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