Aluminum holding

[fishsail18]

Is it ok to use an aluminum holding tank.? I’ve only seen them in plastic.

[boatpoker]

Definitely not, I just replaced a badly corroded aluminum tank with an epoxy/glass one. Urine corrodes aluminum even in fresh water. Add salt water and the corrosion process is much quicker.

[peghall]

Boatpoker is right...aluminum is not recommended for sewage holding. It was the material of choice in the '80s because it was the least expensive material, but by the early '90s it had become apparent that while metal is suitable for water and fuel holding, urine is so corrosive that it can turn any metal--even 316 SS--into a colander in less than 10 years. Thickwalled polyethylene is the the recommended material for holding tanks. Rotomolded tanks are the most common.


If you need a holding tank, Ronco Plastics Ronco Plastics marine Tanks (no relation to the VegoMatic Ronco) is your best source. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. They've been the OEM supplier to most of the "high end" boat builders for decades.



--Peggie

[GrowleyMonster]

High quality plastic. It's the only way. Don't even think about aluminum.

[boatpoker]

We had to cut up the old aluminum tank to get it out. We then laid in four layers of 16oz. biaxial and epoxy and made a top plate of 1/2" plywood wrapped in the same glass. We also changed over to ABS pipe withonly 6" of hose at tank, toilet and seacock connections as I've never been happy with what my wife and I can smell from even the high end sanitary hose. 2 days work (from start to finish $200 all in .... couldn't get a tank delivered in that time.

[HopCar]

What Peggie said. Aluminum is the worst choice for a holding tank. Ronco is the place to buy plastic tanks. That’s where my water tanks and holding tank came from.

[Lepke]

Aluminum is made in many different alloys. There is one for sewage type corrosion. But many boat builders and cheap tank builders just use whatever alloy is locally available. I'm not saying aluminum is better than some kind of plastic tank, just that it's possible to make a holding tank correctly using a proper alloy.

[Jim Cate]

Originally Posted by Lepke

Aluminum is made in many different alloys. There is one for sewage type corrosion. But many boat builders and cheap tank builders just use whatever alloy is locally available. I'm not saying aluminum is better than some kind of plastic tank, just that it's possible to make a holding tank correctly using a proper alloy.

curious: what is that alloy? I'd like to see what is in it that makes it ok for sewage+salt water storage.

Jim

[fishsail18]

Thanks for all the great info. I was also thinking of making one out of fiberglass however I'm not sure what to use for the fitting flanges?
any thoughts about fiberglass?
TY

[peghall]

If urine can eat through steel, even 316 SS, it will eat through any aluminum alloy. It's not the sheet metal that fails first...the first leak is always a weld--sometimes more than one--at a seam or a fitting, typically in 2-5 years. Meanwhile urine is gradually turning the bottom of the tank into a colander, a process that takes an average of about 10 years.


When thinking about building a FG tank, especially if you also use plywood, weight can become an important consideration. Water and waste weigh 8.33 lbs/gal, so even a 25 gal tank adds 200 lbs plus the weight of the tank. That can be enough to affect the boat's trim depending on the tank's location. You also need to compare the cost of all the materials (not to mention the value of your time to build a tank) vs the price of a top quality plastic tank that'll last as long as the boat will last, ready to drop into place and attach the plumbing to the fittings in the locations you've specified....



Just sayin'....



--Peggie

[boatpoker]

Originally Posted by peghall

If urine can eat through steel, even 316 SS, it will eat through any aluminum alloy. It's not the sheet metal that fails first...the first leak is always a weld--sometimes more than one--at a seam or a fitting, typically in 2-5 years. Meanwhile urine is gradually turning the bottom of the tank into a colander, a process that takes an average of about 10 years.


When thinking about building a FG tank, especially if you also use plywood, weight can become an important consideration. Water and waste weigh 8.33 lbs/gal, so even a 25 gal tank adds 200 lbs plus the weight of the tank. That can be enough to affect the boat's trim depending on the tank's location. You also need to compare the cost of all the materials (not to mention the value of your time to build a tank) vs the price of a top quality plastic tank that'll last as long as the boat will last, ready to drop into place and attach the plumbing to the fittings in the locations you've specified....



Just sayin'....



--Peggie

The only weight added for our epoxy tank was a 3X5" sheet of plywood, a gallon of resin and four layers of glass. I didn't work it out but I guarantee that was less than the original tank that I removed. Since I'm retired, 2-days and $200 Cdn. ($140US when I did it) was not only a sound financial decision, my tank will last at least as long as my boat and the fittings will never leak or expand/contract at different rates because they too are glass/epoxy ..... just sayin'
.

[james247]

Originally Posted by fishsail18

Is it ok to use an aluminum holding tank.? I’ve only seen them in plastic.

Plastic are better however if you are converting an existing tank it needs to be lined with an epoxy tar to create a barrier. It's a messy exercise best sprayed in from an inspection hatch.

[fishsail18]

I'm going plastic, I have the room for a 15 gal. tank. I'm thinking that's large enough for the two of us? we also have a electrscan in the other head for areas where we can pump over.

[peghall]

15 gal should be enough along with the ElectroScan--provided you're mostly in waters where you can use it...and it looks like you are. If you decide to spend an extended visit in waters where you have to use a tank, it could be a bit skimpy.

Ronco Plastics has over 100 non-rectangular tanks in their catalog Ronco Plastics marine Tanks They can be oriented any direction 'cuz there is no top or bottom until the fittings go in and you'll decide where to put 'em. So rotate and flip end for end...you may find you have room for a tank that's few gallons larger. And btw, they sell direct.

--Peggie

[fishsail18]

Hey Peggie, I have the Raritan seaera electric head I’m wondering if the head can pump up about 4’ to the top of the holding tank? I would like to mount the tank vertical in the corner of the lazarette. Do you think this will work? Thanks very much.