Originally Posted by mottseng
John_Trusty, this was exactly what I was looking for. This past weekend we had 5 people on board 3 have never been on a boat. Everytime I looked someone was using the head and went through 4 rolls of double ply. Needless to say I had some major stinky problems to work our later. I too have a tank that slopes leaving a few gallons behind.
That would likely be 3.75 rolls too many. My standard briefing to new pax includes instructions on how to use the head, what can go in it, what CAN'T go in it, how much water to use (or not), how much paper is acceptable... and the warning that whoever clogs it fixes it. Agree, or leave the boat now.
The comment about usable capacity is also very germane. Our "40 gallon" tank will hold about 25 gallons, mostly due to the nature of the inlet and outlet hoses. You might consider learning
ground truth, by purposely filling it once and keeping track of how much went in during that test.
And then you can do some guess-timating about ground truth for typical flushes, too. Use a known measure to introduce a known quantity of water into the bowl, something that sorta replicates what happens when one uses the toilet controls. Then do some math...