Custom Holding Tank

Mithril · #1 (**permalink**) 03-13-2007

With the refit of my 36' steel sailboat I am planning on having a custom holding tank fabricated. Currently, I plan to have about 25- 30 gal between the wall to the head and the hull. I read a few articles on this forum so I plan to stay away from a metal tank but there seems to be some difference in opinion about which plastics to use.

What is the best material to use for a holding tank?

Some have suggested Polyethylene or High Density Polyethylene. I have also heard some recommend Polypropylene, several of the custom tank fabricators I have spoken to suggest using this material.

How thick should the material be? .25”, .375”, .5” ?

I need the holding tank to last as long as possible, because it will be a major ordeal to replace it (the cabinetry on the port side of the salon would have to be removed to gain access).

I plan to have an inlet, a discharge outlet, and a vent outlet in the holding tank. What other fittings do you think I should include? Iinspection port, a relief valve like domestic’s TankSaver, a second discharge outlet?

Anything else I should consider?

Thanks in advance for your input.

Here is a rough design for the tank as I currently see it. It would be 20 inches deep.

PBzeer · #2 (**permalink**) 03-13-2007

You would want the least permeable material you can find, as to how thick, I can't really say. You only need inlet, outlet, and vent, but you might want to put in something for a volume guage, such as the Tank Tender. I'd also try to keep the hose runs as short as possible.

sailingdog · #3 (**permalink**) 03-13-2007

I would go with the high-density polyethylene and make the walls of the tank a 1/2" thick. Remember that sewage weighs about 84 lbs/cubic foot, and the tank has to be strong enough to not bulge or sag with a full load.

I would also include an inspection/cleanout port on the top of the tank, provided the top of the tank will be accessible. This way, you can clean the tank thouroughly once in a while. Also, it allows you to clear the tank if it should get clogged. I used a clear Beckson deckplate/inspection hatch on a holding tank I recently helped modify, so that it doesn't have to be opened to do a rough inspection...

A volume gauge of some sort is highly recommended. One of the biggest dangers with a holding tank is overfilling it and having the pressure of the contents blow a hose clamp off someplace or having the contents clog the vent hoses.

I'd install two vent hoses, rather than just the one... to help keep the tank well ventilated. This helps keep the odor of the tank to a minimum.

sailortjk1 · #4 (**permalink**) 03-13-2007

To add to what SD has said, I would not burry the tank under "cabinetry that has to be removed to gain access." Try and find another location, or fabricate something into the cabinetry that will allow you better access. Its not something that you burry and never touch again, stuff happens, and when it does, you will want to be able to reach it.

sailingdog · #5 (**permalink**) 03-13-2007

Yes, I'd agree that you'd want to leave the hose fittings and the access port accessible, regardless of where the tank is located. If one of the hoses breaks loose or bursts, you don't want to have to disassemble the boat to get to it... and you will want to get to it quickly.

shantijwk · #6 (**permalink**) 03-14-2007

Mithril
Send your drawing to Ronco Plastics for a price quote or look up existing designs in their on-line catalog. They make custom water and waste holding tanks. WWW.ronco-plastics.com
They come highly recommended by Peggy Hall the "headmistress"

Mithril · #7 (**permalink**) 03-15-2007

Thanks SailingDog and all for your input. I do plan to have an access panel for the top of the tank to get access to hoses etc and for cleaning/inspection. It is just that removing or replacing the tank would be very difficult because of it's location.