Wednesday, October 29, 2014

Fresh Water Flush--a Miniature Water Closet?

Over at Sail Delmarva Drew has had a wonderful idea! Why not flush with fresh water - if only while at the dock (and let's be realistic here - most boats are at the dock the majority of the time):
Seawater contains bacteria and sulfate. Flush with seawater, deprive the bacteria of oxygen for long, and facilitative (those that can go aerobic or anaerobic) start using sulfate (SO4-) as an electron source. The result is H2S (rotten eggs) and the head stinks. The cure? Either use the head every 8 hours (thus, smell is not a problem when living aboard or actively cruising) or flush with fresh water.

But you can just plug the freshwater pipe into a manual head; water will blow through, flooding the compartment, and bugs can swim up-stream. The water closet, introduced a century ago, prevents both by employing a float valve and creating an air gap (bacteria can't fly, as a rule).

What about a miniature water closet? $38 from McMaster Carr

Made for commercial ice machines, this unit holds enough to flush 3' of 1.5 inch line--not enough for most manual heads--and refills at 0.33 gpm, or about 1:20 seconds per full flush (12 strokes or so). Clearly, a larger size, holding about 60 cubic inches, is what is needed. It could also be expanded by mounting a 1-3' length of 3" pipe under it to serve as a reservoir (4' of hose can be flushed for every 1' of 3" pipe). Mount it to a bulkhead somewhere handy and you should be good to go. A pair of valves would allow switching from seawater and isolation.

But I haven't tried it. I'm happy enough with sea flush and a rinse with potable using the shower head before going home.


  1. I like the idea of flushing water recycled from the hand washing sink in the head. I am not sure how well it would work in practice, but may give it a try one day. You plumb a tank under the sink so the water from the sink fills the tank and when the tank fills then the overflow is plumed to the sea-cock. If there is not enough water to flush, just run the sink for a few seconds.

  2. The idea of using the sink water twice is certainly appealing...



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