What I did to improve on the factory holding tank:
1) cut open the floor on the port cabin sole, being careful to cut a straight edge at the seat base and centerboard and keep it in one piece, so it could become a hinged cover later.
2) removed factory tank.
As you can see, this tank was a ticking time bomb.
3) removed all factory discharge hoses, except the one from the Y valve to the bow pump-out port.
4) removed Y valve
5) installed new 20 gallon polypropylene plastic Ronco tank in large space beneath V-berth. Tank was fitted (all fittings in top) with 1" vent, two 1.5" outlets with suction pickup tube, and one 1.5" inlet from toilet, all with 90-degree fittings. (Adding a second pickup tube so there was one for OB discharge and one for pumpout eliminated the need for a Y valve and all its complications.)
6) ran new inlet from toilet through bathroom cabinet, through forward bulkhead into space beneath berth on port side, then through to center section where tank was mounted.
7) all other discharge lines were run to forward space beneath v-berth where overboard pump was mounted from factory (vent, hose out to deck outlet (no pump needed on that line), outlet to discharge pump to overboard through-hull).
8 ) space where tank used to be was refinished. Centerboard trunk was also refinished with paint tinted to match hull liner (removed padded vinyl). Floor edges refinished and fitted with stainless piano hinge on seat-base side, mated to a stop on the centerboard side, so it could be walked on and used for storage. Space within sanded, painted, and used for storage (our "wine cellar" for canned and bottled goods, mostly).
The stop or ledge on the centerboard that supports the floor had not yet been added in the above image. I used a piece of angle aluminum and epoxied it in place.
After all the trim was fitted it looked almost as if nothing had been done.
9) new 1" vent went to new through hull at side of bow, beneath v-berth (as high as possible). This would allow some small amount of water to enter the vent when crashing into waves in rough conditions, for each wave. This required that the vent line have no dips in the mid-sections to trap water. The vent must be clear to prevent anaerobic conditions that lead to bacteria growth (it's anaerobic bacteria that cause odor -- vent is not just to relieve pressure). Because it let in water, that portion of the project could be improved upon, but it was a minor issue.
I left out a lot of detail in this post. If anyone is going to try this project I encourage you to contact me for advice that will help you do this more quickly and effectively. I encountered some issues and learned a few things I'd be happy to pass along.
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