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Cutting Holding tank access hole

Discussion in 'Technical Discussion' started by Danvilletim, May 18, 2017 at 12:20 AM.

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  1. Danvilletim

    Danvilletim Member

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    Location:
    isleton, ca
    We have a plastic tank made of a lower tank and upper tank that are joined via a. 3" 90 degree. Something has fouled the bottom exit pump out path. It will take forever to remove and re plumb all the fittings. I want to cut an access hole but concerned with being able to properly seal a plastic tank.

    Keep in mind the access needs to be in the lower tank which will have positive pressure once it fills and starts filling up the upper tank.

    Suggestions on how to seal up such an access hole? On a time crunch to get this fixed before boat gets loaded.
  2. Capt J

    Capt J Senior Member

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    do you not cut an access hole in the bottom tank it will never seal. Are you sure the macerator pump is not bad??? what style macerator pump do you have? If it's a sealand, they have a ductbill that goes bad in them sometimes and they won't move fluid. It would take something very serious to clog a 3" fitting. Try to pump it out with a pump out, that might suck it the other way. Or, Take the hose going to the macerator pump if that's what's clogged, make a fitting or hose with a compressed air fitting and blow it with compressed air (careful not to do too much at once) and see if you can unclog the fitting that way.
  3. 30West

    30West Member

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    Holland, Michigan
    You very likely would need to seal it up with a mechanical plate and gasket arrangement, I'm not sure if there are products you can install without access to the inside. That might leave you with a cleanout access plate if you can make it work. Maybe a plate and gasket on the outside, and a pair of half-round nutplates on the inside you can hold with a string, through two of the four holes on each. Have to look around to see if such a thing exists.
    [​IMG]

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    Last edited by a moderator: May 18, 2017 at 8:40 AM
  4. Pascal

    Pascal Senior Member

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    If by plastic you mean poly tanks not fiberglass then I would not cut anywhere but a time the top which won't work since you have pressure from th upper tank. The only way to really work with that material is the origianl plastic weld they use installing the fittings. Nothing really stick to that material. A plastic access plate will never seal reliably, maybe a SS on like pictured above... risky

    I agree with J, getting a clog in the outlet fitting is very rare even though reading your post i think you mean the clog is in the smaller outlet, not the 3" link.

    I'd look at the pump first. As J said, duckbills could have reversed if pumped against a closed sea cock.

    If not the pump, you could shut off the valve on the outlet, disconnect th pump, attach a piece of hose to the valve (long enough to extend above the top of the tanks, reopen the valve and back flush with a dock hose.
  5. Danvilletim

    Danvilletim Member

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    isleton, ca
    Thanks for reply. It is poly tank. I don't believe there is a macerator between tank and deck pump out. In testing it sucks fine for about 7-10 seconds and then stops with no vacuum. Water via a hose flows freely into the tank from the deck fitting. Again if we add suction it works for a few seconds and then stops. Mechanic thought it was sucking something into the exit of the tank.

    The 3" I mentioned is the connector between the small and large. The hate the idea of cutting but we are running out of options. Hell maybe there is a macerator. But I have never had to use it or turn it on.
  6. Pascal

    Pascal Senior Member

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    Are you sure the dock pump out is working with engines auction?

    If it does, and if it works for a few seconds and then stops it s almost always a clogged vent. Do you have a charcoal filter on the vent? How old is it? If the tank has been filled, the filter could have gotten wet and the charcoal solidify

    No vent, no pump out

    You also risk pressurizing the tank while flushing which will cause the tank to bulge out

    Most boats have macerator so you can discharge where legal. A must have especially in area with no or very few pump out like the Bahamas

    They can be connected to the tank thru a second outlet or thru the same outlet with a Y valve
  7. 30West

    30West Member

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    Stops sucking, but no vacuum in tank? Could the hose up to the pump out fitting be sucking flat?
  8. Danvilletim

    Danvilletim Member

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    Vent is not blocked. We removed filter and able to blow air into the tank. It is a wire reenforced hose.
  9. Pascal

    Pascal Senior Member

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    Try a different pump out station

    Actually I have a similar issue here... the pump out at our slip is very slow. Takes 4 hours to empty a 200 USG tank. No blockage, no vent filter (lazzara used two 1 3/4 or 2" vents. Yet super slow. It starts fast and then slows after a few seconds

    Eventually it empties. It s not the tank pick up as it is share with the overboard pump thru a Y valve and it works fine. Pretty sure it s the marina system
  10. PacBlue

    PacBlue Senior Member

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    The roto-molded polyethylene fittings on these tanks are usually "friction welded" to the material. They cut a hole, and the fitting typically has a flange, and then is spun to the surrounding surface adjacent to the hole and then welds itsef to the surface material.

    Seen this at their factory: http://www.ronco-plastics.com/

    They offer the same inspection plates as noted above.

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