Dark blue liquid in bilge

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Senior Member
Aug 9, 2021
Vessel Name
Sea Dog
Vessel Make
1991 DeFever 49 RPH
So I have discovered a sizable puddle of dark blue liquid in my forward bilge - near the holding tank. It’s roughly the color of toilet bowl cleaner. I can’t think of anything on the boat that could generate that color. I’m not using any chemical in the holding tank that might generate that color, and I can’t think of anything on the boat that might have leaked that has that color. I’ve felt it and it feels vaguely slimy or oily, but I don’t have the nerve to taste test it. Is there some weird algal bloom or bacterial growth that could produce that color? Has anyone ever seen anything like this?
It would need to be extremely hot to cultivate that color of Alge. Most likely something of that color was left in the bilge. Someone forget a bottle of Power Aide Sports drink in the engine room? Possibly some hard candy fell out of a pocket? Old bottle of windex?
I would check the contents of the holding tank and see if they are that color. Probably not but a simple thing to check and eliminate.
I pumped some holding tank contents overboard and it came out the usual brown color.
If an oil it would be a surface sheen vs well mixed color change as this appears, at least in the photo.
Toilet chemicals are rather deep color and that would be my first guess.
Do you have an overboard discharge line with a valve that it could be a residual from long ago?
The vertical PVC pipe has a short section of dark blue below what appears to be pinkish... any leaks in that area or was the level that much higher at one time.
I’d be checking the steering or any other hydraulic system onboard. Those fluids can be that color. Some can also absorb small amounts of water, kind of like brake fluid will.
I have seen pockets of uncured resin appear, but mostly when removing hull blisters.
Has the boat been winterized at any point recently? It looks a bit on the dark side, but the color is close to the blue -100* propylene glycol antifreeze.
Put it in a glass half full of water, if it is oil repellent then it floats, if not it is water based.


I don’t winterize the water system, so it’s not propylene glycol antifreeze. It’s not steering fluid, I use the Hynautic fluid that is not that color. The stabilizers are well aft of that spot and I can’t think of any other hydraulic lines above that bilge. There is no 2-cycle oil on the boat. It’s not residue from the overboard sewage line because I use that regularly. I believe the blue color on the sewage line is PVC sealant, not related to this.

Next step is to go through the under-sink cabinets in the heads and see if anything has leaked.
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I am guessing that you will find broken bottle of holding tank treatment while you are searching under the sink.
Looks like the blue goo for the toilets. You said you have blue water in the toilet. A leak in the bowel, or in water supply. A bottle of concentrate sprung a leak.? Smell it the blue stuff has a pretty distinct odor.
Scoop up a little in a container, put the lid on, and shake it. It looks like liquid laundry detergent which should bulb with bilge water when shaken.

Mystery solved. Found this never-opened but empty bottle under the sink in the guest head, with a blue mess under it and a tiny hole in the bottom of the bottle. Thanks everybody!


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