Bilge Pump siphoning water BACK into boat - SailNet Community

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  #1  
Old 10-19-2010
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Bilge Pump siphoning water BACK into boat

Hey everyone - I have been poking around on and I cannot find a post that relates to exactly our issue, so I will post it here (I apologize in advance if there is already a discussion).

Here is our issue - our automatic bilge pump, we discovered - is siphoning water back into our bilge when we are under power (and when the butt of the boat goes down)...it was a very "un-fun" discovery that you can read about here: Windtraveler: Taking on Water is NOT Fun...

Anyway - we already have an anti-siphon right at the bilge - about 3 feet from the bilge pump itself. We also have a loop in a lazarette, but obviously this is not enough...we think when we are butt down and motoring at 6+ knots, there pressure building over the course of the hose and then, filling the bilge...

Should we add another anti siphon closer to the back of our boat, by where the bilge exits the boat? Or is there something else we should do?

Thanks in advance!

Best,
Brittany (& Scott)
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Old 10-19-2010
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The anti siphon loop and siphon break needs to be as high up in the bilge pump output hose as possible, and definitely above the waterline on both tacks...

If the anti-siphon device is below the waterline or at a lower point in the hose, it won't break the vacuum and stop the hose from siphoning.
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Last edited by sailingdog; 10-19-2010 at 12:56 PM.
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Thanks Sailing dog, yes, the loop is high enough...but what about the antispihon valve? Should that be somewhere specific? Right now it is about 3 feet above the bilge pump itself, and we are wondering if we should move it somewhere else? Thanks again.
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Old 10-19-2010
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It should be at the top of the loop.
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Repeat of what 'dog posted: the siphon breaker MUST be at the *very highest* part of the discharge line AND the height of the 'loop' must be 'well above' the highest heeling angle so that siphon break stays ABOVE the heeled water line.
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Okay, thanks everyone...does anyone see any issue with putting two in...one at the top of the loop, and one where the original is (near the bilge) to prevent more backflow?

Thanks!
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To clarify, two VALVES, not loops....one valve at the top of the loop, one nearer the bilge pump itself...

Thanks!
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To clarify: If the anti-siphon valve is not at the very highest point of the line, at the top of the highest loop, water pouring down the hose into it will close the valve, just like the pressure of water coming up from the pump does... It won't be at a point where the water in the hose is only generating a siphon vacuum, so it won't work properly.

I would remove the lower one. It becomes another point of failure, and you really want to reduce those in a bilge pump. It also probably adds more friction to the output hose and reduces the amount of water the pump can move. What you should do is have a continuous, smooth run from the output barb on the pump to the inside part of the siphon break at the highest point the hose runs. Use a smooth wall hose to reduce the resistance to the pump.
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Last edited by sailingdog; 10-19-2010 at 01:00 PM.
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It sounds to me like the O.P. may actually haver a check-valve in place, not two anti-siphon valves.

If this is your one and only, or primary bilge pump, I would get rid of the check valve.

As for the reason why water is siphoning back into the boat, I would suspect that your anti-siphon valve is gunked up with salt or other deposits and is not functioning properly. I.e., it is no longer open and therefor isn't breaking the siphon. You should at least confirm that it's working properly.
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From his description, he only has one of whatever he has, and if it is a check valve, it ain't working properly. Granted, he really shouldn't have a check valve in the bilge pump line, and he should have an anti-siphon valve in it...

Quote:
Originally Posted by JohnRPollard View Post
It sounds to me like the O.P. may actually haver a check-valve in place, not two anti-siphon valves.

If this is your one and only, or primary bilge pump, I would get rid of the check valve.

As for the reason why water is siphoning back into the boat, I would suspect that your anti-siphon valve is gunked up with salt or other deposits and is not functioning properly. I.e., it is no longer open and therefor isn't breaking the siphon. You should at least confirm that it's working properly.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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