Valving for overboard by macerator OR gravity

[Channelside]

I am replacing the holding tank on my new old boat and the new tank's bottom will be a couple of inches above the water line. I want to be able to discharge by gravity but there is a perfectly good macerator pump teed off of the pump out line about even with the top of the tank. There is a Y coupling at the seacock so I could put the gravity line in one side and the macerator discharge in the other. My question is, if I tee off of the tank discharge to the pump out/macerator line and the gravity line to the seacock, do I need a valve between the tank outlet and the seacock. Will I wind up pumping round and round when I use the macerator?

[jrd22]

The macerator is probably pulling from a fitting at the top of the tank connected to a tube that goes down to the bottom, at least that's the way it should be set up. Not sure that a gravity flow from a fitting down low on a holding tank is going to work well (at all) if the bottom of the tank is only a few inches above waterline. If you don't like depending on the macerator why not install a manual pump? If you do hook up both like you describe I would install a valve just so you know it's pumping overboard.

[Sea Diamond]

If the macerator is drawing from the top of the tank then I agree a tee off this line will not drain the tank by gravity.
However, if your macerator is teed off a discharge line coming from the bottom of the tank, I'm wondering if the original design was to have the discharge line proceed up to a deck pumpout fitting, and use the macerator when offshore. The Y at the thruhull may have originally been to allow straight discharge from the head when offshore. As you probably know, this is a very common setup.
I think you're on the right track with your concern about closed loop pumping. I'd also be thinking about the elevation of the tank and the possibility of syphoning seawater when heeled or in heavy weather. Does the macerator have a vented loop? Finally,with a deck pumpout as well as a macerator pumpout, do you need a third option that may be inviting more serious issues?

[Channelside]

Change in plans, found the mascerator pump to be on its last legs today so I am eliminating it and considering a gravity only overboard from a fitting at the bottom of the tank. I do have a macerating head so the stuff in the tank should be pretty finely ground. The drawback is that the tank bottom is only about 3 inches above the static waterline so the driving head near empty will be minimal. If it proves necessary, a manual pump would be pretty easy to install between the tank outlet and the seacock. All that said, my next question is, if I install a pump, is there any need to put a valve between it and the pumpout? Will the suction of a marina pumpout damage a manual pump?

By the way, sorry I started this in this forum I am a total newbie. If I can figure it out, I'll move somewhere more appropriate.

[Sea Diamond]

Maybe talk with your local yacht store to get an answer on the need for an isolation valve. I'm guessing it would be a good idea to protect the pump.
I'd still be concerned about the syphoning issue - can you be absolutely sure the seacock would always be closed except when draining overboard in calm seas. Failure to keep the valve closed could have serious (flooding) results. You might want to ask the experts if this is a good place for a vented loop. fyi - pumping 20 gal. by hand takes a while.

[Sea Diamond]

Sorry Channelside - I had forgotten the static head on the seacock is only a couple of inches. Doubt its a good application for a vented loop unless you go with a pump so the downstream loop could be raised up above the waterline to incorporate the vent.
Plumbing is lots of fun, eh? (sorry that's a Canadian thing).

[Channelside]

Thanks for all the replies, I think I have a plan and yes, it will involve a siphon break where siphoning may be a problem. I will be glad to have this behind me so I want to do it once. Thanks again.

[Sea Diamond]

I'd love to see a sketch of the proposed layout if convenient.