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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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