Because we use our A27 Sport Cruiser inshore, offshore and also visit the Bahamas I will plumb the head with both the holding tank (pump out) and through hull discharge. I have seen three different diagrams for the plumbing:
#1. The Y valve is before the holding tank allowing the choice of A) waste going directly to the through hull or B) waste going directly to the holding tank for pump out. Each option is isolated from the other.
#2. The Y valve is after the holding tank. Waste always goes into the holding tank and then from the tank the choice of A) pump out or B) thru hull.
#3. Two Y valve's: one before and one after the holding tank. I guess this would allow A) waste going directly to the through hull or B) waste goes directly to the holding tank for pump out or C) waste going into the holding tank then to the thru hull.
I am definitely leaning toward #1 with the Y valve before the holding tank. #2 is what I think was original construction with the Y valve after the tank. #3 just seems like unnecessary overkill and confusion to me. Regardless, before diving in I'd like to hear what others have done and hear your thoughts & experiences as well.
I should also note that we have plenty of room to mount the Y-valve in the head before the aft bulkhead. I removed the sink & cabinet allowing full width in the head... for the head. I never did use that teeny tiny sink when the larger galley sink is 3 feet away. We didn't loose any storage as I installed two small hatches on the horizontal "shelf" accessing the void along the hull and glassed in a "bottom" (the TP is always dry

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