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Old 15-01-2022, 09:57   #1
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Simple pumpless holding tank discharge with gravity and syphon

Has anyone seen a simple marine holding tank with a gravity discharge into a seacock that draws from the top of the tank?

I'm hoping to eliminate a pump, and only use the syphon affect of a manually primed discharge hose.

The discharge hose would a few feet longer than required and fastened with a strap except in the rare instance of manual evacuation. The hose would be primed before manual evacuation and then the seackock opened. The hose would be elevated high enough to start the flow of the primed section of the hose out through the seacock. This would cause nearly the entire contents of the holding tank to empty through the seacock.

Benefits: No pump, pump & valve maintenance, simple. No waste lingering in hoses.

Drawbacks: Extra manual effort during (rare) manual HT discharge, including priming the hose. Room needed for extra feet of hose, and to manually lift hose high enough to start flow and syphon.

Haven't been able to see anything described like this on a boat forum, though remember a childhood friends' family had an RV which worked like this. Wouldn't a simpler system be better than anything with valves which don't last more than a couple seasons exposure to antifreeze? What other issues are there?
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Old 15-01-2022, 10:21   #2
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Re: Simple pumpless holding tank discharge with gravity and syphon

Why draw from the top? I suspect it will be problematic. I've done it but with a bottom outlet going straight to the seacock nearby and it worked well.

With the bottom gravity drain you can flush with water anytime you want.
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Old 15-01-2022, 12:09   #3
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Re: Simple pumpless holding tank discharge with gravity and syphon

The newer boats have a gravity discharge system with the holding tank higher than the toilet. The discharge system flows from the bottom of the tank. You can just leave the dump valve open when you are offshore.

It works OK because they have fitted larger diameter piping to the dump side. I would not try that with a 1.5 inch waste pipe.

The higher the holding tank is, the more critical the joker valve becomes.
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Old 15-01-2022, 12:16   #4
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Re: Simple pumpless holding tank discharge with gravity and syphon

The entire tank would need to be above the waterline to empty its contents with a siphon as proposed. Given that, you could simply drain it via gravity with a drain mounted on the bottom. The tank would still need to be above the waterline but this would eliminate the trouble of priming the siphon.
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Old 15-01-2022, 12:26   #5
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Re: Simple pumpless holding tank discharge with gravity and syphon

Quote: "The higher the holding tank is, the more critical the joker valve becomes."

And we all know how joker valves reliably make perfect seals even when someone flushes toilet paper. Or in one case in my memory, cherry pits. Sounds like an invitation to an "OH, SH#T" moment.

Why not gravity from toilet to tank, and then gravity to the discharge? You might have to elevate the throne. If you have to go up and over the top, remember to flush with lots of water so when the joker valve does as joker valves reliably do, it's not such a bad day after all.

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Old 15-01-2022, 12:41   #6
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Re: Simple pumpless holding tank discharge with gravity and syphon

The higher the holding tank is, the more critical the joker valve becomes.

Not necessarily. Any marine toilet working anywhere near factory specs can lift bowl contents up to 4'. So if the toilet discharges up and over a loop--not a vented loop, just a loop-- that's just barely higher than the inlet fitting on the tank (which surely won't be higher than 4') and is flushed in the dry mode, gravity will get it the rest of the way and there should be very little back flow to the toilet except for clean water used to rinse behind the flush. Has the added advantage of reducing the amount of pumping needed, increasing the number of flushes the tank can hold.



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Old 15-01-2022, 12:57   #7
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Re: Simple pumpless holding tank discharge with gravity and syphon

Have a look at these holding tanks. Not cheap but simple to install.

https://www.tek-tanks.com/product/80...al-waste-tank/
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Old 15-01-2022, 13:28   #8
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Re: Simple pumpless holding tank discharge with gravity and syphon

Tek-Tanks makes top quality tanks, but they're in the UK...StanL hasn't said what part of the world he's in, but if unless he's on that side of the pond, shipping would be a deal breaker.


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Old 16-01-2022, 09:51   #9
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Re: Simple pumpless holding tank discharge with gravity and syphon

French catamarans have been using gravity powered holding tanks for years. Fresh water electric toilet pumps to the top of a tall tank, thru hull empties from the bottom. Works well in a seaway.
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Old 16-01-2022, 11:04   #10
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Re: Simple pumpless holding tank discharge with gravity and syphon

Quote:
Originally Posted by StanL View Post
Has anyone seen a simple marine holding tank with a gravity discharge into a seacock that draws from the top of the tank?

I'm hoping to eliminate a pump, and only use the syphon affect of a manually primed discharge hose.

The discharge hose would a few feet longer than required and fastened with a strap except in the rare instance of manual evacuation. The hose would be primed before manual evacuation and then the seackock opened. The hose would be elevated high enough to start the flow of the primed section of the hose out through the seacock. This would cause nearly the entire contents of the holding tank to empty through the seacock.

Benefits: No pump, pump & valve maintenance, simple. No waste lingering in hoses.

Drawbacks: Extra manual effort during (rare) manual HT discharge, including priming the hose. Room needed for extra feet of hose, and to manually lift hose high enough to start flow and syphon.

Haven't been able to see anything described like this on a boat forum, though remember a childhood friends' family had an RV which worked like this. Wouldn't a simpler system be better than anything with valves which don't last more than a couple seasons exposure to antifreeze? What other issues are there?
Can’t beat Gravity as a fool proof discharge? I regularly fill my holding tanks with detergent and fresh water, let them slip around for a few days and drop them. Sensitively of course.
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Old 16-01-2022, 11:04   #11
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Re: Simple pumpless holding tank discharge with gravity and syphon

Geminis use gravity discharge but from its it’s from the bottom of the tank. I can’t imagine a siphon having any chance of working with a little bit of drop and the big pieces of material. In any case, overboard lines require a vacuum break that makes a siphon thank impossible.
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Old 16-01-2022, 12:09   #12
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Re: Simple pumpless holding tank discharge with gravity and syphon

more tank/head design info needed- but head does the maceration into tank and then let flow out bottom.
I would assume using a clamshell on the thru hull would permit setting up for a syphon, but I can not envision dried slurry not becoming difficult to motivate. Ideally tank bottom is above waterline so not regularly backflushing into tank with seawater.
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Old 16-01-2022, 12:24   #13
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Re: Simple pumpless holding tank discharge with gravity and syphon

You'd be eliminating a pump, but introducing a need for a method to prime (as you stated). Wouldn't that also involve a seal, moving parts, hose connection, etc?

I don't know how it would work. Second thing I did on my boat was toss the head and all the nonsense and replaced with composting. Best upgrade I've done. (I'm not suggesting you do that though, it's a personal opinion of trade-offs, not a better vs worse thing)



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Old 16-01-2022, 15:38   #14
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Re: Simple pumpless holding tank discharge with gravity and syphon

Another person here unsure as to why you want the top discharge? Our new to us boat has a gravity discharge but from the bottom of the tank. Works very well and easy to use ... we prefer it to the discharge through a macerator on our previous boat.
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Old 16-01-2022, 20:21   #15
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Re: Simple pumpless holding tank discharge with gravity and syphon

Peggy, what is the dry mode?

Normally, with holding tank below toilet, you don't need to add additional inlet water beyond what is necessary to clear the bowl and the joker has no back pressure. With elevated tank the joker has to hold back a 3 to 4ft head of sewage. To avoid sewage backflow through a marginally defective joker, you could do a lot of extra pumping to clear the discharge hose after each flush thereby greatly reducing the number of flushes for the tank capacity.
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