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-   -   Convert existing steel freshwater tank to blackwater holding tank? (

patrickstickler 09-24-2010 07:13 AM

Convert existing steel freshwater tank to blackwater holding tank?
I have no blackwater holding tank on my boat (an old English boat with just a sea head) and have a large steel freshwater tank under the vberth exactly where I'd install a blackwater tank if the other tank wasn't already there.

I'm thinking about just using that steel tank for the blackwater holding tank, and installing two new collapsible freshwater tanks in the bilge (there are two perfect spots on each side of the for them).

The main benefit of having the blackwater holding tank in the vberth is that it would be trivial to configure a gravity drain to the current sea head thruhull. If I put a holding tank in the bilge, I'd need to always pump it out when offshore since it would be below the water line.

My only concern is whether the salt in the seawater will cause the steel holding tank to rust out (since the head flushes with seawater, not fresh). Would that be a problem?

tommays 09-24-2010 08:21 AM

I would think if its steel and still alive it must have some type of internal coating ?

I would think that seawater would find the weak links in the tank and you would have a pretty crappy event at some point :)

billyruffn 09-24-2010 12:20 PM

Something in the back of my mind told me this wasn't a good idea. I googled something like "steel waste holding tank" and found the following at Caveat: this is a commercial site and they're selling polyprop tanks, but the following is consistent with my recollections.


Although you’ll see aluminium and stainless holding tanks, no metal of any kind should ever used to hold sewage. Urine is the most corrosive material that is possible to put next to any metal. If you are in doubt (ladies will have to take our word for it), notice the dividers between urinal stalls in men’s toilets. If that facility has been open for more than a week, no matter how clean and well-maintained it is, even though the dividers are stainless steel coated with enamel, you’ll see rust stains from the bolts that attach the dividers to the tile.
I'm not sure he's correct about "pee" being the most corrosive material -- hydrochloric acid is probably just a tad worse.

mitiempo 09-24-2010 12:35 PM

I'd second billyruffn. I doubt the welds would last a year. And if you think a leaking water tank can be messy....

steel 09-24-2010 04:20 PM

I don't see why stainless steel wouldn't work. High Cr stainless holds up to salt water. It should work for urine too.

ronspiker 09-24-2010 04:46 PM

The issue is how long will it be before it fails and when it does will it be a catastrophic failure or a small leak in some inaccessible location.

tdw 09-24-2010 05:10 PM

I'm with BillyR and TomM....get a new tank. An old 'steel' (does this mean 'black iron' ?) is a recipe for whiffy.

mitiempo 09-24-2010 07:55 PM

It's not the stainless but the welds. Ever notice most corrosive materials are shipped in plastic barrels.

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