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How to Make Plywood Tanks
I recently had to replace the holding tank in my boat. Although it didn’t leak it had become quite smelly. By smelly, I mean a smell that was beyond awful. It literally smelled worse than anything I have ever experienced. I wanted to ask myself…What is this incredible smell you have discovered? Pardon the pun from Star Wars, I couldn’t help myself.
Along with a smelly tank cam smelly hoses, so it was time to replace the entire system. Since I had to replace the tank I also wanted to increase the capacity. The stock tank on our 30’ sailboat was 15 gallons. Not nearly enough for our 1-2 week summer trips and just barely big enough for weekends with guests.
The first step was to remove the old tank and the hoses. After the tank was removed I measured up both the tank and the space it fits into. Then I jumped online to look at stock tanks. I couldn’t really find a good upgrade in size. The largest stock tank I could find was 18-21 gallon. I really wanted to double our capacity to 30 gallons if possible.
After some discussion with my father, who made his own holding and fuel tanks 25+ years ago, I decided to build one. This way I could build it to fit the space for maximum capacity. Another advantage is that you can build a shaped tank, say one that would fit perfectly under a seat and shaped to fit next to the hull. My tank was located under the V-berth, so I could go wider and taller. After measuring it up I determined I could get very close to a 30-gallon tank and still keep it rectangular.
Making a plywood tank is relatively simple. Basically, it’s made from ½” marine grade plywood. The interior of the tank is lined with fiberglass cloth. The tank is built with butt-joint construction and epoxy fillets on the inside. This makes an extremely strong tank. The outside joints were then covered with fiberglass tape to strengthen the joint further.
The fittings I used were stock PVC fittings. I had one fitting for the head intake, one for the pump out, and one for the vent. To install the fittings I simply drilled oversized holes in the tank. Then I placed a temporary fitting, which had WD40 sprayed onto the threads, centered inside the hole. The hole was then filled with epoxy which had been thickened with just a little silica. After the epoxy dried (24 hours) I could easily twist the fitting out, thus leaving a perfectly threaded hole for the final fittings to be installed.
After the tank was made I coated the entire box with a 2-part epoxy primer from Interlux. This kept it looking clean and tidy. Plus this can be an important factor if you plan to sell your boat later on. Shipshape is a good thing!
The finished tank was then installed with new tie down straps, new intake and pump out hoses and clamps, and a new vent line.
The tank worked perfectly. No leaks, more capacity, and at a fraction of the cost of buying a tank as well. Check out our 3-step video sequence to learn more about how to build your own holding tank.
Video Article created by Kim Downing
Step 1: Create Your Own Fiberglass Panels
Watch an overview of how the tank is built. Plus, you’ll learn how to make your own fiberglass panels for the top, sides, ends and bottom of the tank.
Step 2: Create Threaded Holes for the Fittings
Learn how to make custom threaded holes for hose fittings. Plus, we dry assemble the tank and get it ready for final glue-up!
Step 3: Glue-up the Tank and Finish the Outside Surfaces
Now it’s time to glue the tank up. We applied epoxy fillets to the inside corners along with fiberglass tape to the outside corners. This made the tank both strong and leak proof. The tank also gets a final coat of epoxy primer to protect the exterior and keep it looking clean.