I am fitting out a 46' ketch
at the moment and have designed a very similar setup. My primary question: is a vented loop necessary? If the tank is full, the discharge valve must be closed, so no need for the vented loop because there's no connection to sea. If the discharge valve is opened, the holding tank will empty, so a siphon will not commence. Basically, I see the holding tank and its breather valve as a big vented loop.
Two other questions. If the tank fills and head
is again flushed, the breather will eventually fill with poo, so what would be the best level indicator or device to warn the tank is nearing capacity? A friend suggested a vacuum style reader similar to that used in washing
machines. Most devices I've seen are designed for fuel
or other clean fluids, so most likely to gunk up, so a vacuum might be the answer. Or would the head
backfill and struggle to flush - to indicating an issue?
Lastly, I was going to place a y-valve on the hose between the tank outlet and the overboard
skin fitting. One route
would empty overboard
, the other would direct to the pumpout. I'd have a separate ball valve on the skin fitting. I could use the pumpout deck
fitting to flush pressured water back up the line into the tank to clear blockages.
All good theories, but eager to hear an experienced point of view.
The bottom of the head is regular waterline and one third up the poo tank is the heeled waterline, if that helps paint
the picture. I'm using a TMC electric
head and have been told it will only pump up 1.5m - when the impeller is new. The top of my tank is currently 1.2 above the base of the head, so I'm trying to steal back every centimetre I can. A vented loop will cost me 15cm. If it's not needed, I could possibly plumb into the side of the tank at the top, rather than looping over and that will save me even more precious cms.