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Old 03-03-2012, 19:11   #1
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Bulkhead Mounted Gravity Holding Tank

I know the subject of holding tanks has been discussed many times on this site and I for one was very interested in the input from everyone. After much research and reading the posts on this and many other sites I was able to design a system that worked for our CSY 44 WO. I wanted a gravity, through tank, system so I could eliminate three way valves, vented loops, etc. The most difficult design feature to overcome was where to install a tank so I could keep hose runs as short as possible and close to vertical to the existing thru hull. I decided that a bulkhead mounted tank was the only way to achieve this and Ronco had a perfect fit. The tank has a 9 gallon capacity and is 15” X 32” X 5”.

I just completed our install and have to say I am very pleased with the results.

The tank was installed on the aft bulkhead so it partially sat on the upper counter of cabinet and the discharge piping was routed through the cabinet to the thru hull under the sink. I secured the tank to the bulkhead by constructing a frame and shelf out of 3/4” marine plywood, glued and screwed together then West System the whole thing. The frame was then thru bolted and screwed to the aft bulkhead. The discharge from the head runs up the bulkhead next to the door frame to the top of the tank and the 1” vent comes out of the other end and is routed into the upper cabinet then out the side of the hull.

The plumbing is all 1 1/2” schd. 40 PVC from the head to the tank and tank to thru hull with the exception of some short pieces of sanitary flexible hose to allow for movement. I installed a Groco 1 1/2” brass valve inside the cabinet and close to the tank as possible that is closed when tank use is required or can be left open for a flow through system when not. Tank venting is supplied by 1” PVC from the tank to a thru hull. I added a valve to the vent line with the thought that in the event of a blockage, the valve could be closed which would pressurize the as the head was flushed in hopes of dislodging any blockages. I don’t know if it would work and hope I don’t need to find out. Thought it was a good idea to have just in case.

During the first test of the tank, I filled it full with fresh water to check for leaks. Then opened the valve to check for leaks down stream and check the discharge rate. When I opened the valve the tanked emptied in about 5 seconds.

After three weeks of use everything seems to be working very well and I can focus on the fit and finish of a tank cabinet.





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Old 03-03-2012, 20:27   #2
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Re: Bulkhead mounted gravity holding tank

I added a valve to the vent line with the thought that in the event of a blockage, the valve could be closed which would pressurize the as the head was flushed in hopes of dislodging any blockages. I donít know if it would work...

It won't...and you don't want to find out what'll happen if you do let the vent become blocked. So be sure to backflush the vent every time you pump, dump and/or wash the boat to make sure that doesn't happen.
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Old 03-03-2012, 22:27   #3
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Re: Bulkhead mounted gravity holding tank

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... So be sure to backflush the vent every time you pump, dump and/or wash the boat to make sure that doesn't happen.
Sorry to sound a little slow, but so that I and my vent are clear, what do you mean when you write "backflush"?
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Old 03-03-2012, 22:54   #4
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Re: Bulkhead mounted gravity holding tank

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Originally Posted by hlev00 View Post
Sorry to sound a little slow, but so that I and my vent are clear, what do you mean when you write "backflush"?
I believe she means to run water back thru the vent line into the tank, making sure it's clear. The gases can build up in a tank and cause it to leak or even burst. Sometimes even catch fire.

Had an in-law family member once with a 45' powerboat where the holding tank blew up. What a stinking mess that was.
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Old 04-03-2012, 00:33   #5
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Re: Bulkhead mounted gravity holding tank

Self draining tanks are a great idea, but why such a small tank in a medium size boat. It seems you wouldnt be able to be in a no discharge zone or many places very long before in was full. I have a 38 gallon tank and dont worry much about it. I'm constantly surprized by the tiny tanks in 50ish foot boats. Going to a small tank just to have it pumpless seems like a backwards step.
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Old 04-03-2012, 00:54   #6
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Re: Bulkhead Mounted Gravity Holding Tank

How do you pump the tank out?
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Old 04-03-2012, 04:35   #7
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Re: Bulkhead Mounted Gravity Holding Tank

"...and the discharge piping was routed through the cabinet to the thru hull under the sink."
Will you be using that sink to brush your teeth?! Just wondering.

You might want to consider whether or not you need a valve between the basin and the discharge line.
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Old 04-03-2012, 08:29   #8
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Re: Bulkhead mounted gravity holding tank

Backflush: stick a hose nozzle (attached to a hose, of course) up against the vent thru-hull and flush out the vent line back to the tank.

If your vent thru-hull doesn't allow this, replace it with a thru-hull that does.
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Old 04-03-2012, 08:36   #9
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Re: Bulkhead Mounted Gravity Holding Tank

He didn't say it IS the sink drain thru-hull, only that it'a thru-hull under the sink. If he did tee into the sink drain. that would indeed be a MAJOR no-no that even violates USCG regs prohibiting any common plumbing--even vents-- between toilet discharge and/or waste tanks and potable water plumbing.

It IS ok for a toilet INTAKE to share a sink drain thru-hull, 'cuz the intake only supplies sea water going INTO the toilet.
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Old 04-03-2012, 11:30   #10
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Re: Bulkhead Mounted Gravity Holding Tank

Ok, obviously not enough information in the original post. Let me clarify a few things.

“It won't...and you don't want to find out what'll happen if you do let the vent become blocked.”

I understand the seriousness of a blocked vent. However I have not found substantial proof that the tank, especially being so small in volume, would not become just another part of the plumbing and allow the head to provide some extra push if the vent was closed and the thru hull opened. For this reason I felt it better to install the valve now in the hopes the theory was true, rather than wish I had one at a later date.

“but why such a small tank in a medium size boat”

Our cruising area is in the eastern Caribbean. The boat is currently in the BVI where there are no requirements for holding tanks nor are there pump out stations. Our tank size allows us to close the discharge while in mooring fields or marinas and then sail far enough from shore to dump the contents. When sailing in the BVI it is more of a personal obligation rather than a legal one. Large tanks take up a lot of space and are difficult to design using gravity as a discharge method. I doubt we would be ever cruising in a NDZ in the near future and therefor went with the simple, smaller tank and system.

I did plumb a T prior to the valve with a stand pipe for a future deck fitting in the case we need to pump the tank.

“He didn't say it IS the sink drain thru-hull, only that it'a thru-hull under the sink.”

Correct! The original thru hull is under the sink. Our sink actually drains into the bowl of the head. Plumbed this way by the PO. We left it that way for now because it allows us to fresh water flush the bowl and use the head while on the hard if necessary.

The reason for the additional valve directly under the tank is so I didn’t hold effluent in the line from the tank to the thru hull. The head will clear the pipe from the head to the top of the tank with a few extra pumps. A benefit, if you want to call it that, of having the tank on the bulkhead above the head is that you can HEAR when the line is clear while pumping!

Thanks for all the input past and present, it has been very helpful.
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Old 04-03-2012, 12:03   #11
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Re: Bulkhead Mounted Gravity Holding Tank

I hope you have an air gap* between the sink discharge & toilet bowl.

* An Air Gap is a physical separation between the free flowing discharge end of a potable water supply pipeline and an open vessel.
An approved air gap shall be at least double the diameter of the supply pipe measured vertically above the overflow rim of the vessel, and in no case should be less than 1 inch.
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Old 04-03-2012, 12:08   #12
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Re: Bulkhead Mounted Gravity Holding Tank

Our sink actually drains into the bowl of the head. Plumbed this way by the PO. We left it that way for now because it allows us to fresh water flush the bowl and use the head while on the hard if necessary.

Ooops...that means gray water goes through the toilet. NOT good, 'cuz hair, soap scum, dirt, toothpaste, etc etc is not good for the toilet pump. It also creates a "scum" on the inside walls of the discharge hose that traps salt and other minerals, waste, etc, resulting in a buildup that's gonna cause problems.

So you really need a tee in the sink drain line that'll let the toilet and the sink share the same thru-hull while allowing the sink to DRAIN out that thru-hull. Put the tee as close to the seacock as possible. This will allow you rinse the sea water out of the WHOLE system, add white vinegar to the whole system too, to prevent sea water mineral buildup (and antifreeze)...and also still allow you to use the sink as a source of flush water while on the hard or any other time that you're in raw water that you'd rather not pull through the system.
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Old 04-03-2012, 12:20   #13
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Re: Bulkhead Mounted Gravity Holding Tank

Gord, you're a bit late to the party. He said above that he isn't using the same thru-hull for the sink drain and toilet discharge. But even if he were, the sink drain has no contact whatever with the potable water system...only with USED fresh water. But even if the sink and tank did share the same thru-hull, an air gap would only prevent a backup into the sink...it wouldn't prevent bacteria from migrating from the plumbing into the sink anyway, so there'd be no point in installing one.

Besides, I dunno what other countries require, but it's illegal in the US for the sink drain and anything that carries sewage to share ANY plumbing...even any common venting. So here at least, an air gap would only be another component in non-compliant plumbing.
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Old 04-03-2012, 13:40   #14
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Re: Bulkhead Mounted Gravity Holding Tank

So you really need a tee in the sink drain line that'll let the toilet and the sink share the same thru-hull while allowing the sink to DRAIN out that thru-hull. [/QUOTE]

Peggy, I'm assuming you are talking about the 3/4" head intake thru hull. Thanks that would work out just fine for our head. Sounds like a good plan!
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Old 04-03-2012, 13:57   #15
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Re: Bulkhead Mounted Gravity Holding Tank

Peggy, I'm assuming you are talking about the 3/4" head intake thru hull.

The SINK DRAIN thru-hull is the usual one for both sink drain and head intake. It's also 3/4" on most boats. However, if the head intake thru-hull is close enough to the sink to let the sink drain line remain short enough to drain the sink quickly AND is also readily accessible to let you close the thru-hull to flush fresh water through the system to rinse it out, I can't see how it could matter which hole in the boat you use, so if the head intake thru-hull works better than the head sink drain, use that one.
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