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poplariv · 12-02-2012, 11:47

We have an aluminum trawler with integral black and grey water holding tanks, I have some concern with corrosion and age....the boat was built in 1968 by palmer johnson a reputable shipyard......it has inspection covers which I will inspect at next haulout. Should the tanks be left full of fresh water over the winter to help with corrosion etc?

**Cotemar** · 12-02-2012, 11:56

Tanks should never have water in them over the winter.
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belizesailor · 12-02-2012, 12:05

I had problems on a previous boat, built in 1984, which had an aluminum holding tank. This tank ruptured at the seams when an overzealous head user over filled the tank (that was not a good life experience). When we removed the tank it was clear that the seams had been weakened by corrosion.

Flushing the tank as clean as possible and then filling with anti-freeze like solution might help neutralize the compounds that contribute to corroding the aluminum...also resist freezing and cracking your tank over winter. Of course, if the tank has been there since 1968 then it probably already has some corrosion damage.

If feasible, I would eventually replace with rotomolded tanks...no seams...no corrosion...non-permeable...tough. This was my solution.

Lagoon4us · 12-02-2012, 12:16

I cannot think of a worse choice of materials for a holding tank than aluminium!
Ally as far as tanks go unless lined is only good for holding diesel. Or Hydraulic fluids.
Cheers Frank.

poplariv · 12-02-2012, 12:17

Freezing is not an issue as the boat stays in the ocean all year and we're in PNW so the boat stays above freezing even without heat. thanks

poplariv · 12-02-2012, 12:31

The black water tank is welded right into the hull and acts as a second hull in that area, its around 150 gal in size. Previous owner flushed out tank with fresh water at end of season but there would still be some residual sludge etc that might be corrosive I suppose. Its better than not doing it. I'm just trying to find someone with experience with this. Aluminum would not be my first choice either but its something I have to be aware of now.

Kettlewell · 12-02-2012, 12:32

Peggy Hall, the "Head Mistress," recommends against metal holding tanks due to the corrosive nature of what goes into them.

Lagoon4us · 12-02-2012, 12:40

Are there baffles in there that will preclude using a liner?

You may be best if it's possible to open them up fully at the top, clean and repair any corrosion and drop in a FRP tank made to fit?

Bit hard without photo's? Cheers.

poplariv · 12-02-2012, 13:21

Theres an 18" inspection hatch easily opened...will look into on haulout this spring. It will have stiffeners and I'd be surprised if there were no baffles. cheers

Lagoon4us · 12-02-2012, 13:30

Be very careful tanks like that generate BAD gases try to vent it lot's before putting your head in there.

The problem with Aluminium is it relies on oxygen to form an oxide to protect itself.

When you get build ups of crap in the corners, lint, dirt, sludge it takes that protecting oxide away and that's the problem.

If the stiffeners/frames allow then maybe you can remove most of the tank sides and baffles leaving enough for an area to drop a new tank into?

PM me some pic's when you open it up but again be aware of gases in there. Cheers

poplariv · 12-02-2012, 13:37

thanks for the warning, I'll use a flashlight and mirror.

peghall · 12-02-2012, 20:33

When your boat was built, the industry had -0 experience with sewage holding...it was assumed that any materials suitable for water or fuel holding would be just fine for sewage and gray water holding. It took about 10 years after the law requiring holding tanks went into effect (1980) for the industry to find out how wrong they all were. By the mid-90s, almost no OEM waste tanks were still metal...almost all went to PE, some to fiberglass (some of those are only just now finding that SS is even a bad choice for pickup tubes).

Why? Because urine is so corrosive that it typically eats through a weld at a seam or fitting within 2-5 years, and turns the tank--ANY metal tank..aluminum or even 316 SS--into a colander in an average of about 10 years. That does assume that the tank is actually used, btw. Repairing the first leaks is chasing your tail...'cuz the first leaks are always only the FIRST leaks...so replacing the tank is the smartest, least expensive, thing to do when you discover the first one...'cuz you WILL have to do it sooner or later...and sooner is cheaper.