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Old 28-02-2024, 12:58   #1
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Blackwater plumbing & Holding tank sanity check

There's a few things we don't like about our existing blackwater (toilet output) plumbing system:
  1. The existing hoses are "stink saturated" and smell bad. There also seems to be a fair amount of crystallized gunk built up inside the hoses.
  2. The exisiting hose configuration is complicated with unnecessary elbows and tees.
  3. To dump the holding tank at sea, we have to pump it out with a manual diaphragm pump. In contrast, charter boats we've been on just require opening the sea cock and letting the tank drain out with gravity.
  4. Sometimes, If we leave the sewage sea cock open (so the toilet can pump directly overboard), wave action seems to back-drive the diaphragm pump and backfills the holding tank.
  5. Of course the holding tank isn't big enough.
We've got a plan to install a larger holding tank and would like to use a plumbing configuration like Nigel Calder's recommendation that eliminates a Y valve and uses the holding tank itself as the anti-siphon (Figure 12-29A in 4th edition Mechanical & Electrical manual). In this configuration, the toilet always pumps into the holding tank, the holding tank is above the heeled waterline, and the discharge goes through two valves (one at the tank and one at the thru hull).


Our limitation is that the top of our holding tank will be above the static waterline, but not the heeled (on starboard tack) waterline. Obviously this could be a bad configuration if we accidentally left the sea cock open while sailing, so we're planning to have an anti-siphon break between the toilet hand pump and the holding tank.



Did we miss anything important in this planned configuration?


Attached diagrams show the existing configuration and our plan for the future. We'd appreciate any suggestions or feedback.
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Old 28-02-2024, 15:48   #2
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Re: Blackwater plumbing & Holding tank sanity check

All of the gravity drain systems I have seen have larger than 1.5 in dump valves and thru hulls. Ignore this at your peril.
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Old 28-02-2024, 16:26   #3
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Re: Blackwater plumbing & Holding tank sanity check

If you are going to perform a redesign of your holding tank, you should pick up a copy of Peggie Hall's book. It isn't so much about engineering as it is practical advice about vent line sizes, vent location, tank location, etc. that will make the difference between a system that will always smell and you can't fix, and one that requires almost no maintenance.
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Old 28-02-2024, 17:09   #4
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Re: Blackwater plumbing & Holding tank sanity check

Somebody's got to say it, so it might as well be me .

Have you considered a composting head? It solves all your problems, and is waaaay simpler and safer. It's perfect for a boat of your size, with a smallish crew.
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Old 28-02-2024, 17:33   #5
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Re: Blackwater plumbing & Holding tank sanity check

Thanks for recommending my book and I agree that he'll find very useful. But no book can answer every question, so whether he buys it or not, I'd be happy to help him one-on-one to solve the problems he laid out, plus any he didn't include or may not even know exist, and prevent creating new ones. I don't charge anything...I do it because I enjoy sharing what I've learned in 30 years and often learn something new in the process.


So Mike...if you'd like my help, send me a PM.


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Old 29-02-2024, 13:14   #6
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Re: Blackwater plumbing & Holding tank sanity check

Happy with Airheads in 3 boats.
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Old 29-02-2024, 18:21   #7
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Re: Blackwater plumbing & Holding tank sanity check

Quote:
Originally Posted by NorthCoastJoe View Post
Happy with Airheads in 3 boats.
Me? I'm happy if no "Airheads" ever come aboard, they need to stay on land.
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