Self-draining holding tank

Jun 2, 2007
403
Beneteau First 375 Slidell, LA
Yes, again, but with a twist. I understand that plugging of the overboard drain line while the valve is closed is the biggest problem with this type of installation. It so happens that our boat had an electric macerator type head on board when we bought it. I'm thinking of plumbing the head discharge to a tee in the holding tank overboard drain line, with a valve between the tee and the tank, and retaining (of course) the thru-hull valve downstream of the tee. The thought process is that if (when) the drain line plugs, I can use the head discharge pressure to blow it out by closing one valve at a time and running the head. I don't really think a plug would be so solid that I'd be risking blowing a hose off a fitting or anything. I would have a dip tube from the top of the tank for pumpouts.

Does this sound workable?
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Why would the macerator pressure not do this for you already?
I would think that after running the macerator you would want to pump raw water, via the head, into the holding tank and run the macerator again to insure the hose is clear.
With that said, I would use a three way valve instead of a tee and be sure you have an anti siphon loop from head to three way valve to keep from flooding the boat. which introduces the extra pumping to clear the hose on the vertical part of the loop on the head side to keep it clear. Kinda moving the problem from one place to another IMHO.
KISS and you are probably not going to discover a better way to do it given so many folks have tired so many things on this topic.
 
Jun 2, 2007
403
Beneteau First 375 Slidell, LA
Sorry, I didn't explain myself very well. Inside 3-mile limit normal lineup would be thru-hull closed, tank valve open, head pumps upwards into tank. Pumpout thru dip tube if necessary. Outside 3-mile limit, open both valves to drain tank (blow out line if necessary) then close tank valve. Head pumps overboard. This was actually about as simple as I could come up with.
 
Dec 2, 1997
8,710
- - LIttle Rock
You're over engineering it...

Sorry, I didn't explain myself very well. Inside 3-mile limit normal lineup would be thru-hull closed, tank valve open, head pumps upwards into tank. Pumpout thru dip tube if necessary. Outside 3-mile limit, open both valves to drain tank (blow out line if necessary) then close tank valve. Head pumps overboard. This was actually about as simple as I could come up with.
The simple solution, no plumbing mods needed:

Flush the sludge out of the tank and the overboard discharge line after each tank pumpout or dump out by putting a few gallons of water into the tank via the tank pumpout fitting ('cuz putting it down the pumpout line sends the water into the tank at the bottom to stir up the sludge and hold iit suspension so it can be drained). If you have a washdown pump, use it to add the water to the tank when you're outside the 3 mile limit. You don't have to flush it out EVERY time...every 5th or 6th pump/dump is often enough.