Made-to-Measure Tanks

By Jan Mundy

Figure 1 - A tank constructed of plywood and “waterproofed” with multiple coats of epoxy resin. Baffles prevent the contents from surging in the tank and scallop-shaped cutouts in the corners vent each compartment.

When you need extra tankage consider making your own. Potable water and holding tanks made of plywood coated with epoxy resin are inexpensive, simple to build and are easily custom-made to fit any tank shape or hull contour. Epoxy is the ideal tank material. It waterproofs the plywood yet won’t contaminate drinking water and is resistant to head chemicals. I’ve used wood-epoxy water tanks on Nutcracker for 12 years without any bad tastes or odors. The tanks were constructed with West System brand epoxy, but I’m sure other resins would net similar results.

Carefully measure the space for the tank, then make a cardboard template. Assemble the pieces using masking tape to hold it all together. Dry-fit the template, then trace the patterns onto the plywood. Building a rectangular tank is straightforward: cut the ends, sides, bottom, top and baffles out of 1/4" marine-grade plywood for tanks up to 40 gal (151L); use 5/16" or 3/8" (8mm or 9mm) ply for larger tanks. Because the bottom and top overlap the sides and ends, you’ll need to deduct double the thickness of the plywood from your height measurements. Decrease the width of the sides by the same amount to allow for the overlap of the ends. To build a tank that conforms to the contour of the hull, laminate multiple sheets of thin veneer to the required thickness over a wood form (mold), then join as described below.

Tanks larger than 5 gal (19L) should have internal baffles to prevent the liquid from surging inside the tank. The number of baffles depends on the size of the tank. A 40 gal (151L) water tank, for example, requires two or three baffles spaced 12" (30cm) or less apart to ensure sufficient support. Cut deep scallops in the lower and upper corners of each baffle (see Figure 1) so the contents flow freely between compartments.

A removable wood-epoxy water tank fits snugly in the vee-berth and has two inspection ports for cleaning. A stainless-steel cable with adjustable turnbuckle attaches to the bulkhead and leads around the tank to a floor-mounted pad eye to hold the tank securely in place. A rubber gasket under the wire on the forward edge protects the tank from chafe.

Assemble the tank in two sections: the main tank with the baffles, then the top. Both are assembled separately and completely finished, then the top is glued in place. Join the sides, ends and bottom panels using cleats (1"/2.5cm) triangular pieces of wood) attached with thickened resin. Use staples, small copper nails or pipe clamps to hold the panels in place until the glue sets. Alternatively, bond the panels together using fillets: epoxy thickened with colloidal silica to a peanut-butter consistency that, when applied to an inside corner, forms a cove-shape bead over the join (Figure 2) . For added strength on larger tanks, cover the fillet with fiberglass tape and resin. The goal is to have rounded, smooth corners. With either method, apply thickened epoxy to all plywood mating surfaces. Scrape or sand off any excess epoxy, then sand the wood (and fillets) with 120-grit paper. Coat the interior of the box and the underside of the top with a minimum of four coats of unthickened resin. Apply three coats “green on green” — recoat when the resin is just slightly tacky and before it reaches its final cure stage (about 2-1/2 to 3 hours at room temperature). After the third coat let it cure thoroughly, then sand to a glass-smooth finish with 120-grit paper. This ensures an easy-to-clean surface. Because epoxy is transparent, add a white pigment (paste or powder) to the last coat at the rate of 5% by weight so you can easily see the scum when cleaning the tank.

Cut holes in the top for large clean-out ports: install 4" or 6" (10cm or 15cm) screw-in plastic inspection ports, positioned so they provide access to the entire tank for cleaning. On tanks with multiple baffles, locate the ports between each baffled section. Mark the placement for the vent, fill and discharge hose fittings, then glue backing blocks made of 1/2" (12mm) stock to the top exterior where marked. When cured, drill the holes. A typical potable water tank has a 5/8" (16mm) vent, 1-1/2" (3cm) fill and 1/2" (12mm) outlet; a holding tank has one or two 5/8" (16mm) vents and 1-1/2" (3cm) inlet and pumpout.

Glue the top to the tank, and hold in place with staples or clamps until set. Apply three coats of resin to the exterior of the tank, followed by two coats of an enamel or polyurethane paint. Install the hose fittings with sealant, then securely mount the tank in the boat.

A word of caution: wood-epoxy tanks are not recommended for fuel storage due to the risk of fire.