BLUF: Crusader 270s with 7-year old Barr risers. 5 years in substantially fresh water, then 2 years in salty water. In the water all year, nothing done to the engines after running. External riser/elbow joints show rust, one set even had evidence of a visible drip. Removed risers to at least replace the gaskets, and ended-up doing a borescope inspection, the results of which were frightening. Staring riser replacement in the face, I decided to glean what info I could from the corpse of one riser.
The outside:
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The inside. Exhaust enters from the right, cooling water enters through pipe nipple:
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You can see a lot just by looking. As far as rot-through goes, not even close. But what is intriguing is what happens after shutdown. The cooling passages are blocked by a stainless plate on the lower right-hand side, as my engine has closed cooling. Thus, water drains out on the left side cooling passages, which are connected to the exhaust elbow, and thence the sea. Note the markedly different levels of corrosion between the upper and lower cooling water passages. Water drains out of the top portion after shutdown, but remains trapped in the lower portion. It shows. There is practically no deep corrosion or blockage in the upper passage. The lower passage is rusty as all get out, and water flow is nearly blocked. Just squint and look at the color difference between the two. If you look carefully, you can see increased corrosion in the upper passage on the far right-hand side, where it is below the siphon break on the left-hand side.
Two things come to mind from the cut-away picture: Fresh water rinse, and post-run drain-down of the cooling passages. I will be replacing the lower drain plug with a valve, plumbed for easy reach, and connecting fresh water to the T-fitting that already exists in my engine raw-water intake, to facilitate after-run freshwater rinse of the entire cooling system.
