Steve Lortie wrote:
I am considering making my own black water tank. Blackwater being
toilet waste.

Strongly consider Wayne's suggestion. Ronco Plastics (no relation to the
VegoMatic Ronco btw) makes TOP quality poly tanks for a very reasonable
price.

I would like to place the tank under the sole between the centre
stringers.

That's a good location for a water tank, but not for waste holding tank.

What size is your boat? Sail or power? Where's the head--forward or aft?
Also, unless you're restoring a houseboat, 45 gals is a LOT of waste
to store aboard...a lot of weight--close to 400 lbs including the weight
of the tank. Why do you think you need a waste tank that big?

I should be able to place a
toilet right over the tank to eliminate pumping it from toilet to
tank. Good idea?

Maybe...maybe not. What kind of toilet do you have in mind?

--
Peggie
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Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://shop.sailboatowners.com/books...ku=90&cat=1304