Re: Macerator set up
Your holding tank should have 2 places for fittings at the bottom of the tank. One can be used to go from the tank to the through deck pump out fitting. Another would go to the macerator, which would then go through a through hull fitting. Mine came from the factory as described above. I don't have a Y or T valve. Instead, I have a double-switch for the macerator at the helm. I have to hit both switches at the same time and hold them to discharge through the macerator and overboard through the through hull fitting.
BTW- It's 3 miles offshore to discharge unless marked on the charts otherwise, not 25 miles.
You could take the line going from the holding tank to the through deck pump out and put in a valved Y valve. From the Y valve, one line would go to the through deck pump out fitting and one would go to the macerator. You'd leave it on the through deck setting for 99.9% of the time and simply pumpout.
I would recommend you pumpout the holding tank, then fill it with water and pump it out again, then repeat several times before beginning such a project. Also, since the holding tank has been used, I would go for option #2. Option #1 that I listed (No Y valve) is going to be a messy proposition at this point. By cutting some of the sanitation hose going to the throughdeck pump out fitting, you will stand a much better chance of avoiding unwanted spills. Plus, I've always wondering how LEO's would view my setup if I went inland. I suspect it is fine, though I've heard of some states insisting that the Y valve be zip tied or otherwise locked in the pumpout position. I have no way of doing that on my boat.