Order a 20 gallon “super premium” polyethylene holding tank from Trionic, model SP-2020.
http://www.trioniccorp.com/index.php/premium-tanks.html
It will increase the holding capacity by 6 gallons, from the standard 14 gallon tank. It is a good deal at $220, which includes of the fittings. It fits almost perfectly into the existing space.


Old Tank Removal

Remove the teak trim board from the center of the rear berth.

Use a jig saw (and keyhole saw or oscillating mutli-tool) to cut rectangular hole about 18” to the right from the center and no more than 16” high from the bottom. If you cut any higher than 16” you will cut into the aft locker hatch board support. (photo 1)

Disconnect the inlet, outlet and vent hose from the holding tank. You may need to cut them. Dont worry, you should replace the hoses.

Remove the screws (large phillips head), that hold the tank to the shelf. It can be difficult to access the rear ones, but with some effort, you can get to them.

Remove the holding tank. Hold your breath and dont spill.

Remove the rubber strips on the shelf that were below the holding tank. Youll need to do this to fit the new Trionic tank. It needs every bit of height to fit. (photo 2)


New Tank Preparation

Remove the PVC vent elbow supplied with the tank and replace it with a straight PVC pipe. Buy a 8” or so long piece of PVC pipe and use PVC glue to glue on a new straight fitting with threads. Then screw the barb that came with the tank into the fitting on your new straight pipe. Dont re-insert the vent pipe into the tank yet. The tank wont slide into place with the vent pipe. Youll insert the vent pipe from the aft locker, once the tank is installed.

Remove the inlet and outlet elbows from the tank. They simply unscrew, but hold the tank fitting while you unscrew the elbows so you do not turn the fitting along with the elbow.

Remove the hatch board support in the locker shelf, the one toward the front of the boat. Youll need this out of the way in order to screw the inlet and outlet elbows back on to the tank once it has been installed.


New Tank Installation

Insert the new tank and position it so that the vent hole is aligned with the vent hose hole in the aft locker shelf Dry fit the new tank, making sure to keep the vent hole aligned with the shelf hole. Make sure the tank will fit so that you can attach the hoses. Mark the position of the tank on the shelf using a pencil. Remove the tank.

I used 1”x2”s to frame the tank on the shelf, front, back, and sides. I attached the 1x2s to the shelf with long stainless wood screws. I used two webbing straps with ratchet buckles to hold the tank in place. I ran the straps under the 1x2s on the tank sides and then ran them up and over the tank with the ratchet buckles placed on the top of the tank. Its easy to tighten the straps from the aft locker once the tank is in place.

After your 1x2s are cut to length. Drill holes in them for the screws. I drilled the holes a slightly larger diameter than the screws, so that the 1x2s could be easily pulled tight when installed. I placed two screws about six inches apart on each of the side pieces, two sets to surround the two straps. I first attached the rear 1x2 and then the starboard side 1x2. I drilled pilot holes in the shelf, using the 1x2s as guides. Make sure your two straps are in position underneath the starboard side 1x2. (photo 3)

At this point I installed a hose on the pump-out tee, that was a little longer than I thought I may need, while the tank was out of the way. It can be hard to reach the pump-out tee when the tank is installed. I knew I could cut the hose to the correct length once the tank was in place.

Slide the tank onto the shelf and check the fit.

Now screw the inlet and outlet elbows into the tank. I remember this was a pain, trying to get the room to screw them in, but with enough messing around, it is possible to screw them both in. Youll have to screw them in as tight as you can. When the hoses are installed, the removable shelf above the tank may just rest on the hose and elbows, or if you are lucky the shelf will clear the elbows.

Now install your port and forward 1x2s. Again, make sure your straps are in the proper location under your port 1x2. I left the port 1x2 a touch loose by not tightening the screws hard. I did this so that I could start to tighten the straps using the ratchet buckle and in order to pull the straps snug under the tank. Then I tightened the screws in the port 1x2 and used the ratchets to tighten each strap around the tank.


Finish Up

All that is left is to install the new hoses. One from the head to the tanks inlet elbow, and then connect the pump-out tee hose (you already have in place) to the tanks pump-out elbow, trimming the length as needed. I also installed a new hose from the manual waste pump to the wast sea-cock. This sounds easy, but it can be a struggle to route the hoses and tighten the clamps. You will definitely want to buy very flexible waste hose, not the cheap and harder white plastic waste hose. (photo 4)

Reinstall the hatch board support in the locker, that you had removed.

Install your PVC vent pipe in the vent hole grommet though the top of the locker and connect the vent hose to the pipe. (photo) (photo 5)

If youre lucky, you have no leaks and you can flush the head with confidence.

The last step is to construct a cover for the hole in your rear bunk. I made a cover that I can slide open so that I can view and access the tank. Since the tank I translucent, you can check waste level pretty easily.