Black bilge water

[Nan]

My 1986 Pearson is getting black, rank smelling water in bilge. Seems to be isolated in forward bilge.
She was hauled Fall 2025 after yard winterized (changed engine &transmission oil, ran banfrost through). Rank smelling water first appeared when I checked her out and put on Winter Cover on Christmas eve, and there were black flakes in bilge water then. Now it’s rank smelling and black opaque. Mast was hauled 2023, stepped cabin top with SS support to keel.
I have Yanmar 2GM20F, overhauled in 2017, including Upper Elbow, Exhaust.
Can anyone give me any ideas where to start?
Thank you. Nan

 

[Don S/V ILLusion]

Holding tank? Hoses to/from holding tank? All hose fittings?

 

[Nan]

Holding tank? Hoses to/from holding tank? All hose fittings?

Thanks. I don’t think that’s the case as the black water appears whether it’s empty or not. However, as soon as this thunderstorm passes, I am headed to pumpout station…to at least rule it out.

 

[Rich Stidger]

I had that issue of black bilge water a couple years ago. I never found the cause. But I cleared it by flushing with fresh water until nearly clear, and then adding Zaal Noflex Digester. That product was recommended by someone, so I went with it.
Black bilge water has not returned.

 

[Tally Ho]

I would be checking your black water tank (assuming you have one, and the rank stuff could be sewage). I had a leaky fitting on my black water tank and had to remove the tank, clean it out, remove the fittings and rebed them all.

You could fill the black water tank with clean water, and start searching for the leak (maybe add some food coloring to make it easier to see), and to verify it is coming from the holding tank system.

For it to be that rank, I can’t imagine it is coming from the engine. Unless your boat floats in a skanky pond, I am going with holding tank system.

Greg

 

[Peggie Hall HeadMistress]

A wet dirty bilge is a primordial soup that can make the entire boat smell like a swamp or even a sewer. The cure: CLEAN THE BILGE--actually CLEAN it instead of just dumping bilge cleaner or NO-Flex into it (Would you just dump Dawn into a sink full of dirty greasy water, swish it around a bit, then pull the plug and expect the sink to be clean? Then WHY would you expect that to give you a clean bilge???). with plenty of detergent (Dawn will do), water and elbow grease...or better yet, a power washer that can get into places you can't reach with even a long handled scrub brush. Rinse thoroughly with plenty of clean water.

--Peggie

 

[capta]

Over the years I've had and operated boats with this problem. I honestly cannot remember how I finally found out what it was, but believe it or not, it is living organisms! Bacteria, fungi, (including molds) and yeast can thrive in diesel fuel, particularly where water is present, collectively known as "diesel bug".
Therefore, I'd recommend cleaning out your bilge with soap and water and identify any diesel leaks, if you have them. It is distinctly possible that the yards guys dumped a bunch of fuel/oil in the bilge when they did the work.