanmar style exhaust elbow on the 25XP broke off at thread to the exhaust flange. It was discovered 30 sec. after the engine started but the water was visible in the now exposed exhaust flange. The engine was flushed right away out of caution and found no problems. I was advised I should've closed the raw water seacock and let the engine run for a bit to expel the wet exhaust before shutting down the engine. Right, like I will remember that next time!

But if the water inlet is at the location as shown in the Beta engine, then water should not back-flow into the engine during such event. So why do most marine engines' exhaust elbows have the water inlet at the unfavorable location?

 

[Nick J]

I think the Yammer style elbows have it on the engine side to cool the exhaust sooner. In the Beta example, the loop is only going to carry hot exhaust gasses and should probably be wrapped with insulation. having the injection point on the engine side doesn't actually mix the raw water with the exhaust until after the turn. This requires a double wall construction though and it's often what fails. When the inside wall fails, it's difficult to catch and can lead to injecting water into your exhaust manifold if left unchecked like you pointed out above.

Here's a look at my old Yanmar elbow from the exhaust side. You can kind of see the internal divider separating the exhaust (bottom, or left side in the pic) from the water.

 

[Alan Gomes]

I was advised I should've closed the raw water seacock and let the engine run for a bit to expel the wet exhaust before shutting down the engine.

Great way to fry your impeller.

 

[Pete the Cat]

As noted the elbows that appear to inject toward the exhaust manifold are really internally jacketed and the actual water comes out away from the manifold. Keeps the elbow cooler. More expensive to engineer and produce, but I think it is less likely to fail because particulate matter is does not get the heat to congeal. I removed my elbow on my Yanmar 3JH4E after 1500 hours: you could not tell it from new.

 

[G Kiba]

Great information guys! I have yet to experience the wonders of diesel exhaust and cooling problems. Probably just around the corner after fixing something else expensive or difficult to do. The joy of boat ownership!

 

[Kenneth K]

Here's another variation in exhaust riser/elbows. I think the height on this was about 9":


And another view of the water jacket in a Yanmar elbow. On mine, the top of the elbow (the water passage) stays only warm-to-the touch even after the engine is fully warmed up. I've never bothered to wrap the lower exhaust pipe pipes with insulation.

 

[jtsai]

If your exhaust elbow components look like this, grab it and shake it a bit to see if the coupler breaks off. The elbow looks OK, but the threaded coupler was the point of failure. I would have a bigger mess if I had not done a pre-trip inspection after ignition on and saw water and steam in the engine compartment. One of my winter projects is creating a peep hole at the port side of the engine box to gain visual without having to remove the entire assembly. The starboard side access is through the door under the sink.

My question remains, if you peer into the engine compartment and saw water and steam spewing everywhere, what do you do?
1. shut off engine immediately and risk water back flow into the engine block
2. shut off the raw water intake and allow the exhaust pressure to expel water, then shut off the engine

 

[Christian Williams]

Recent thread on same topic.

T-fitting in Raw Water Circuit

We have overheating issues with our Universal 5432 and substantial amounts of steam mixed with the exhaust. I had a mechanic check the freshwater/coolant side of the heating system at the beginning of the summer and everything appears to be fine. When he sea-trialed the engine for an hour, it...

ericsonyachts.org

 

[Kenneth K]

My question remains, if you peer into the engine compartment and saw water and steam spewing everywhere, what do you do?
1. shut off engine immediately and risk water back flow into the engine block
2. shut off the raw water intake and allow the exhaust pressure to expel water, then shut off the engine

Not immediately knowing what the problem is, I think most of us would kill the engine immediately then start troubleshooting after.

Your problem had me scratching my head for a few minutes.

- My first thought was: If the break was only at the coupler below the elbow, the water passage should have been left intact and water would still be pumping into the lift muffler. So I didn't get why you saw water spraying out.


- My second thought was: The problem of hydro-locking the engine usually occurs during failed starting, when there are no exhaust gasses to push the water away from the engine and through the exhaust system.

- Finally, I put the two together: With your engine was running, water must have completely filled the exhaust hose between the elbow and the lift muffler. All the exhaust pressure from the engine was escaping from the broken coupler so there was no buildup of exhaust pressure to force the water up out of the muffler. The water you saw spraying out couldn't occur until the whole system had filled with water.


So, here, I think, is your answer--if you had continued to run the engine (versus shutting it down) no water would have been expelled (from the engine) because all the exhaust pressure was escaping. During the time it would take to locate and close the raw water shutoff, more water would have been pumped into the hose, but none of it would have been "expelled." So, the concept of "clearing" the exhaust line no longer works when the exhaust gasses can escape. I think you were right to shut the engine down first.

As to damaging the engine? Once water backs up to the the engine side of the elbow, it will start filling the exhaust manifold. The exhaust passages from the valve head look like they are slightly higher than the bottom of the manifold, so the manifold would collect a little bit of water before it started entering the valves and damaging the engine.

In your case, some of the water was escaping at the broken coupler, too, so this probably bought you some additional time.

Still, nice save!

 

[jtsai]

- Finally, I put the two together: With your engine was running, water must have completely filled the exhaust hose between the elbow and the lift muffler. All the exhaust pressure from the engine was escaping from the broken coupler so there was no buildup of exhaust pressure to force the water up out of the muffler. The water you saw spraying out couldn't occur until the whole system had filled with water.
View attachment 48545

So, here, I think, is your answer--if you had continued to run the engine (versus shutting it down) no water would have been expelled (from the engine) because all the exhaust pressure was escaping. During the time it would take to locate and close the raw water shutoff, more water would have been pumped into the hose, but none of it would have been "expelled." So, the concept of "clearing" the exhaust line no longer works when the exhaust gasses can escape. I think you were right to shut the engine down first.

Kenneth, thank you for the diagnose. After have gained some understanding on the mixture elbow construction and recalling that hectic event, your third illustration was what happened. As I recall the coupler was cracked at the threaded section but not completely displaced so there was build up pressure in the exhaust hose. In addition, the exhaust elbow assembly does not have tall enough riser (which has been corrected), the content in the exhaust hose flooded the mixture elbow and to the engine exhaust side once the engine side of the pressure is reduced due to the crack.

This has puzzled me for a long time because I had the same experience on a Sabre 28 with Westerbeke 13 engine. It had a tall riser and the exhaust mixture did not find its way back to the engine exhaust side. Now I understand the importance of riser. Thank you all!