Mixing Elbow Question

[pierview]

I just attended a Max Boring diesel seminar and the instructor stated that the mixing elbow gets clogged and should be replaced every 5 years.

I'm far from an expert but this was the first I had ever heard of this. On my C30, which was 20 yrs old, I never replaced it. Is the elbow something others out there are replacing or servicing on a regular basis? It looks like it is not something that would come off easily.

Its supposed to hit 52 degrees this afternoon so I hope to get down to the boat to take a look, but I don't really know where the elbow is on the U35 (the Boring class emphasized Yanmars). Any suggestions?

[stu jackson c34]

The Max Boring guys work a lot on Yanmars, which have a reputation, as I've learned on the web, of having elbows that get clogged. Best reason I can see is that their elbows are right on their engines, whereas ours are at the further end of the exhaust riser. A friend with another C34 asked me the same question years ago and I told him to take the waster injection hose off and eitehr look up or use a mirror and what he'd find is a clean pipe.

Where our risers fail is either right ast the nipple or just above it 'cuz the weak spot's the weld.

Different problems for different engines.

[pierview]

Following up on my own question, I subsequently found the following on-line....

Mixing Elbow- This is the term for the portion of the exhaust system where exhaust gas is mixed with cooling water. The mixing elbow is usually attached to the exhaust manifold. This lowers the temperature of the exhaust gas. The mixture is then discharged overboard.
Term Note: Mixing elbows are sometimes refered to as exhaust risers.

The portion where the hot exhaust gas and cooling water mix is especially prone to corrosion. This due to the cotrasting affects of a hot and cold substances. Further, if an engine runs in a salt water environemnt the mixing elbow may need to be replaced due to corrossion from salt water.

Should you need to replace your mixing elbow there are a few things you'll need to know.

1. Mixing elbows come in 2 basic shapes. One is what we term a 45 degree. This is typically a straight pipe that angles downward. The other is a 90 degree or U shape. Here the pipe is bent so it forms a U. Another name for this type of setup is an exhaust riser. For most Yanmar engines this is what you need to know. This along with your engine model # will allow your parts supplier to spec the correct part.

2. If you have a Universal or Westerbeke it's not as simple. These engines do not come with a mixing elbow. This means there is a much wider range of options for these engines. Configurations range from simple black pipe to mixing elbows that are produced by Westerbke.

Here's what it's helpful to know if you have a Universal or Westerbeke. Again, we start with whether the part is a 45 degree or 90 degree configuration. Next, we need to know how it attaches to the engine. If it is a recent engine it will have a steel collar and clamp. If it is an older engine it will attach via pipe thread. If it is pipe thread we need to know the specification of the pipe thread. Then we need to know the diamater of the exit end of the part. This tells what size exhaust hose can be fitted. In summary you need to know A. 45 or 90 degree B. How attached to engine C. If pipe thread, diamater D. Diamater of exit end.

Once you have your dimensions check for the appropriate in our Marine Diesel Direct Store. 3. Yanmar mixing elbows often require a few other pieces. With some Yanmars the mixing elbow has a threaded joint and an elbow or flange, that go between the actual mixing elbow and the engine. Often these must be replaced as they become corroded and are difficult to remove.


I then called our local (for NJ) distributor in Marblehead and the service people told me that (at least in the newer engines) the elbow piece is actually aluminum, and mine was wrapped in some sort of tin foil like insulation by Catalina (according to them). This will not scale like the cast iron elbows and should be fine as long as I don't detect any leaks.

Has anyboody tried to drain their muffler through the little pet cock drain on the side of the muffler? Mine is tight and I'm in the process of spraying with WD40 but its in a very awkward spot and I'm wondering what I'll find when/if I get it freed up.

[plaineolde]

I have not drained my waterlift muffler in a number of years. I used to drain it after I'd run antifreeze through the raw water system, but what came out was largely antifreeze, so I didn't see the point. It's been at least 5 years since I've drained it and the Md. winters have not damaged it.

Just as an fyi, I believe that you turn the drain cock clockwise to drain it, eg., screw it in. That seems counter intuitive. Mine is a bit tight to start, but once started, screws right in. If I'm off on that, someone please correct me.

[LCBrandt]

The main purpose of the drain petcock on the aqualift muffler is to prevent engine damage.

OK, I just made a dramatic statement, but now let me explain. Cranking the engine causes the raw water impeller to drive water through the engine, heat exchanger, and then into the muffler. Usually the engine starts in a few seconds, and the engine exhaust mixes with the water and 'blows' it through the exhaust hose and on out of the boat. IF the engine is cranked for excessively long periods of time WITHOUT starting, the water being pumped into the exhaust system can accumulate in the riser, back up and enter the engine via the exhaust valves. Water in the cylinders, being non-compressible, would damage the engine as it cranked. (Disclaimer: Others with personal or anecdotal experience are asked to elaborate on this, as I am speaking from a theoretical perspective only.)

It is said that if the engine has been cranked for thirty seconds without having started, you should drain the muffler before continuing to try to start the engine.

I have seen students attempt to start an engine without realizing that the T-handle was never returned to the normal position at the end of the previous day. New charterers, for example, might be prone to overlook this. And if the engine fails to start next morning, they might think that maybe they just need to crank, and then crank a little more.

[LCBrandt]

I refer you to JibSheet issue #1, and Chooch's excellent write-up "Exhaust Riser Replacement Fun."

[sarahsue]

Larry

You are spot-on with your assessment, and 30 seconds is about right for a good safety margin. And a very valid point regards returning the engine stop T-handle to its original position. I have been caught out on this one on a previous Catalina; went below and madly bled all lines before realising what happened.