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Old 31-08-2013, 23:22   #1
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Self Draining Holding Tank or Catch and Release of Blind Mullet?

Hi There,

I'm thinking of making a self draining holding tank (only about 20 liters or so), and was wondering if anyone else has done this and what issues you may have had.

Basically the thought was to make a small holding tank that will be located about 1-2 foot above the water line of the boat.

I have an electric macerating toilet, and currently it pumps out at about 1-2 feet under the water line.

Current plumbing is normal vented loops that go well above the water line and are long enough that i could put a tank under the vented waste loop on the top of the hose and still have the bottom of the tank above the water line.

My thinking was that when needed i could pump a couple of number two's into the tank, then when in an appropriate area open the valves and let the poo self drain to freedom.

Also if needed i could run the electric toilet when valves are open to flush clean the holding tank.

At present i only forsee the waste to be in the holding tank for maybe a day or two at most before flushing so i dont expect much of a sludge buildup.

Can anyone see problems with this sort of system?

Thanks in advance,

Matthew
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Old 31-08-2013, 23:53   #2
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

I've had this system on two boats. I've also had an RV style head and compositing head. The simple gravity feed you describe is by far my favorite. Simply open the seacock, and it's gone. No additional pumps or y-valves necessary.

I've been on a few boats with this system and have best liked the seacock under the vanity if possible. When I installed this system myself, I placed the inspection port on the holding tank directly above the outlet, so in the unlikely event event the out take ever clogged, it would be a straight through snake.

I find simply leaving the through hull open under way from time to time provide plenty of cleaning action.

Great system.
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Old 31-08-2013, 23:55   #3
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

I had a waste tank which was installed below the water line and had to be pumped out. I had so many problems with it that I stopped using it and eventually cut it out in order to do hull maintenance.

I will be rebuilding a tank in the near future and had intended to do exactly what you propose. I will leave the overboard open whilst at sea and let the tank drain straight thru, only closing it when near shore as a holding tank.
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Old 01-09-2013, 00:40   #4
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

Ours is the gravity type. Just be carful how you design it, all the waste must flow out, so the tank needs to be sort of funnel shaped with a larger sized exit waste hose to prevent clogging. The tank also sits rather high up, behind a cabinet in the head at eye level in order to create a good forceful gravity push when the valve is opened I suppose. So first, the electric toilet needs to pump up and over the top of the tank to prevent backflow.
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Old 01-09-2013, 01:05   #5
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

I have that now and it's near-perfect. Agreed on some of the points above:

- Have the inspection hole above the outlet hole so you can just jam a coat hanger through if you need to.
- Have it on a slope of sorts so the outlet drain is the lowest part of the tank.
- Have the inlet at the top.

It functions like a anti-siphon valve so you don't need one of those (probably still need one on the intake side of things).
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Old 01-09-2013, 02:51   #6
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

Brilliant...

Thanks for the reply's so far.

I was hoping that it would be a good system to install, and i have taken on board the points raised so far, re: access for cleaning etc.

Anyone else care to share i am still interested in what you may know.

Thanks again,

Matthew
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Old 02-09-2013, 16:42   #7
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

Just a bit more on one of my previous comments. On one boat I owned that had a gravity fed drain, the bottom of the tank was at about the heeled water line. Leaving the seacock open form time underway would continually flush raw water in and out of the tank, doing quite a bit of self cleaning.

Give thought to the convenience of the seacock - that was my under the vanity point. One boat I owned had the seacock at the bottom of the Lazarette which was a royal PITA to get at.
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Old 02-09-2013, 17:07   #8
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

I'd recommend adding a pump-out option. I have the same arrangement except there is deck access for dropping in a pump-out hose.
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Old 02-09-2013, 17:14   #9
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

Quote:
Originally Posted by markpierce View Post
I'd recommend adding a pump-out option. I have the same arrangement except there is deck access for dropping in a pump-out hose.
Speaking of pump outs, I should add on here that I recently had a backup and a problem with the gravity fed systems is just that: gravity. Six gallons of sewage, backed up for unknown reasons. Not a lot of ways of addressing the issue without a lot of mess.

I hopped in the water with a coat hanger and jammed it into the thru-hull, no joy.

I waited for a decent tide, got on the up-current side, and used a plunger against the hull. The whole six gallons of sewage went flying out in front of my face, yanked away from me by the tide.

I felt pretty good about myself that day.
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Old 02-09-2013, 17:29   #10
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

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Originally Posted by resilientg View Post
Brilliant...

Thanks for the reply's so far.

I was hoping that it would be a good system to install, and i have taken on board the points raised so far, re: access for cleaning etc.

Anyone else care to share i am still interested in what you may know.

Thanks again,

Matthew
My Gemini was designed that way. It also had a pump-out option.

I like the idea and it worked for me. I would not want one any other way.

My only suggestion is to make it as big as you can. Custom shaped tank? = $$ but............
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Old 02-09-2013, 17:58   #11
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

For not too much money you can get the Todd tank and then get the relocation kit to at least put the holes exactly where you want them. Money well spent.
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Old 02-09-2013, 18:57   #12
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Here is what I did on a gravity-draining tank. The tank was tall and skinny so it drained well.I did install a pump out option. For this, I had a 1 1/2" threaded fitting placed on the top of the tank. A 3/4" pvc pipe went inside the tank almost to the bottom, so the pump out would get everything. On the outside of the tank on top, I put an elbow with a plug on it. I did not install any pump hose nor a deck fitting, because it was not worth the headache of routing the hose and installing the deck fitting. Instead, I got a length of sanitation hose long enough to reach out the window. On one end I put a deck fitting and on the other end hose barb to male thread. If I ever want to pump out, I can remove the plug, screw in my hose, put the other end out the window, and pump it out.
If i were to do it again, I would have installed another threaded fitting with a plug in it, as a clean out. But with a macerating head, have never had a clog.
The input was also on the top of the tank.
Don't forget, a long piece of 4" PVC will work as a legal holding tank. Durable, cheap and easy to make.
48" of 8" pvc is a ten gallon holding tank.
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Old 02-09-2013, 20:20   #13
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Re: Self Draining Holding Tank or Catch and Release of Blind Mullet?

A trick I have used to clear waste from the discharge hose is to place a ball valve in the vent hose. With the vent valve closed pumping water into the tank will force out waste stuck in the discharge hose. Also, pumping water into the tank with the tank discharge valve closed and the vent valve open will clear the vent hose.

This arrangement can also be used in direct discharge head setups by modifying the anti-siphon valve to be a vented loop with an overboard vent hose.

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Old 16-11-2013, 09:01   #14
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I have my new 16 gallon tank installed above waterline and new lavac head. Bottom of tank has 1.5 inch threads for gravity drain. Problem is I only have about 10 inches to the hull which with the angle is scarcely room for a proper sea com.

Would like to just put a barb fitted through hull and then run 6" of hose to tank. No sea com.

This whole assembly is uncovered and visible for inspection each time we go on the head.

Bad idea ?
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Old 16-11-2013, 09:36   #15
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I have my new 16 gallon tank installed above waterline and new lavac head. Bottom of tank has 1.5 inch threads for gravity drain. Problem is I only have about 10 inches to the hull which with the angle is scarcely room for a proper sea com. Would like to just put a barb fitted through hull and then run 6" of hose to tank. No sea com. This whole assembly is uncovered and visible for inspection each time we go on the head. Bad idea ?
How are you going to keep the poo in the tank with no valve?
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