fireman: walters option is a good one, but there is a second option too.
set it up to have the overflow of center hard tank fill the pnp bag. Take the footwell cover out and take the overflow hose off from the bib going out of the hull. run this hose straight up to the lower connection on pnp bag. (you may need to drll a hole in the floor under the chilax seats to run this overflow hose into the pnp compartment. if you do, cut the hole as close to the pnp bags low connection as possible. this is important because this overflow line will be your pnp drain line and or fill line). Next, run a hose from farthest forward top connection on pnp bag down into the footwell and connect it to the open vent line bib. this will be your vent line for both the hard tank and pnp.
At this point you COULD be done. the system SHOULD work without adding anything else, but you may need to test it. As the hard tank fills, water will overflow into the bottom of the pnp sack. when you go to drain the system, the pnp sack water will backflow into the hard tank and be pumped out. This is how my rears are set up, as well as Eds rears. You may run into the same problem ed and I ran into with our rears.... as the pressure of the water builds in the sack above the hard tank and pump, the empty pump cannot hold back all the pressure and water started to flow out of the empty hose out of the boat. Ed and I just put ball valves inline on the empty hose, keeping this valve closed when filling. I have a feeling that trying to fill a 950 lbs sack through a 500 lbs hard tank will result in the same problem we experienced.
this problem can be solved with a single inline ball valve that would have to be closed when filling, and opened when emptying. This means you will need to put it somewhere it can be easily accessible and I do not have location recomendation for you. doing it this way will leave you with a pnp opening on top that will need to be plugged.
The other fix, that would be completely hands off when filling or emptying is this: on the hard tank overflow line as it comes up through the floor into the pnp bag, split it with a "y". take one of the lines and run it to the top connection on the pnp bag. this will be your fill line and will not need to worry about the pressure forcing water past the empty pump. on the second line connected to the "y", put a one way valve in line and connect that to the bottom of the pnp bag. the valve needs to be placed so that it will allow water to drain out of the bag. this way, when you fill your hard tank, it overflows into the top of the pnp bag, the one way valve at the y will prevent water from going into the sack at the bottom. when it is time to empty the ballast, the drain pump will empty the hard tank and the pnp sack will gravity drain out of the pnp bag, through the one way valve, and into the hard tank.
both these options are very close in set up, but I think that the 2nd option will be a little bit cheaper and it will not require turning a valve everytime you want to fill or empty.
i hope this makes sense...it makes sense in my head
