Simple pumpless holding tank discharge with gravity and syphon

[David B]

Originally Posted by StanL

Has anyone seen a simple marine holding tank with a gravity discharge into a seacock that draws from the top of the tank?

I'm hoping to eliminate a pump, and only use the syphon affect of a manually primed discharge hose.

The discharge hose would a few feet longer than required and fastened with a strap except in the rare instance of manual evacuation. The hose would be primed before manual evacuation and then the seackock opened. The hose would be elevated high enough to start the flow of the primed section of the hose out through the seacock. This would cause nearly the entire contents of the holding tank to empty through the seacock.

Benefits: No pump, pump & valve maintenance, simple. No waste lingering in hoses.

Drawbacks: Extra manual effort during (rare) manual HT discharge, including priming the hose. Room needed for extra feet of hose, and to manually lift hose high enough to start flow and syphon.

Haven't been able to see anything described like this on a boat forum, though remember a childhood friends' family had an RV which worked like this. Wouldn't a simpler system be better than anything with valves which don't last more than a couple seasons exposure to antifreeze? What other issues are there?

Why not from the bottom? We have a holding tank on both heads. Each tank is very simply plumbed. The heads pumps to the top of the tank - the tank empties through the bottom (75mm or so from memory).
There is also a vacuum pump-out port to deck discharge, and an overflow on the top as well.

No pumps other than the heads macerator pump - works beautifully, and when off-shore, the gravity discharge takes all of 4 seconds - 5 at most if we are heeling the wrong way.

[peghall]

The dry mode is a waterless flush. Manual marine toilets and optional flush panels for electric macerating toilets provide that choice. Assuming that the loop isn't higher than 4' above the toilet, "dry" flushing" can send most flushes over the top of a loop that's slightly above the top of the tank with minimal pumping...follow that with half a bowlful of water to rinse the hose. That routine can often increase the number of flushes a tank can hold by 50% or more compared to the usual simultaneous pumping water and waste allthe way to the tank. This is true whether the tank is above the tank or below it.


--Peggie