Originally Posted by Rogerh
Skid, do you by chance have a photo of the stack unit you are talking about?
No, but it's just a standard vented loop made of Marelon plastic (Perko brand), which is connected with a short (~3") section of hose atop the "Y" valve inlet. Double clamped at each end. So one end of the loop butts directly against the inlet to the Y valve. Routing is: toilet discharge to loop to Y valve.
The amount of fresh water in the bowl after a flush is roughly the volume of the hose between the toilet outlet and the top of the loop....about a 5" diameter puddle in our case...depends on the instant you release the flush button. Guess I could add a flush timer, but the normal users know exactly when to release.
Our boat originally had large radius bronze loops in remote locations...lots of dirty water sitting in the hoses. I purchased the plastic loops because the radius was smaller (shorter/narrower), which made the loop/Y valve "stack" easier to fit in the small space behind the cabinets. In one bathroom, I had to remove a vent blower to make space for this plumbing. By the way, most of the new black water plumbing is now PVC pipe and all of it drains down hill to the holding tanks or through hulls....no low spots for black water to accumulate. That was a bear to accomplish on our boat as it required rerouting in some very tight spots.