A 12 x 12 inch square section running 32 inches long: the BOAT27W is 19 gallons shaped like a beam, and it slides into the chases, quarter-berth margins, and engine-room corners where a conventional tank footprint never fits. If the space you have is deep and narrow instead of wide and shallow, this is the molding drawn for it.
The H.D. wall makes it heavier-built than Ronco's light-duty series - this is the marine-duty shell - and the polyethylene is FDA-compliant, rated for potable water. Ronco has molded tanks in Tustin, California since 1974. Fittings are installed per order at the locations you mark: print the drawing below, set your inlet, outlet, and vent with the fitting planner on this page, and it arrives welded to spec.
Water is heavy in any shape - about 158 lb when this tank is full - and a tall narrow section concentrates the load on a small base, so give it a solid shelf and a restraining strap. See the other narrow-profile options in boat tanks, or the complete catalog on the Ronco Plastics hub. OneSource Plastics is an authorized Ronco Plastics dealer.