The B556 spreads 20 gallons across a 26 x 25 inch near-square base only 8 inches tall. It is a plinth of a tank - the shape you want under a berth platform or cockpit sole where the structure above needs to stay put and only a sliver of height was ever available. Low, wide, and stable: nothing about this molding wants to tip or shift.
It is a dual-role marine blank like the rest of the B-series. Supply fittings make it a fresh water tank in FDA-approved potable polyethylene; waste fittings make it a low-profile gray or black holding tank. Ronco installs your choice per order, spin-welded or threaded to your marks. With 8 inches of depth over a broad floor, pickup placement decides the last usable gallon - print the drawing below, mark the low point of the actual installation, and set the layout in the on-page fitting planner.
It ships at 25 lb and carries about 167 lb full, spread across almost the whole footprint, so the load per square inch is gentle. Keep the support continuous and the tank will outlast the boat's upholstery several times over. Molded in Tustin, California by a maker at it since 1974.
Other low-profile formats are in RV holding tanks and boat tanks; the full Ronco range is on the Ronco Plastics hub. OneSource Plastics is an authorized Ronco Plastics dealer.
Technical drawing: download the Ronco Plastics B556 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.
