Some LNVTs with over 4000 hours on their Cummins engines have reported wet muffler failures. Autopsies of these mufflers turned up melted components. After the obvious causes were eliminated, broken impeller, blocked thru-hull, etc, it was thought that the melting was due to poor muffler material selection by the manufacturer. However, the cause may very well lie in the exhaust's wet injection elbow. Over time carbon build-up in the elbow reduces the raw water's cooling efficiency. Cooling efficiency is important because depending on RPM the exhaust's temperature is between 350 and 800F. The muffler, which is only about 3' away from the elbow and made of FRP, can withstand temperatures to around 250F. Thus an engine running at high RPM that has carbon build-up in the injection elbow could conceivably go over-temp in the muffler. A recent inspection of Nellie D.'s (63) 4500 hour elbow showed significant carbon build-up. When the carbon was removed from the elbow, extensive crevice corrosion was found. So much so that the elbow had to be replaced. It's probable that the crevice corrosion was exacerbated by the carbon because it allowed a very caustic micro environment to form.
I now have many good reasons to bi-annually inspect for and remove carbon build-up on the inside of the elbow, not the least of which is its $1600 replacement cost.
For some good background information I recommend the following articles: