Holding Tank Question

[russpatterson5]

My holding tank can currently only be emptied via pump-out. I want to add the ability to empty out through the thru-hull when not in restricted areas. My question is, do I have to have a macerator pump to make that happen, or will it drain via gravity/physics the same way the scuppers and sinks do?

Also, why do the macerator pumps just have a 1" output? The thru-hull/seacocks are always set up for 1 1/2". Is stepping it up from the 1" to the 1 1/2" at the seacock the normal way to handle that?

[Andreas W]

You will not need a macerator pump. Gravity will do the job. 1 1/2" hose is a good diameter for the discharge.

[Dsanduril]

Is the bottom of your holding tank above or below the waterline? This is the key question as to whether or not a gravity drain will work. If the bottom of the tank is above the waterline then I prefer a gravity drain, much simpler. Use a 1.5" through hull and valve, that way it is not a choke point, same size as everything else in the system.

If the bottom of the tank is below the waterline then you'll need a pump to empty it. Since macerators chew everything up into very small chunks you can manage with a 1" discharge. Some prefer a macerator even with an above the waterline tank as it chews up everything prior to discharge, but we had a gravity drain tank for 30+ years and never had a problem getting it to dump (pun intended).

Also, for gravity drain you really need an outlet on the bottom of the tank. This may be difficult/impossible. In that case a pump can use a "stinger" installed from the top (same as your pumpout fitting) to pump out the tank. Last thing you want to do is siphon drain your tank using a stinger - how do you start the siphon?