Hmm, I like this idea. Running that airline down into the bottom of
the tank with lots of holes would also let you aerate stuff if you
were in a no discharge zone. A 12 V air pump is 1/10 the cost.

What kind of tank do you have? I would have to pump against the head
pressure of the tank being below the waterline so would need 2 - 3 psi
which is a fair amount for a plastic tank. I would sure hate to bust
something.

--

Roger Long



"Terry K" wrote in message
oups.com...
My waste tank is purged to the sea or, with an external hose
connected,
to the Wall Mart plaza storm sewer like RVers do it, or an external
tank, like my porta pottie from the truck, using air pressure. We
don't need no steenkin' macerator pump, nor no 'nuther pump neither,
except the 12vdc tire pump, which pressurizes the system enough (1
or 2
lbs overpressure) to blow the "ballast" out the marine discharge. I
use
a plastic ball valve as a sea cock, and it doesn't leak. Nor does
the
joker valve, even with pressure on.

The only time I open the cheap plastic ball valve / sea cock / port
/
sea valve is when emptying the holding tank. The only valve, aside
from air vent valves in the system, is the one sea cock ball valve.
There is a tee between the head, through hull / sea cock, and
holding
tank pipe.

Two minutes of pumping air into the tank with the air vents closed
and
the sea cock open and it all goes out. Then, it farts to let me know
it's done.

Terry K

The devil is in the details. Thinking about it for another few minutes
last night, I realized that I don't want to subject all these plastic
hoses, joint, and screwed fittings that I'm trying to keep absolutely
odor tight to pressure. If it was a steel tank with hard piping,
maybe. Even then, I would want a pressure relieve valve and those are
very low pressures to have one work reliably.

A wad of toilet paper going up into the (too small) vent on our
plastic tank could also turn this into a bomb that you don't even want
to think about.

Slightly off the subject:

I remember a shipyard here in Maine needing to fill the built in tank
in a
just completed fishing dragger with water to test it. The water at
the
dock was off so they ran a long hose down the hill from the shop and
connected it to the tank. They were smart enough not to hook it up to
city
water pressure and ran the water into a funnel. They weren't smart
enough
not to hard mount the hose to the tank and the vent hadn't been
installed
yet. The weight of water in the hose, with a vertical drop of about
25
feet, bowed the top of the tank up about 3 inches. Since this was
also the
floor of the crew's quarters, all the accommodations had to be ripped
out
and a huge section of the boat cut out and rewelded. It cost about
$100,000
to fix.

--

Roger Long