Dear Boat Doctor,

I am having a problem with a black water smell in the bilge. Well at least I am pretty sure it’s a black water smell, I don’t really know. Do you some ideas how I can diagnose my problem?

Dave Rheingans

Fort Lauderdale, Florida

Dear Dave,

This is a bit of an open-ended question, but I think I can help. First off, smell emanating from a holding tank is distinctive and one we are all attuned to. If you think you have a black water problem, you likely do.

The most obvious thing to check for is a leak from the tank itself or one of the hose connections. It should be pretty obvious, and I assume you would have caught this by now.

The most likely culprit is your head hose, the line from the head to the tank. This hose is the most likely to be holding effluent, and if a hose is holding effluent the smell will eventually leak out. The best way to diagnose it is to place a wet rag on the hose, and then smell the rag. The rag will collect and concentrate the odor and you’ll have your culprit.

If you have smelly hoses your only option is to replace them. I would choose the best marine head hose you can find. Best usually means most expensive, don’t be surprised to see a price tag of $10 per foot or more. Shields, Trident and Raritan are all top-notch brands.

Even the best quality hose will smell if the effluent sits in them, the key is to keep them empty. The only practical way to do this is pump the head a lot, and pump a lot of seawater through it. Yes, you will fill your holding tank more quickly, but the effluent will be more diluted and will smell less. And, most importantly, the liquid that is left in the lines will be mostly water.

Another good solution is to use a holding tank deodorizer, any of the commercial brands will help.

On the off chance you have a smell emanating from some other bilge source, I’ll give you a few things to track down. The mostly likely sources arise from draining the ice box or shower to the bilge. Either of these will introduce organic material into the bilge, a warm and wet location, and will result in smelly organic growth. The answer is to not do that, drain those areas into a contained sump or get it overboard directly via a dedicated pump.