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Old 07-04-2016, 15:50   #1
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Anti Siphon Loop -- Bilge Pump

I have done a thorough revision of my bilge pumps, and one fault I found is that my large bilge pump does not have an adequate anti siphon loop.

I am not really sure I can get the top of the loop above any possible waterline -- I try to sail her flat but the rail does get in the water sometimes.

Will a vent prevent siphoning even if the top is below the waterline on a sharp heel?

And in general, do people put vents in anti siphon loops for bilge pumps? I don't believe having ever seen one.

All advice gratefully received.
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Old 07-04-2016, 16:23   #2
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Re: Anti Siphon Loop -- Bilge Pump

IMHO if the outlet is low, you CANNOT have a vent. Because you will lose the airlock and flood the boat.

Loop it up as high as you can and you will be fine. In our boat the loop goes to the deck level then falls down to the outlet. We of course do dip the edge of the deck in the sea at times (topsides are 2' here ...) not a drop of seawater in the bilges ever.

PS It DOES help if you have your outlets fore or aft rather than midship - just think about it ... which part of the rail digs in first and stays wet longest ... ;-)

So loop it. IMHO

b.
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Old 07-04-2016, 16:29   #3
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Re: Anti Siphon Loop -- Bilge Pump

Sailors who don't actively maintain their own boat will tell you to not install a non-return valve on a submersible pump. Not a problem on my boat because I use all well filtered belt-driven pumps.
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Old 07-04-2016, 16:56   #4
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Re: Anti Siphon Loop -- Bilge Pump

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Originally Posted by Blue Stocking View Post
Sailors who don't actively maintain their own boat will tell you to not install a non-return valve on a submersible pump. Not a problem on my boat because I use all well filtered belt-driven pumps.
We discussed this recently --

Maintenance pump = non-return valve is fine

Power pump = not fine!

This being my large pump intended to shift water in volumes, rather than just sip up the rain water which comes down the mast, there is no non-return valve.
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Old 07-04-2016, 22:45   #5
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Re: Anti Siphon Loop -- Bilge Pump

The trick with any anti syphon loop on a monohull is to put it as close to the centerline as possible - that way the line is usually always above the waterline even when heeling.

We have an anti syphon vent on our bilge even though our through hull is well above the waterline - I figured a belt and suspenders approach wasn't too much redundancy. Will we ever need it? Not likely. It may have added complication where none was needed but I sleep well at night.
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Old 08-04-2016, 04:54   #6
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Re: Anti Siphon Loop -- Bilge Pump

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The trick with any anti syphon loop on a monohull is to put it as close to the centerline as possible - that way the line is usually always above the waterline even when heeling.

We have an anti syphon vent on our bilge even though our through hull is well above the waterline - I figured a belt and suspenders approach wasn't too much redundancy. Will we ever need it? Not likely. It may have added complication where none was needed but I sleep well at night.
Well, my through hull is above the waterline, but not when the boat is heeled far over. No way to get the loop anywhere near the centerline without completely rerouting the discharge through the transom. That would add a great deal of hose length and resistance.
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Old 08-04-2016, 05:27   #7
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Re: Anti Siphon Loop -- Bilge Pump

Ive seen a high volume crash pump rigged with a low outlet just above the waterline and a seacock that is kept closed until it's used. This way the head is much lower so the pump can work really efficiently. The switch was in the same locker as the seacock so that you remembered to open the seacock. I guess if you could find a low restriction non return valve it would be a good safety feature in case of a pump failure..

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Old 08-04-2016, 05:52   #8
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Re: Anti Siphon Loop -- Bilge Pump

I've had our boat for 25 yrs never thought to have an anti syphon loop. the hose just lie in the bottom of the boat and exit at a through hull at the stern. Never have had water come back in. Oops strike that now I've probably tempted Loki I'll get wet!
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Old 08-04-2016, 05:58   #9
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Re: Anti Siphon Loop -- Bilge Pump

Quote:
Originally Posted by Snowpetrel View Post
Ive seen a high volume crash pump rigged with a low outlet just above the waterline and a seacock that is kept closed until it's used. This way the head is much lower so the pump can work really efficiently. The switch was in the same locker as the seacock so that you remembered to open the seacock. I guess if you could find a low restriction non return valve it would be a good safety feature in case of a pump failure..

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We discussed non-return valves recently. I will never, ever have a non-return valve on a discharge line used by a high volume pump.

However, the idea of just keeping the seacock closed is what I've actually been doing since I discovered this problem about a year ago. I keep the seacock closed and I keep the switch in the "off" position, not "auto".

The idea is that the bilge alarm will alert me to the need of the big pump in time for me to open the seacock and switch on the pump.

Like all the seacocks on my boat, this seacock is instantly accessible -- in 10 seconds. Behind the port side salon settee.

I think this isn't the worst solution in the world, but I would prefer for it to be fully automatic, which is why I started the thread.
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Old 08-04-2016, 06:55   #10
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Re: Anti Siphon Loop -- Bilge Pump

We were on a really great beam reach in the Bahamas in probably 3-4 seas. My wife went down below to find water up over the floor boards. Calmed my wife down by telling her we are on the Bahama Banks, so if we sink, we will just sit on the spreaders until we're found. If you have never experienced anything like that, it is quite a fire drill between manually pumping and searching for where the water is coming in. We had the boat for a couple years and had been in all types of conditions. We flattened out the boat and the bilge pump pumped the water out, with no more coming in. We anchored for a couple of days cleaning up the mess, lost a couple of power tools, but that was really the only damage. The through hull was above the static water line and the discharge hose was up high, but no vented loop. Sailing we kept an eye on the bilge pump, and on a port beam reach again, water was coming back in through the bilge pump. Added the vented loop, and never had a problem for the next 8 years we owned the boat, a Pacific Seacraft 37.
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Old 08-04-2016, 08:08   #11
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Re: Anti Siphon Loop -- Bilge Pump

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Originally Posted by keepondancin View Post
We were on a really great beam reach in the Bahamas in probably 3-4 seas. My wife went down below to find water up over the floor boards. Calmed my wife down by telling her we are on the Bahama Banks, so if we sink, we will just sit on the spreaders until we're found. If you have never experienced anything like that, it is quite a fire drill between manually pumping and searching for where the water is coming in. We had the boat for a couple years and had been in all types of conditions. We flattened out the boat and the bilge pump pumped the water out, with no more coming in. We anchored for a couple of days cleaning up the mess, lost a couple of power tools, but that was really the only damage. The through hull was above the static water line and the discharge hose was up high, but no vented loop. Sailing we kept an eye on the bilge pump, and on a port beam reach again, water was coming back in through the bilge pump. Added the vented loop, and never had a problem for the next 8 years we owned the boat, a Pacific Seacraft 37.
OH, my God.

That's exactly the nightmare which flashed through my mind when I discovered how my bilge pump was plumbed.

And to think I had sailed thousands of miles with the sea cock open, and the rail in or near the water. I sure was lucky.
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Old 08-04-2016, 08:50   #12
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Re: Anti Siphon Loop -- Bilge Pump

Thanks DH for raising this. My bilge pump hose is looped up under the sink to the level of the underside of the worktop and no valve on the skin fitting. No problems so far, touch wood, but it's something I've been slightly concerned about. Before I fit a seacock which will be hard to both fit and manipulate because of the lack of access, I think I'll try a vented loop.

On previous boats I've had a 220V submersible pump aboard for emergencies. Nowadays there are such pumps with over 400 l/min output admittedly at zero head and I'm thinking of adding one of these too which might help and at least give some reassurance.

Cheers,
Mike
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Old 08-04-2016, 09:25   #13
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Re: Anti Siphon Loop -- Bilge Pump

Quote:
Originally Posted by Dockhead View Post
...No way to get the loop anywhere near the centerline without completely rerouting the discharge through the transom. That would add a great deal of hose length and resistance.
It beats sinking your boat.
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Old 08-04-2016, 09:40   #14
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Re: Anti Siphon Loop -- Bilge Pump

I have a 1975 Tartan 41 with a bilge pump outlet just a few inches above the static waterline, near mid-ships. I typically keep my pump on auto to get rid of any leaking rain water. On starboard tack just 15 deg of heel causes back flow. The PO put a ball valve the discharge line. That works if you remember to close it, which I have not always done and nearly ran my battery down on an over night passage. ( I typically day sail and don't worry about it ) The bilge pump was pumping out, back flow, pump out, repeat... for ~15 hours before i noticed it....
Now the comment about putting the loop near center line has me thinking I can add a loop under my galley near the sink. It's not that high, but it is above the waterline even when heeled. Note that I bought a check valve and did not install after reading previous thread. My bilge is not clean and I don't need to deal with clogs in my bilge pump line.
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Old 08-04-2016, 09:42   #15
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Re: Anti Siphon Loop -- Bilge Pump

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It beats sinking your boat.
Yeah, I think it's not the only way to prevent that.
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