The typical installation has the Y valve on the output from the head. One branch of the Y goes to the fill port at the top of the holding tank. Output from the bottom of the tank goes to the deck pumpout fitting.
The other branch of the Y goes to a through hull fitting, which allows you to pump the head overboard. There should be a seacock on the through hull to prevent the boat from sinking in the event of a failed hose. With this arrangement it isn’t possible to pump the tank overboard. The Y valve only lets you pump the head over when offshore. The tank always needs to be pumped out. An alternative arrangement I’ve seen has the head pumping directly into the inlet for the holding tank. The outlet from the tank goes to a Y valve. One branch of the Y goes to the deck pumpout fitting. The normal operation is to fill the tank and then pumpout. The other branch of the Y goes to a macerator pump, which is plumbed to a seacock and through hull. When the tank is full and you are suitably offshore, switch the Y to the macerator branch and pump the tank overboard. In either event, I don’t think you can drain your holding tank overboard without a macerator pump. The tank is generally at least partially below the waterline, so gravity will only drain the tank down to the waterline. Rick Brass Washington, NC From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of David Knecht via CnC-List Sent: Wednesday, July 31, 2019 9:47 AM To: CnC CnC discussion list <CnC-List@cnc-list.com> Cc: David Knecht <davidakne...@gmail.com> Subject: Stus-List Offshore holding tank dump I hope to never have to do this, but our holding tank was rather full coming back from a cruise and we were well offshore, so I wondered about dumping the tank out there. I realized I have never tried to do it and was not sure how it is done. I don’t have a macerator pump that I can see. The Y valve is zip tied so taht would have to be cut. There is a hose from the holding tank that goes to a large stopcock normally kept closed. I am presuming that if I opened the Y valve and stopcock, the downhill pressure of the water in the holding tank would push it out the opening. Is that correct and legal? Thanks- Dave S/V Aries 1990 C&C 34+ New London, CT
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