Here is a link to some ideas at the West Marine web site..

http://www.westmarine.com/webapp/wcs...tionsystem.htm


One can get complex, or simple as needed. You should 1st consider how
you want to use the system. You mentioned there are few pump outs;
but do you want to only be able to empty the tank at sea? And when
using the head, do you want to have the options to directly discharge
overboard as well as be able to pump into the tank?

1) The simplest setup would be to plumb the head into the top 1.5"
stub with no down tube. Then run a single hose from the other 1.5"
stub, the one with the down tube, to a deck mounted pump out plate.
When using the head, it always goes into the tank, and then you use a
shore based pumpout to empty. Simple, and little to fail (err, leak
:-)

1a) And alternative is to run the down-tube stub through a pump
(manual or electric) through a seacock. The head still goes into the
tank, but to empty you go overboard. BTW, a manual pump would be
fine for a small 6 gal tank.

2) Getting more complex (and flexible) is to add a 'Y' valve to the
down-tube. One of the outlets of the Y valve would go to a deck
mounted pumpout plate, the other side to a pump and the seacock. In
this way you can empty the tank using a shore based system, or out at
sea. I have seen some referances to using a simple 'T' instead of a Y
Valve. The idea being that closing the seacock would allow the deck
pumpout plate to be used, and making sure the deck plate was sealing
well would allow the pumpout pump to be used. Seems attractive to me
in that there is no 'Y' valve, but have need seen it in real life so
no idea if it realy works....

3, 4, 5..) There are still more combinations possible, ala a Y valve
from the head to the tank, or overboard; treatment systems, etc.

But in your case, it sounds like Simple is best. I would suggest 2 as
the most flexible while still keeping simply. If you are sure you
will never use a shore based pump out, then 1a is attractive as it
eliminates the Y Valve. You should also check local regulations: some
places do not like the idea of a system which is not able to use a
shore based pumpout. And some require a 'lock' on any overboard
options, e.g.: a locking Y valve, or perhaps a lock on the seacock.

Other comments: In any case, I would not worry about getting sea
water into the tank to flush it, just us the head to add sea water
while you are emptying the tank. I do seem to remember someone
writing about a setup where their tank, pump, and all was below the
seawater level. When running the pump it emptied the tank, then the
stopped the pump and sea water backflowed back into the tank. Then
the ran the pump again to empty.. But do not remember enough about
that to be sure. (They were using a discharge pump which allowed
seawater to backflow. A diaphragm type pump, like manual pump, would
not allow this)


And the vent; they say Air is the important to keeping the tank from
smelling. Use the oversized host (perhaps 1"?) from the air vent, run
it to somewhere up high with as little bends as possible. Make sure
there are no 'dips' in the hose routing which could collect water and
thereby blocking the free exchange of air.

I hope this helps. And good luck!

-al-




On Mon, 03 Jul 2006 07:58:20 GMT, ray lunder wrote:

Ahoy, I'm trying to figure out how to install this plastic holding
tank. So let's be optimistic and say the tank is half full and you
want to pump it all out and you're 3 miles out, etc. You open both sea
cocks, divert the Y valve to the holding tank and start pumping. But
doesn't this just fill the holding tank with sal****er as it flushes
out the waste? I want the tank to be clean and empty, right? Sorry if
this is dumb, I've never had a tank before. It's 6 gallons with two 1
1/2" stubs, one of which goes down to the bottom of the tank, and one
3/4" vent. Where does the vent get installed? All the stubs are on top
of the tank. Thank you.