Vertical space is the most-ignored real estate on a small boat, and the Ronco Plastics B503 is built to claim it: a compact-footprint, upright 21 gallon molding weighing 26 lb, made for lockers that are deeper than they are wide.
It ships as a dual-role B-series blank. Fresh water fittings produce a potable tank, its polyethylene food-contact grade; waste fittings produce a gray or black holding tank in the same tall shell. The upright format makes fitting elevation critical, a pickup too high strands water, a vent too low burps, so Ronco installs every fitting per order, spin-welded or threaded at exactly the heights and positions you mark.
Full, the B503 holds about 175 lb of water at 8.34 lb per gallon on its small base; brace it against tipping as well as sliding. Print the dimensioned drawing below, measure your locker depth honestly, then set the ports in the on-page fitting planner.
Molded in Tustin, California by Ronco Plastics, molding since 1974. The 21 gallon holding tank is the committed-waste alternative, and the boat tank line spans every other proportion; delivery is quoted at the freight cost estimator. OneSource Plastics is an authorized Ronco Plastics dealer.
Technical drawing: download the Ronco Plastics B503 dimensional drawing - print it to plan your install and mark exactly where you want fittings located; Ronco installs spin-weld or threaded fittings to your marked spec.
