How to make tank constructed of plywood
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Holding boat tanks
Made-To-Measure Holding boat Tanks
A "waterproofed" tank constructed of plywood with multiple coats of epoxy resin and when you need extra tankage for your boating needs consider making your own.
Note: wood-epoxy tanks are not recommended for fuel storage due to the risk of fire.
How to make a plywood Holding tanks
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 151L (40 gal); use 8mm or 9mm (5/16" or 3/8") 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 19L (5 gal) 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 151L (40 gal) water tank, for example, requires two or three baffles spaced 30cm (12") 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.
In two sections
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 (2.5cm/1") 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. 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 10cm or 15cm (4" or 6") screw-in plastic inspection ports, positioned so they provide access to the entire tank for cleaning. On holding 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 12mm (1/2") stock to the top exterior where marked.
When cured, drill the holes. A typical potable water tank has a 16mm (5/8") vent, 3cm (1-1/2") fill and 12mm (1/2") outlet; a holding tank has one or two 16mm (5/8") vents and 3cm (1-1/2") 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.
Tankless Water Storage
If you need to replace your hot water boat tank and installing an on-demand system.
What are the pros and cons of this change? Are there special installation issues I should be aware of?
Instant hot water without a storage tank is a nice idea and it is technically feasible to have enough hot water instantly available for a decent shower. It just requires large amounts of energy on demand. That means either propane or 110-volt AC power (220 volts is even better but harder to come by). Diesel fired heaters also exist but I am assuming you have gasoline engines. Propane is the traditional method of doing demand hot water on boats and relatively simple to add if you already have an approved propane system installed.
Note that there are no American Boat and Yacht Council (ABYC) approved on-demand hot water heaters. However, propane and boats have a difficult relationship. Any installation should meet the relevant ABYC (U.S. Coast Guard does not regulate LPG on recreational boats) standards. A 110-volt AC power supply is always available when you are plugged in at the dock but underway or at anchor it can present problems. These appliances draw up to 30 amps at 110 volts AC thus requiring a decent sized onboard generator when away from the dock (assuming you don't mind turning it on each time you want hot water).
Since you are dealing with high voltages, this installation may also require a professional marine electrician to keep your insurance company happy.
Think about how the tank sits in the boat
Both with pump nozzle and with 5 gallon hug. During the first 2 to 3 gallons into the tank (at any level) has will gurgle and spit back out of the filler tube.
The bottom line is volume, what ever volume the fuel is filling that much air must come out of the vent. If it's done this from day one you could find the vent or fill line marginally kinked off. For example the vent hose could have been cut too long. So maybe it is kinked in a corner, sometimes it's as simple as shortening the hose. Think about how the tank sits in the boat. If there is any rise of the bow, air will trap toward the front of the tank. As you fill, you're pumping aerated fuel in and the trapped air has to have somewhere to go. If you fill more slowly, it'll help. Usually get my crew to the bow or have everyone get out to assist in getting the air out.
Should be able to remove the vent line at the ends and purge air thrue it at around 20psi, or seal filler neck and give short burst of air into tank, you should hear it escape at the vent.