As I understand this, no bubble stream from a poo pump sucking an empty
bowl is ever gonna percolate much watery waste up a 3 foot loop
discharge, vented or no. Well, mabey no, since syphon action would
pull the poo up the unvented loop, but not efficiently, especially once
the donating vessel empties and the syphon is sucking air from the
source instead of from an air valve designed to short circuit the
phenomenon.

With all the frantic bubble pumping concluded, there is going to be a
significant amount of water that never makes it over the top of the
loop and tumbles back to the bottom, to rest against the duck bill and
foot valve, which might very well leak back into the bowl. My duck bill
leaks not at all, even (especially?) when tested with the holding tank
air pressure I use to purge the tank and test for withered wrinkled
duck lips and fawltey towers foot valves. Drink decent beer, it doesn't
rot the rubbers, Billy.

Tough titty. Get used to it. The only way to prevent scaling in the
discharge is to rinse copiously, every time, leaving only clean rinse
water in the tubes. Eliminate it with vinegar.

For arguement's sake, I value my time at one decabuck per hour. At 10
bucks per hour, how much air must a poo pumper pump to achieve the cost
effectiveness of replacing the bottom end of the poo out pipe once
every two years or so, or if it ever scales itself shut? Factor in a
cheap and easy quick fitting arrangement, so pipes are easy to inspect
and quick and easy to change using MJ couplings and even rigid pipe
where possible. Rigid pipe is safe, my house hasn't sunk yet, and the
pipes ( I really do go around snorfling pipes!) don't stink.

I am confident in my recipie, and am prepared to plunge the plumbing
whilst standing on my head in a puddle of poo in the bilge, again. I
have done it before and frankly, it didn't bother me much, really.
Youse are all a bunch of wussies. You survived a messy childbirth, so a
little internal goo applied externally ain't gonna hurt you, unless
someone in the crew brings cholera aboard in his gut sack. Where was
he eating?

I would not consider spending a thousand bucks to avoid smelling a fart
every few years. Mind, my system has required no maintenance since I
cobbled it up three years ago, though I did change the old bellowed car
radiator hose connecting the head to the port / sea "tee" just out of a
sense of unease. An engine oil change regimen is more trouble,
messier, and more environmentally challenging. A purgeable poo tank
enables supervised purging, only opening the sea discharge on occasion,
as required to maintain vessel ballasting trim.

Point for discussion:

-Keeping the sea valve thru hull open unattended is more dangerous than
not having an anti syphon vented loop.

Terry K