what's that black sludge in the water in the bilge?

exhaust system

What's that black stuff in the bilge? Definitely NOT oil, the material settled to the bottom in little flakes...we've had black sludgy stuff in the bilge in past years, and a topic on a forum described exactly the same crud in the bilge we had.
photo of exhaust hose deteriorated at transom
Exhaust hose deteriorated at transom

On Reality Check the culprit was a leaky exhaust hose at the transom, not the exhaust riser, and the fix was replacing the entire hose.

The following topic gave me the direction for problem determination.

The following topic is from the SailboatOwners.com forum

07-05-2006, 01:05 am #1 Craig Catalina 36 - black water in bilge ?
We are the third owner of a 1985 Catalina 36. Having had the boat only a couple of months, I have learned that it is recommended that you run the engine at high throttle and let it "stretch its legs" frequently. Having just done this, the next day we noticed the bilge had black water in it. After examining closely, it was definitely NOT oil. While rinsing and cleaning, the black material settled to the bottom (almost like sediment in a wine bottle) in little flakes or specks. The engine compartment shows no evidence of any fluid leaks and the oil level is full. Has anyone else had this experience? What was it? Reading past tech notes from the c36 association, they indicated that the engine needs to be run at high rpm otherwise there can be carbon build up. I am inclined to think that this is what is going on but would like to make sure and also am curious why it would show up in the bilge if this indeed a burning off of built up carbon.

07-05-2006, 01:41 am #2 Mick C36 A reply to Catalina 36 - black water in bilge ?
Check the exhaust system starting at the manifold. Possibly the 'elbow' has developed some leaks, and is allowing exhaust to enter the engine room. This is a common failure point especially on earlier models.

07-05-2006, 01:42 am #3 Stu Catalina 34 C34 San Francisco
Check the coupling A reply to Catalina 36 - black water in bilge ?
between the exhaust riser and the manifold and the riser and the muffler. The C36 guys'll help, assume you've posted there, too. The "old" C36 website has a good explanation of a riser replacement.

007-05-2006, 03:23 am #4 Stu Catalina 34 C34 San FranciscoHere's the link on exhaust risers A reply to Catalina 36 - black water in bilge ? http://www.catalina36.org/articles/maintenance/ExhaustSystem.htm (link is broken) from the C36 website

07-05-2006, 05:39 am #5 Craig Thanks for the responses A reply to Catalina 36 - black water in bilge ?
The puzzling thing to me is we had the exhaust elbow examined and tightened as recommended by the surveyor prior to sale, there was no smoke, no fumes, the water had no odor to it to suggest it was exhaust related. We have operated the boat for nearly two months with no indications of exhaust leaks or fumes and no black in the water until running at full throttle. There is no evidence of any leaks or other problems around the engine block at all.

07-05-2006, 06:32 am #6 Stu Catalina 34 C34 San Francisco What happens over time is A reply to Catalina 36 - black water in bilge ?
What happens over time is that things do change. The connection at the exhaust nipple could well be silting over. Who knows, you're there, we're not. There are a gaggle of C36 owners in your neck of the woods, including Tom Soko who is or should be your technical guru. Why not call him up and ask him, or some others with lots of experience, to come take a look? Wish I could pop over, but we're a fair distance away! I do think it's more than coincidence that both Mick and I came to the same conclusion independently.