After spending a couple seasons filling and drain nearly 2k lbs of ballast every time we were out I wanted to automate it bit. The issue with this on a direct drive boat is the lack of compartments to hide bags in, as well as the amount of space the rolled up bags took up on my boat at least.
For a standard DD configuration I would do trap doors on both sides as pictured below. This allows the bag to be connected and ready to fill, and keeps it from taking up precious storage space on my small boat when empty.
In order to supply the bags you need an intake under the water line, a shut off, I needed a street elbow, and I needed to make a manifold for the fill pumps. I believe I went with 1-1/4" bronze mushroom thru hull, my elbow and ball valve are stainless, I know different metals...
i am also running an additional T and one more pump.
In addition to the fill pumps, on an aerator system you need a drain pump and an outlet, as well as a vent. I did not combine the two and have 3 thru hulls per side . These can be located closer to the bag and cut down on needed hose length.
I would recommend using quick connects on your bags as well, flow-rite is the manufacturer and they are very cheap to order straight from them.
I am running a spring check valve on each vent line, the pressure need to overcome the spring stops the bag from syphon draining once the pumps are off.
Also on the fill lines, I have spring check valves, one fills from the bottom and the other from the top. The top fill bag can syphon draining, the bottom fill bag will gravity drain back out the intake without the check valve.
With the bottom fill bag, I had a priming issue, idk which of the two solved the problem, but I added a vent prior to the check valve, and I turned the outlet on the pump to the top.