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

[COLOR="Blue"]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. [COLOR="Red"]If you have a Universal or Westerbeke [/COLOR]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. [/COLOR]

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.