Hello For our holding tank the following was advised by Covey Island Boatworks: Lavac pumps directly into top of holding tank Holding tank has an independent suction shore pump out pipe from the top of the tank to 2" above the bottom of the tank for vacuum pump outs At the bottom of the tank there is a 1-1/2" rigid outlet with a ball valve which leads directly to the through hull. The tank is also fitted on the top with a 1/2" ball valved vent pipe For operation: At sea - air vent left closed, valve to seacock left open, pump pressure runs sewage through the tank and overboard NDZ - Air vent opened, seacock valve closed, so tank fills. To dump where legal, we open the vent and seacock and let it gravity drain. This seemed to be a very simple system with no Y valve or macerator pump to contend with. Covey Island Boatworks says this is how they plumb all their boats, none of which we could come close to affording, so we figure if it is good enough for them it should be good enough for us! Lavac just arrived, so we will be installing soon. Darby & Mike m/v Some Goode