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Old 14-07-2015, 08:47   #1
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Check valve for bilge pump just inside the hull.

My catamaran has a number of bilge pumps. The discharge hoses loop high in the compartment and then discharge not far above the waterline. The trouble is, on a rough day, water sloshing against the side of the hull can be forceful enough to squirt water through the discharge hose back into the bilge. I know that check valves are sometimes used just upstream of the pumps but I do not consider this a good idea (reduced flow, air locks etc...). I assume that a vented loop will prevent a siphon problem but won't stop the forced water as I have described?

Is there any harm in putting a check valve just inside the hull? The outgoing water is already on its way 'downhill' by then so it will have a very minimal affect on the discharge rate and the valve will prevent water from going the wrong way.
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Old 14-07-2015, 08:59   #2
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Re: Check valve for bilge pump just inside the hull.

We have them installed exactly like you are contemplating. Another idea is to put flapper valves on the outside of the thruhull if convenient to do so.

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Old 14-07-2015, 09:57   #3
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Re: Check valve for bilge pump just inside the hull.

You do not want to put a check valve in your bilge discharge line. They can stick closed. I like the idea of a taller loop or an external flap valve or both.
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Old 14-07-2015, 10:41   #4
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Re: Check valve for bilge pump just inside the hull.

For clarity, the type of check valve we put in was a tube with a rubber flapper in it that can rest against a stop. The valve is pointing down - so the flapper is always hanging open until a wave pushes into the thruhull.

I don't see anyway this can stick closed, and the resistance to pumping is miniscule.

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Old 14-07-2015, 11:50   #5
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Re: Check valve for bilge pump just inside the hull.

I'd advise against putting any kind of check valve on your primary 'oh ****' pump

For a secondary or nuisance water bilge pump, sure
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Old 14-07-2015, 12:06   #6
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Re: Check valve for bilge pump just inside the hull.

Mark,

I agree with you. Bearing in mind that we are not talking about a check valve near the pump, I don't see that the rubber flapper can be any more prone to jamming than would a flapper on the outside of the thru-hull.
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Old 14-07-2015, 14:02   #7
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Re: Check valve for bilge pump just inside the hull.

Why wouldn't a clamshell cover over the thru-hull work?
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Old 14-07-2015, 17:30   #8
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Re: Check valve for bilge pump just inside the hull.

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Why wouldn't a clamshell cover over the thru-hull work?
It can, as well as one of those external flapper valves I mentioned earlier. In our specific case, neither of those will work in the location of the thruhull that needs it, so I put in the in-line flapper valve. Other thruhulls on our boat have either a clamshell or an external flapper.

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Old 14-07-2015, 17:32   #9
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Re: Check valve for bilge pump just inside the hull.

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I don't see that the rubber flapper can be any more prone to jamming than would a flapper on the outside of the thru-hull.
It won't be, and it will have the added benefit of lasting longer.

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