Eighteen gallons is a deliberate size: big enough to be a boat's primary water supply for coastal cruising, small enough to pass through a real locker opening and sit in spaces that 30-gallon tanks only dream about. The B345 serves it as a bare marine blank at 31 lb - a solid mid-weight molding, with enough wall to be trusted and little enough mass that installation is a one-person afternoon.
Ronco ships it without fittings and installs them to order, which is the quiet superpower of this whole line. Your plumbing plan comes first; the tank conforms to it. Print the drawing below, mark fill, vent, and pickup with the fitting planner on this page, and each port is spin-welded or thread-installed at the Tustin, California plant that has molded marine tanks since 1974. In fresh water service the polyethylene is FDA-compliant, safe for potable supply; as a dual-role B-series shell, waste fittings turn the same blank into an 18-gallon gray or black holding tank.
Count on about 150 lb when full - manageable, but still worth a flat landing and a strap that would hold in a knockdown. Ronco molds this capacity in more than one shape, so scan the boat tank category before settling, cross-shop trailer builds in RV water tanks, and see everything else on the Ronco Plastics hub. OneSource Plastics is an authorized Ronco Plastics dealer.
Technical drawing: download the Ronco Plastics B345 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.
