[Mithril]

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.