The brackish water is corrosive do to the salt content. While the engine itself is protected from the corrosive salt water, the steel components of the raw water side of the cooling system are vulnerable to "rust". About 6 months ago, we a large cast iron exhaust elbow replaced on the starboard engine. It had leaked and corroded the mounting bolts so badly that we could not remove them. Thus, we had it replaced professionally, which means that it was also expensive. This time the leak had barely begun and corrosion was minimal, and we were able to remove the old elbow relatively easily. However, it seems that hot salt water leaking from the exhaust system is HIGHLY corrosive so those little casual "lookabouts" before each cruise can save a LOT of work and expense down the road. The first picture shows the badlycorroded underside of the exhaust elbow where the leak occurred. 
The port Velvet Drive transmission had previously been removed for repair, but the absense of the transmission made overall access much easier. Note that the black (3.5 inch inside diameter) exhaust hose also appears to have failed or been cut at this location. If the hose had leaked first, then this could have caused the rapid corrosion and failure of the elbow from the outside. The Admiral did a wonderful job of scraping, wire brushing, and using emery cloth to remove the rust, and the she applied a coat of the venerable "Lehman red" spray paint (second photo). Along the way we also replaced two additional hoses on each engine.
The final picture show the "Captain" in an unfamiliar role of exhaust elbow repairman. If he looks relaxed and relatively happy, it is because the picture was taken early in the adventure. Well, the final result looks good, but it remains to be tested until the transmission has been repaired and replaced. So, time will tell. If the repair was successful, we did it for about half of what the professional version cost and also gained more knowledge about the inner workings of OUR boat.
