Ballast and check valve question

Ok so I am using this diagram as reference. I realize I do not need everything they have listed but I do plan on tying in the drain and vent lines together. My question is, if you have the Drain pump/line attached to the bottom fitting of the sac, wouldnt water continually drain out of the bag while under way? What is the point of the check valve in the vent line?

Wouldnt it be better to attach the fill line and put a check valve on that on the bottom of the bag so it doesnt drain out and put the Drain line/pump on the top of the bag? But would you have priming issues with that?

I am planning on doing a three bag system with a total of 6 aerator pumps so I want to get it right the first time. Let me know what works for your setups.

Oh and dont get started on "just shell out the extra dough and run a reversible pump" I dont wanna thats why.

Yes, you would have priming issues if you don't put your drain line and drain pump on the bottom of the bag. The drain line won't self drain, because your line goes up and out your vent/drain port on the boat. There simply isn't enough force to push the water up and out without the pump pumping.

I would avoid putting a check valve and fill line on the bottom - you want the fill line to fill your sac with as little resistance as you can. The fill pump would have to pump water against the force of a full sac and through the slight resistance of the check valve - not good.

Also, the check valve in the diagam is placed on the vent line so that your drain line goes out the boat instead of being cycled back into the bag via the vent line. This check valve is important.

Follow the diagram and you're set. It's been tried tested and true, so trust it !

Yeah that makes sense. Thanks for your input

Ok so just to clarify. Don't put a check on fill line and only put one on the vent. So I need a total of three check valves with three bags. Another thing with this setup , am I going to need a ball valve at the intake manifold in order to stop water coming in at all times? I would suppose it would be a necessity for safety anyways to prevent the boat from flooding in a catastrophic failure but in theory I would need to get into the engine compartment and turn the ball valve to off every time I fill the ballast right? Is there a way around this?

I wouldn't put a check valve on the fill line. The drawing has it because the fill line can in theory siphon itself and empty the bag out the thru-hull in the bottom of the boat. I don't have a check valve on my fill line sacs, and have never run into this. Keep your fill line up high and you should be fine. Just check-valve on your overflow line before the Y to the drain.

Ball valve on your thru-hull intake is a very good idea if you ever do have a catastrophic failure (hose leak, pump leak, etc. Defintely make sure you use one. I personally leave my ball valve open all the time so I never touch it, but it would be a good idea to close it if you leave the boat on the water for any length of time to avoid the rare chance of a leak and the bilge pump can't keep up.

Also, not sure what boat you have, but I have successfully added a T to my existing factory intake thru-hull. From there I attached the factory pump and my second pump, just as they have depicted in the diagram. Works great, and avoids having to drill another hole in your boat.

I did this system last year for the two rear bags in my supra24v. I ran my fill lines all the way to the top of the engine bay and looped them and my 800 pound bags still siphons it self down to about half way. So this year I had to add the check valve on the fill lines. I'm not saying it happen to every body but on my bag it did. I would just follow it and do it right the first time they know what there talking about.