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Old 11-19-2012
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Question Orientation of Flexible Holding Tank

Hello all:

The PO of my boat had purchased an Avon flexible holding tank, fittings, macerator pump, and hoses to bring the boat up to current standards (currently, all waste discharges to the ocean). I have decided to install these components since I have them, having to add only a Y valve and vented loops. I have read enough from those who have flexible holding tanks to be comfortable with its use.

My question is the positioning of the tank. It appears that they are designed to be installed lying flat, with the hose fittings at the top. If so, how does the waste exit the tank at pump out? I am new to boating and have never experienced a holding tank pump out or flexible tank. My RV discharges by gravity, so this is all new to me. Will the pump out create a suction that would keep the discharge fitting in contact with the waste? Does the fact that the tank is flexible necessitate that the top will always remain in contact with the discharge opening since the tank will naturally collapse as it empties?

Neither of the 1 1/2" fittings have a pipe that would protrude into the tank and reach down into the waste as I have seen diagrammed in top-fed discharge systems with rigid tanks. And, I don't think it would be wise to have the edge of a pipe coming into contact with the bottom of the tank when its empty, anyway. Also, it would seem the the tank vent, the center fitting of the bladder, would break any suction that pump out would create.

As always, your assistance with this matter would be greatly appreciated.

Kermit
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Old 11-19-2012
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Orientation of Flexible Holding Tank

My boat had a flexible Nauta tank and when i had it pumped out it would suck down to the bottom and take a very long time to evacuate completely. So I'd like to hear how this supposed to work.
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Old 11-19-2012
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Re: Orientation of Flexible Holding Tank

Gary:

Was your tank lying flat and was it vented?
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Old 11-19-2012
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Re: Orientation of Flexible Holding Tank

We used to have an old vetus flexible tank before we reconfigured our whole system. One of the issues was when it was pumped out it would sometime collapse on itself somewhat preventing evacuation fully.

Also when installing this make sure it is supported with some kind of strapping system ( with soft sided restraints to the hull ot it will move around when partially full as it could break, put strain on the valves. Remember...1 gallon of water =12 lbs.

dave
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Old 11-20-2012
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Re: Orientation of Flexible Holding Tank

When we bought our current boat it had two flexible tanks for black water. We had so much trouble with them we threw them out. We could never stop them smelling the boat out and eventually gave up. So good luck with your installation (not sarcastic).

We had one tank horizontal in a locker space and one vertically installed, hanging on a bulkhead. The vertical one was discharged from the bottom so no issues there. The horizontal one had a small length of pipe protruding into the bag at the discharge fitting which was located on the top of the bag. The top of the tank settled as the tank emptied, keeping the end of the little pipe in the effluent thereby keeping the suction alive. The end of the little pipe was notched to prevent it blocking when it reached the bottom face of the tank. This worked OK but the bag was never really empty. Close but not 100%.

The macerator pump that we're using (same one but in a rigid tank) is a positive displacement, self priming unit so it always pumps unless the pipe is restricted or draws air. Most of these pumps will say in the fitting instructions that they should never be run dry. That is because they have rubber impellers much the same as a raw water engine pump and the rubber burns away if run dry. Avoid conditions that would see the discharge pipe blocked by contact with the opposite side of the bag.

Many of these installations in "the old days" had no breathers because the conventional wisdom of the day held that the bag expanded and collapsed as it filled and emptied and didn't need to breath. Avoid this at all costs.

If the bag can't breathe it will generate anaerobic bacteria which smell really bad. And if you're putting a breather in, the most difficult aspect is keeping it from clogging due to the flexibility of the bag. Make the breather so that you can blow it clear from the exit end periodically.

As far as securing the bag goes, our boat had carpet lining the areas where the bag contacted the boat and the bags had four strong eyes in the edges to fix them to the boat. It is quite important that they cannot move or abrade because they will wear through with particularly onerous (should that be odorous) consequences.
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Last edited by Omatako; 11-20-2012 at 02:23 PM.
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Old 11-21-2012
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Re: Orientation of Flexible Holding Tank

Omatako:

Thank you for your detailed reply. A flexible tank would not be my first choice, but since a new tank and all associated materials came with the boat, I thought I am really out nothing if it doesn't work in the long run. Worst case, I will have to eventually replace the tank with a rigid one that will fit in the same compartment.

The tank will go in a bow compartment which will be padded and supportive so it cannot move around. I intend to install a proper vent through the starboard side of the hull, just below the deck. I can blow back through it periodically to ensure that it remains clear.

Kermit
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Old 11-22-2012
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Re: Orientation of Flexible Holding Tank

Just an FYI. Most of the C&C tanks were made by Ronco and they still have the molds and specs for them. Our 35 was also in the bow and fits in rather easily. Nnot expensive and capacity is 24 gallons.

Dave
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Old 11-22-2012
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Re: Orientation of Flexible Holding Tank

Quote:
Originally Posted by kjzerr View Post
....I thought I am really out nothing if it doesn't work in the long run. Worst case, I will have to eventually replace the tank with a rigid one that will fit in the same compartment......
This is true, with one big addition. Not working out means it might smell horribly. The thought of remediating a used holding tank is one of the least desirably boat jobs of all time. Therefore, you will find you have to live with that terrible odor until you have the time or desire to deal with it.

Ask me how I know...........

A tip. Be absolutely sure there are zero low spots in your discharge lines and they have a desire to dip when full. They need to empty fully into the tank. If waste sits in them, they will begin to stink and you'll have to replace them too. No way to repair a permeated waste line.

It wasn't clear whether you will have a vent line. One want one to be 1" or larger. There is absolutely zero chance of success without one. The boat will wreak without a properly vented tank, as the odor is directly from anaerobic bacteria. On the other hand, keeping it from clogging and effectively not having one, is going to be a trick.
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