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Gravity holding tank does not drain
I'm new to this but would certainly appreciate any thoughts and/or advice. I purchased a 2001 Beneteau 331 last year, and the Jabsco manual head was plumbed so that a Y valve allowed you to either pump directly overboard, or into the holding tank. The holding tank could only be emptied by pumping it out. I believe this was/is the standard arrangement in the US.
Since I live and cruise in Canada, where pump-outs are less common and discharge is allowed in open water, I replumbed the head so it pumped into the holding tank, and put the Y valve on the holding tank outlet, so the tank could either be pumped out, or drained overboard. I also installed a holding tank gauge, along with new hoses and clamps, etc. Outlet hose is standard 1 ½ inch.
The system worked great in tests - put water in the holding tank, put the Y valve in the drain overboard position, open the through-hull, and whoosh - the tank drained completely. The bottom of the holding tank is a couple of feet above the waterline. However, while on our first longer trip I noticed that the tank was not draining when both valves were open (glad I installed the gauge). This led to some rather unpleasant work with a long stick to unclog the drain. The same thing happened a couple of days later, and this time I could not get it to drain at all. I was able to empty it at a pump-out station when we returned home.
The holding tank vent is clear and the tank discharge hose is relatively short, although it does have a right-angle bend as it leaves the tank. I believe that solids are collecting at the bottom of the tank, and gravity isn't enough to push everything through the outlet hose. It seems as if there are two possible solutions:
1) Install a manual or electric pump to empty the holding tank
2) Install a macerating head
The first option involves providng additional suction to empty the tank, instead of just gravity. However, the best location for the pump would be at the bottom of the tank, and there's virtually no room there at all. To allow the tank to be emptied at a pump out station, the pump would also have to be downstream of the Y valve, which would again put it further from the tank and where any blockage would likely be.
The other option would involve installing an electric macerating head, which would hopefully eliminate the problem by grinding everything up before it goes into the holding tank, allowing it to drain by gravity as intended. It would also simplify 'head handling' for guests etc. I understand Jabsco makes electric heads which have the same dimensions, connections etc. as their manual heads.
I'm leaning toward the latter idea, but welcome your thoughts and advice. I guess the main question on my mind is if the tank will drain without problems if it contains only macerated waste. Can anyone vouch for this arrangement? Thanks for your posts! (I long to go on another trip without carrying a ‘stir stick’…)