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Old 12-10-2011, 08:08   #1
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Gravity Holding Tank Installation Questions

We are still in the design and planning phase of installing holding tanks in our CSY 44 walk over and I had a few questions that hopefully someone will have some answers to..

I like the design of a gravity system with the tank above the healed water line thus eliminating a bunch of other plumbing features.

However, with a gravity tank installation how close to vertical and directly above the thru hull do you need to be for this design to work?

Could a ball valve be placed on the vent line so that when closed the pump on the toilet could be used to aid a gravity discharge design by pressurizing the tank a bit and help push any content out and flush out the tank?

Your thoughts are appreciated.
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Old 12-10-2011, 08:25   #2
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Re: Gravity Holding Tank Installtion ?s

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Could a ball valve be placed on the vent line so that when closed the pump on the toilet could be used to aid a gravity discharge design by pressurizing the tank a bit and help push any content out and flush out the tank?
Not really. You just are not going to make that much pressure from the head end. A macerator for overboard pumping is pretty tried and true. It grinds the waste to make it flow. A decent vent line that does not clog is the key to a good system. You can't pressurize the system well enough partly because the head isn't designed to work that way. You really can't make waste proof check valves either.

I think the best head design is going to be all runs as absolutely short as possible! Long hoses with poop just is asking for trouble you actually can imagine. Run your lines as direct as you can. A macerating head might be the better answer. It sure would flow better and with a good vent the oxidation should keep it better smelling. Poor venting with low oxygen is what makes the really nasty smells.
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Old 12-10-2011, 08:38   #3
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Re: Gravity Holding Tank Installtion ?s

Consider most systems rely on gravity downstream fo the vented discharge loop. There is no pressure after that point due to the vented loop. That said, I agree with Paul - the shorter and straighter the better.

Most of the Moorings boats I've been on have gravity feed tanks. I really like that system. On my boat, I'm sure the bottom of the tank could be below the water line if heeled. The through hull is at the bottom of the cockpit lazarette which is a real PITA to get to. I much prefer the boats that have it under the sink in the head. Simply open the vanity door, and open the through hulll to drain.

I'm in the process of installing a gravity feed system on my Hunter as well. I'm putting the inspection hatch on the holding tank (which is also needed to install the fittings) directly above the discharge out the bottom. That way it can easily be snaked if needed.
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Old 12-10-2011, 08:48   #4
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Re: Gravity Holding Tank Installation Questions

Large diameter hoses is a good idea too. I believe wide, short hoses are the medium through which God wanted human waste to go overboard.
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Old 12-10-2011, 08:55   #5
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Re: Gravity Holding Tank Installation Questions

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Large diameter hoses is a good idea too. I believe wide, short hoses are the medium through which God wanted human waste to go overboard.
Amen to that.

But in the real world if you have a longer run to the through hull from the head or other restrictions that mandate a longer run, the goal would be to avoid any low spots between the head discharge or holding tank outlet and the through hull where the waste finally ends up.
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Old 12-10-2011, 13:44   #6
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Re: Gravity Holding Tank Installtion ?s

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Originally Posted by nautical62 View Post
Consider most systems rely on gravity downstream ...
Although I have no specific experience, I doubt that any gravity flow system will satisfactorily evacuate the solids from a typical holding tank.

When we do shoreside underground waste plumbing, we generally specify a 2% gradient (1/4" fall per foot of run). This is not only code in most jurisdictions, but it is optimal for sewage discharge.

Drainage gradients from 1 in 40 (2% fall) to 1 in 100 (1% fall) will normally give adequate flow velocities.

If the fall is too shallow, it will not drain fast enough, causing blockage. But, if it too steep, then the liquids run down too fast, leaving the solids behind, also causing blockage.

Our application ios complicated by a larger, flat bottomed holding tank; wherein gravity drainage doesn't provide enough force to pick up the solids (settled on the bottom) and carry them out.

As always, I’d defer to Peggy’s expertise.
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Old 12-10-2011, 13:55   #7
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Re: Gravity Holding Tank Installation Questions

we've been gravity draining now for 3 years without a hicup!! The tanks vent also acts as the vented loop. On the downstream side I put a ball valve as close to the tank discharge as possible to avoid waste sitting in the hose between the tank and the thruhull fitting. Normal operation in waters where legal is to just pump through the tank, when doing so I close the valves about once a month and fill the tank. The full tank dumping at once generates enough velocity (in my mind) to periodically flush the nasties. We also pump the head generously to keep the whole system clear...about 15 times for #1 and about 25 times for #2.
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Old 12-10-2011, 15:16   #8
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Re: Gravity Holding Tank Installation Questions

Would not worry about keeping bottom of holding tank above your healed waterline, just so the tank will drain when boat is level.
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Old 12-10-2011, 19:50   #9
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Re: Gravity Holding Tank Installation Questions

with a gravity tank installation how close to vertical and directly above the thru hull do you need to be for this design to work?

As long as the hose from the tank to the thru-hull is straight, you can get away with up to 45 degrees. More than that could be problematical if you're using the tank as a flow thru when the boat is heeled.

Could a ball valve be placed on the vent line so that when closed the pump on the toilet could be used to aid a gravity discharge design by pressurizing the tank a bit and help push any content out and flush out the tank?

That COULD be done...but I don't recommend it.

Apparently you're having a problem finding the best location for a gravity dump tank...if you'd like some help spec'ing it out, I'll be glad to do that...send me an email. I don't charge anything just to brainstorm.
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Old 17-10-2011, 05:49   #10
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Re: Gravity Holding Tank Installation Questions

Here is our gravity tank installation, with the ball valve under the tank (handle is removed due to lack of space). The discharge hose from the bowl goes up into the tank. Hope this helps...

Sailmonkey or Peggy, would you have a suggestion for helping keep the lines clear such as using a water softener?
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Old 17-10-2011, 06:01   #11
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Re: Gravity Holding Tank Installation Questions

Simply pump water into the top line hose vent at the tank top or wherever you think it will clog up and pump seawater into that to flush the outlet all the way to the sea after or during while dumping the tank.
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Old 17-10-2011, 08:57   #12
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Re: Gravity Holding Tank Installation Questions

Fresh water rinse and a weekly cupful of white vinegar flushed all the way through the system should be all that's needed. Backflush the tank vent every time you wash the boat to make sure it doesn't become blocked. If your vent thru-hull isn't a design that allows you to do that, replace it with one that does.
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Old 17-10-2011, 20:08   #13
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Re: Gravity Holding Tank Installation Questions

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Originally Posted by silverp40 View Post
Here is our gravity tank installation, with the ball valve under the tank (handle is removed due to lack of space). The discharge hose from the bowl goes up into the tank. Hope this helps...

Sailmonkey or Peggy, would you have a suggestion for helping keep the lines clear such as using a water softener?
That's the ticket! Thanks for the photo. Is the hose T-ed into the hose below the valve lead to a deck fitting? Couldn't tell where it went.
Thanks again.
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Old 18-10-2011, 18:08   #14
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Re: Gravity Holding Tank Installation Questions

Yes, that goes up to the pump out deck fitting.
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Old 18-10-2011, 18:16   #15
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Re: Gravity Holding Tank Installation Questions

SilverP40, what kind of hose did you use? It looks 2" but I can't tell if it is PVC pipe or hose. Also, what is the inline ball valve that you used under the tank.
Thanks
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