DIY Holding Tank

It’s amazing how much thought has gone into a marine sanitary holding tank. There is a local marine sanitation supply company in Seattle who I can buy a custom tank with all of the fittings preinstalled but they seem to be uninterested in helping me figure out how to go about fitting a new tank. I finally decided to do it without any help, except what the internet can provide. Mind you, the internet has taught me quite a bit about DIY holding tanks. So I broke down and bought a new 10 gallon tank without any fittings. I also bought two pump out kits to prevent having a ‘Y’ valve in the system. So what’s involved in building your own tank? Well back when Satori had her first holding tank installed the tanks used bulkhead fittings. Now a-days we can use Uniseal fittings and PVC pipe fittings to get a clean seal without having to use bulkheads. This eliminates having a 6″ hole for the sole purpose of threading the bulkhead backs from the inside of the tank. The result is a clean seal and a low-profile attachment for compact storage.

Marking the holding tank for drilling
Marking the holding tank for drilling

The first step was to place the holding tank into the locker and create the platform to hold the tank steady. Then once the location is determined, I marked the location of the pump out and input fittings. In the event the entire assembly is too high, I can now just simply cut away some of the plywood bottom of the counter top for a perfect fit. At the time of writing this, the fittings look to fit perfectly (knock on wood).

Using a hole saw, drilling the holes is pretty easy

Once I determined the center marks for the drill bit in the hole saw, I can start drilling each hole. I used a little water and made sure that the drill did not heat the plastic too much to prevent any deformation. I cut three 3″ holes for a 2″ PVC fitting and then another 1 1/4″ hole for a 3/4″ PVC fitting.

First fitting is installed into the holding tank using a Uniseal
First fitting is installed into the holding tank using a Uniseal

Next was to get the pump out kit assembled correctly and prepared for installation. The key to the assembly is to make sure the seal is attached when gluing the PVC fittings together. When I measured the depth of the tank, the instructions stated to take 6″ from the pipe when cutting to fit. I think 5 1/2″ would have been a better fit. If you’re doing the same thing, make sure to cut long at first, then cut again. You’ll want the tip of the fitting at the bottom to barely touch the bottom of the tank as the actual bottom of the tube will still be sitting 3/4″ off the bottom. Mine is more like 1  1/2″ off the bottom so there will always be a little left when pumping out. No big deal.

The pump out assembly is ready for installation. I used hand soap for the Uniseal lube

When I was imagining the vent fitting, I made a mistake with buying the proper assembly. I decided to buy a nylon elbow barb fitting and then a 3/4″ threaded collar to PVC fitting. This ended up much too tall to fit into the locker so I’m going to scratch that setup and buy new fittings to make it much lower profile. Once the vent fitting is installed, the tank is ready to be installed with sanitation hoses and secured for offshore sailing.

Just need the vent fitting and the holding tank is complete

This morning I just ordered the rest of the components for this project. Today I’m also buying the toilet to begin drilling the holes through the floor for the toilet plumbing. I still have more wood finishing to do before I can install the toilet but I’m starting to see the end of the project more clearly. Until the other components arrive, I can focus on finishing the wood and counter top. I set a goal to complete this project before December and as long as the mail-order delivery is next week I think it’s very possible.


2 replies on “DIY Holding Tank”

Hey is that Seattle company that was not very helpful located next to fisheries. Well if it is, we had the same problem. Just curious, Glad your vessel is on the West Coast


Yes you’re correct, the shop next door to Fisheries Supply. They had the toilet I want to use so as far as keeping inventory they have more than anyone else around. I just wish they weren’t just a supplier and had better service. What problem did you guys have?

Comments are closed.