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Old 09-03-2023, 11:35   #1
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Will head tanks gravity drain effectively?

So I need to redo my head plumbing this spring, the previous owner had it set up in a way that I don't really like and doesn't really work for us.

I've got a jabsco manual head and there is a 20 gallon black water tank in the wall behind it. The through hull for the discharge is below the sink next to it and the pump out is directly above it. What I want to do is plumb the head into the inlet of the tank, then where the outlet is I want to go out a few inches an T it off going straight up or down. This way it goes up to the pump out or just flows down to the through hull. This gives me the option to close the through hull when we are at anchorages or places where we can't just flat out discharge, but then allows me to open it and dump it when we get to open water. It also maintains my ability to get it pumped out if we need/want to.


My main question is will a holding tank effectively gravity drain without having a macerator on it? Or will solids eventually build up and block the outlet?
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Old 09-03-2023, 12:17   #2
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Re: Will head tanks gravity drain effectively?

Almost exactly what i have, however I have a y valve, directly overboard or into tank.
I have a y valve below the tank, to deck fitting or overboard via a y fitting near the thru hull.
Gives you the ability to do anything.
On a port tack with a good breeze ia little over canvased i can have water washing in and out if the holding tank to keep it nice and clean.
It’s a design which operated properly leave no fecal matter in any pipe. No pumps to fail.
It’s the best setup I have seen, which is why it’s done that way.
It has always graviity drained perfectly.
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Old 09-03-2023, 12:40   #3
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Re: Will head tanks gravity drain effectively?

Oh, forgot to mention first fitting out of tank is a pvc ball valve, i can remove the tank non empty if the need arose, which it has not in 20 years.
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Old 09-03-2023, 12:41   #4
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Re: Will head tanks gravity drain effectively?

Yes, a head tank will gravity drain without a macerator.

We have Jabsco manual heads into gravity drain tanks. They have worked for 27 years, and that includes routine additions of toilet tissue.

Attention to details helps. If the inlet piping is 1-1/4", make the outlet piping 1-1/2" Be sure to use full port valves.

Ours pumps out through a dip tube that comes straight down from the deck fitting, and drains straight out the bottom to the seacock. I am not sure exactly what you are changing, and what doesn't work on your current system...
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Old 09-03-2023, 12:46   #5
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Re: Will head tanks gravity drain effectively?

Holding tanks can drain via gravity but they must be entirely (bottom at least 6-8" above the boat's waterline) to do it. That can put the inlet fitting on the tank 3 feet above the toilet discharge fitting. Jabsco manual toilets can lift up to 4 feet. Pumping waste straight up that far is gonna make important to learn to use the dry mode lto leave as little in the discharge line to run back down as possible,and also to replace the joker valve to keep it out of the toilet.


If you keep the thru-hull continuously closed for very long, sludge will accumulate in the drain line...not much of problem unless left long enough to pack it. Pumping out won't pull the sludge out of the drain line, so it's important to be able to get into waters where it's legal to open the thru-hull and flush out the drain line...and the tank--fairly often.


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Old 09-03-2023, 12:53   #6
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Re: Will head tanks gravity drain effectively?

[QUOTE


If you keep the thru-hull continuously closed for very long, sludge will accumulate in the drain line...not much of problem unless left long enough to pack it. Pumping out won't pull the sludge out of the drain line, so it's important to be able to get into waters where it's legal to open the thru-hull and flush out the drain line...and the tank--fairly often.


--Peggie[/QUOTE]
Or the first fitting out of the tank could be a ball valve, all the above issues negated.
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Old 09-03-2023, 18:14   #7
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Re: Will head tanks gravity drain effectively?

Quote:
Originally Posted by SailingHarmonie View Post
. I am not sure exactly what you are changing, and what doesn't work on your current system...
Currently it's set up to either go straight out through the seacock or to go to the tank to be pumped out. The reason I don't like this is that we will be in the caribbean starting next year and so we will be wanting to discharge overboard. However, I want to be able to flush into the tank and discharge when I choose to. I'd rather not be dumping sewage overboard while were at anchorage because that just seems rude.
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Old 09-03-2023, 19:09   #8
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Re: Will head tanks gravity drain effectively?

Quote:
Originally Posted by nuru05 View Post
I'd rather not be dumping sewage overboard while were at anchorage because that just seems rude.
Same reason we went with that set-up.
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Old 09-03-2023, 19:56   #9
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Re: Will head tanks gravity drain effectively?

Yep, I installed that setup on our previous boat during refit for 2 years of cruising - worked perfectly. I just installed the same setup on our current boat last summer. Outlet from the tank out the side at the bottom to a PVC T. PVC ball valve going down to the thru-hull and PVC pipe going up above full waste level then a short bit of hose connecting to the deck plate. If need be, I could push a hose down from the deckplate all the way through the tee, valve and out the thru-hull. I leave the thru hull open so get some seawater flushing the valve and lower hose. Close the PVC valve to hold, leave it open to dump. The head always discharges into the top of the tank.
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Old 09-03-2023, 20:09   #10
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Re: Will head tanks gravity drain effectively?

Just want to commend you on your intent. Our holding tank is in the bilge, so we can't gravity drain, but we operate the same way. Pump to the tank 100% of the time, macerator pump overboard as we transit to the next anchorage. It's just right.
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Old 09-03-2023, 20:11   #11
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Re: Will head tanks gravity drain effectively?

That's dding an unnecessary fittinng whose only function would be to protect owner from failure to maintain the system, adding another layer to "whatever can go wrong,will."



A marine surveyor named Charles Kanter introduced the gravity drain holding tank about 30 years ago...he called it "The KISS (keep it simple stupid) MSD." He specified the parts needed:
1 tank
1 outlet seacock
1 inlet seacock (from the toilet)
1 "Tee" fitting (he used the same hose for both pumpout and overboard discharge)
hose clamps on minimum number of joints, double clamped
hose clamps for vent double clamped
Vent fitting
X Feet of hose or PVC pipe


I've never seen any need to add any additional equipment, especially any that require any maintenance, although I think I would run separate pumpout and dumpout lines, eliminating the tee.


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Old 10-03-2023, 00:14   #12
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Re: Will head tanks gravity drain effectively?

Quote:
Originally Posted by peghall View Post
Holding tanks can drain via gravity but they must be entirely (bottom at least 6-8" above the boat's waterline) to do it. That can put the inlet fitting on the tank 3 feet above the toilet discharge fitting. Jabsco manual toilets can lift up to 4 feet. Pumping waste straight up that far is gonna make important to learn to use the dry mode lto leave as little in the discharge line to run back down as possible,and also to replace the joker valve to keep it out of the toilet.


If you keep the thru-hull continuously closed for very long, sludge will accumulate in the drain line...not much of problem unless left long enough to pack it. Pumping out won't pull the sludge out of the drain line, so it's important to be able to get into waters where it's legal to open the thru-hull and flush out the drain line...and the tank--fairly often.


--Peggie
I hate to argue with the queen of the marine head, but the Jabsco manual says different:

Quote:
It is self-priming with a dry suction lift of up to 3 feet (1 meter) and a discharge head to 9 feet (3 meters).
A Jabsco head would have no trouble pumping up to most any gravity drain tank on most of the boats this crowd is interested in.
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Old 10-03-2023, 05:19   #13
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Re: Will head tanks gravity drain effectively?

I switched both heads to gravity fed…… my beneteau black water tanks are set up with a discharge at the bottom which previously was the deck pumpout and one from the top of the tank with a pvc pickup tube inside the tank for the mascerater. I switched the bottom to the gravity discharge and the top pickup tube to the deck pumpout.which both work perfecting. Also, when setting this up, I repositioned the pickup tube inside the tank to sit right above the bottom discharge. Every few weeks or so after a pumpout I use a hose and high pressure flush from deck fitting to flush gravity hose. By switching to gravity fed I eliminated at least 5 feet of stinky hoses, a stinky y valve and mascerater and opened up a ton of new storage under both sinks. For me a no brainer. Good luck!
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Old 10-03-2023, 05:42   #14
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Re: Will head tanks gravity drain effectively?

Quote:
Originally Posted by nuru05 View Post
So I need to redo my head plumbing this spring, the previous owner had it set up in a way that I don't really like and doesn't really work for us.



I've got a jabsco manual head and there is a 20 gallon black water tank in the wall behind it. The through hull for the discharge is below the sink next to it and the pump out is directly above it. What I want to do is plumb the head into the inlet of the tank, then where the outlet is I want to go out a few inches an T it off going straight up or down. This way it goes up to the pump out or just flows down to the through hull. This gives me the option to close the through hull when we are at anchorages or places where we can't just flat out discharge, but then allows me to open it and dump it when we get to open water. It also maintains my ability to get it pumped out if we need/want to.





My main question is will a holding tank effectively gravity drain without having a macerator on it? Or will solids eventually build up and block the outlet?


Iím not sure I would want my head to pump into the bottom of holding tank. That could build up a lot of pressure if you fill it, against your joker valve.
20 gallons of sewage could make a mess if your joker valve fails.
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Old 10-03-2023, 05:44   #15
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Re: Will head tanks gravity drain effectively?

Correction to above.
20 gallons of sewage will make a mess when your joker valve fails.
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