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Old 13-08-2007, 18:45   #16
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Originally Posted by marc
Well, evryone that said that the addition of a vented loop in the system would not help,WAS RIGHT. So it was back to the drawing board for me. I finally hooked up a temporary disharge hose from the pump,up and over the side. It worked. Filled the tank 3 times (with water), and at worked every time. So I re-routed the discharge hose to exit about 16" above the waterline & under the bridgedeck (forgot to mention it's a catamaran). I guess the system wasn't designed to pump out below the waterline. I hope this "fix" will solve my 10 year problem.
Thanks for all the helpful advice,
It has to be the seacock or through hull. Does the through hull have a screen or strainer on the outside? That would match the symptoms of working until the strainer clogged with solids, then eventually washed out to magically fix itself.

Are you sure the sea cock is opening all the way? What diameter is the through hull?


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Old 19-07-2008, 01:32   #17
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This is rather belated, Marc - if you ever get to read this - but frankly, I still find it hard to believe that pumping out higher up was your answer really. The small amount of back-pressure from the few inches under water the outlet was is just so small any pump should have overcome it. Your system sounds in principle very much like mine, after I was forced by regulations here is Queensland, to install a holding tank in my Clipper 34. I use a TMC macerator pump for thru-hull pump-out from a bladder holding tank in my for'd bilge space, which was supposed to self-prime to up to one metre, but never has. So if I go any length of time between pump-outs, I have to prime the macerator pump by reversing flow for a few minutes by pumping water from the toilet back down to it with some juggling of taps and valves - it then works well. I suspect your pump was suffering the same problem, and somehow when you changed your outlet, the setup holds fluid in the line better, keeping the pump primed - or the pump has ended up lower than it was before, or both, or you had a kink somewhere in the original outlet? Am I right? As you found, the anti-siphon valve is only relevant to stopping back flow in, and easily stopped by a tap in the line and/or turning off the seacock. I use both, as that tap is also what allows my priming diversion.
Peter B

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holding tank

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