I really like the gravity feed system in my Beneteau
if you will be cruising in areas where overboard
discharge is legal
and appropriate. It meets both holding and dumping needs as simply as I've ever seen.
If you have the room above water
line - install a hard tank there. If the hose from the head comes into the top, you don't even need a vented antisiphon loop. From the bottom of the tank run a hose to the through hull
which is hopefully a short straight run. To empty the tank, all you need to do is open the through hull
. If it's always legal and appropriate, just leave the through hull open. No pumping required. Run a vent line as typical for holding tanks
. Use good head hose. If you are in the U.S. - have a way to secure the seacock in the closed position to keep yourself trouble free with the coast guard. If you want a pump out option, run another hose from the bottom of the tank to the deck
fitting for that.
I'm not as qualified to talk on the intake filling issue, but it seems to me you either need to raise the head or install an anti siphon loop on the intake side or get a head with a better seal at the dry/wet valve. I added an anti-sipon loop with valve on a previous boat I had and it worked fine. (The suction of pumping closed the valve on the loop to draw water) Other people have told me however, they never can keep the sipon loop valve working correctly with this set up.
If above waterline space is limited, consider either having a custom tank made or make your own epoxy/mat tank to fit.
What I love about the gravity feed system is no pumping, no rebuilding a bag pump (yuck) and it's quiet to discharge.
I'm no expert on heads, but have spent some time on the subject.