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Old 08-04-2016, 09:44   #16
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Re: Anti Siphon Loop -- Bilge Pump

I had a 30ft boat taking on water after pumping the bilge. We were sailing in 20 to 30 knot wind heeling quite a bit. The bilge outlet was just above the waterline. Everytime I pumped, the bilge would fill up. After getting back the first order was to install anti siphon device. Problem solved. Current boat has a midship/waterline bilge thruhull where I installed an antisiphon valve running it
up under the deck. No problem- Tim (Northstar-Tartan 41/43 (1974))
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Old 08-04-2016, 09:54   #17
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Re: Anti Siphon Loop -- Bilge Pump

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I had a 30ft boat taking on water after pumping the bilge. We were sailing in 20 to 30 knot wind heeling quite a bit. The bilge outlet was just above the waterline. Everytime I pumped, the bilge would fill up. After getting back the first order was to install anti siphon device. Problem solved. Current boat has a midship/waterline bilge thruhull where I installed an antisiphon valve running it
up under the deck. No problem- Tim (Northstar-Tartan 41/43 (1974))
Fancy seeing you here Tim.. My Tartan's thruhull is under the pilot berth, so a loop to the deck would require me to run the line aft and up inside the galley cabinets. It that how you did yours? I think under the galley sink would be an easier solution on Diablo.

To the OP, I saw you say it would be a ton of work for a centerline discharge. But you don't need to discharge centerline if you can put an anti-siphon loop near centerline. Then it doesn't have to be very high, just above static waterline, where an outboard siphon loop needs to get up close to the deck.

luck...
Zach T41 'Diablo'
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Old 08-04-2016, 09:55   #18
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Re: Anti Siphon Loop -- Bilge Pump

I guess I don't understand the problem with a vented loop on a bilge pump...?
Be careful! I experienced the same thing as Keepondancin did. I was motoring flat though. My pump discharge was above waterline, but the boat squats when motoring hard, that and a combination of wind waves off the stern and I started siphoning water in the boat.
Fortunately, and with some luck, I happened to go below and pick up the bilge cover as I was curious what my new vacuum gauge on the fuel filter was reading... to my horror, the water was just up to the bottom of the floor boards! A high loop, check valve or vented loop is necessary!
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Old 08-04-2016, 10:03   #19
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Re: Anti Siphon Loop -- Bilge Pump

Two additional safety items- a shutoff valve at thru hull, (antisiphon loop runs in locker behind nav station) and a bilge alarm. The valve is in case the ant isiphon malfuctions and you need to shut off. The bilge alarm let's me know
there is a problem.--Tim (Northstar-TARTAN 41/43 (1974)
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Old 08-04-2016, 10:22   #20
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Re: Anti Siphon Loop -- Bilge Pump

The air inlet for the antisiphon need not be at the top of the loop. You can add a T fitting at the top, or near the top of the loop and run a hose 10' or more into the cabin roof or wherever. I ran it to the aft side of cockpit.
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Old 08-04-2016, 10:24   #21
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Re: Anti Siphon Loop -- Bilge Pump

My thruhull for the electric bilge pump is 12" above the waterline on the port side. It siphoned water in after it ran when I was heeled over.

I found a check valve had been installed half way from the pump to the thruhull.

Here's how I'm fixing it:

Changing the thruhull and adding a marelon seacock. A real seacock, not a ballvalve.

Added a vented loop up high under the deck. It's on the port side. Centerline wasn't practical. If/when I get in extreme conditions again I'll shut off the seacock.

Eliminate the check valve.

Adding a diaphragm pump to keep the bilge water low.


At anchor or in a slip the bilge pump rarely runs. I get water from rainwater and underway from the prop shaft. (Rudder shaft had seepage but that's been repacked.)
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Old 08-04-2016, 10:30   #22
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Re: Anti Siphon Loop -- Bilge Pump

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Fancy seeing you here Tim.. My Tartan's thruhull is under the pilot berth, so a loop to the deck would require me to run the line aft and up inside the galley cabinets. It that how you did yours? I think under the galley sink would be an easier solution on Diablo.

To the OP, I saw you say it would be a ton of work for a centerline discharge. But you don't need to discharge centerline if you can put an anti-siphon loop near centerline. Then it doesn't have to be very high, just above static waterline, where an outboard siphon loop needs to get up close to the deck.

luck...
Zach T41 'Diablo'
Thanks; I understand the principle. But there's no place to do that.


Although -- it might be that I already have that -- as the hose does come right up to the level of the raised salon floor before being led to the discharge point in the topsides. That might be be slightly above the static waterline. Maybe that explains why I haven't had any siphoning in all these years and thousands of miles.


I am starting to think, though, that the safe way to deal with this is just to leave the seacock off and leave it at that. This is not the primary pump -- it is for emergency use only.

And for a REAL emergency, I have a massive 22 000 liters per hour trash pump with 3" fire hose, which I keep in the bilge. Just in case. Hope I'll never need it, but I could use it as a fire pump, and I could also use it to save someone else's boat.
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Old 08-04-2016, 10:36   #23
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Re: Anti Siphon Loop -- Bilge Pump

Thanks for triggering a great discussion. I recently bought a 16 year old Malo 39 which while fitted with an engine powered bilge pump did not have an auto bilge pump of significance. I am in the process of adding a Rule 3700 with Ultra switch and plan to route through the transom with no check value and with an anti-syphoon loop. Well that's the plan! As others have said it is worth aiming to do it right and compromise if no other solution is possible.
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Old 08-04-2016, 10:50   #24
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Re: Anti Siphon Loop -- Bilge Pump

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Thanks for triggering a great discussion. I recently bought a 16 year old Malo 39 which while fitted with an engine powered bilge pump did not have an auto bilge pump of significance. I am in the process of adding a Rule 3700 with Ultra switch and plan to route through the transom with no check value and with an anti-syphoon loop. Well that's the plan! As others have said it is worth aiming to do it right and compromise if no other solution is possible.
The Rule 3700 (or in my case, 4000) plus Ultra switch is exactly what I have.

If I were to do it over again, however, I would use the Predator bilge pump, which is much more heavy duty.
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Old 08-04-2016, 11:20   #25
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Re: Anti Siphon Loop -- Bilge Pump

In my case I have several high drains at the back of the cockpit at seating level and on the side decks. I connected my bilge pump discharge to the top of the hose from one drain with a loop in the bilge hose. If you have a high cockpit or side deck drain. you can use that for the vent to prevent siphoning

One issue with using vented loops is the small rubber air valve, may clog with bilge "stuff" over time.
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Old 08-04-2016, 11:47   #26
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Re: Anti Siphon Loop -- Bilge Pump

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. . .
One issue with using vented loops is the small rubber air valve, may clog with bilge "stuff" over time.
Is that a problem? Do they clog open or closed?

If they clog open I wouldn't think it would be a big problem -- just get a little of the discharge water spraying out instead of going over the side.

But the big question is whether vented loops are needed at all on bilge pump lines. I believe my original install pumps do not have vents in the loops.
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Old 08-04-2016, 12:57   #27
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Re: Anti Siphon Loop -- Bilge Pump

A non-vented loop on a bilge pump line is completely useless. If the discharge port is below the waterline, it will still back siphon. Your pump will fill the loop with bilge water and when it shuts off the weight of the water will flow back toward the pump and start a siphon. You have to have the loop vented or there's no point to having the loop.. save maybe splashing water from flowing down the line... pls. correct me if I'm wrong.
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Old 08-04-2016, 13:06   #28
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Re: Anti Siphon Loop -- Bilge Pump

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A non-vented loop on a bilge pump line is completely useless. If the discharge port is below the waterline, it will still back siphon. Your pump will fill the loop with bilge water and when it shuts off the weight of the water will flow back toward the pump and start a siphon. You have to have the loop vented or there's no point to having the loop.. save maybe splashing water from flowing down the line... pls. correct me if I'm wrong.
A big loop does have some advantage... being that the water incoming has to raise up and over the loop to start a siphon. That wont occur much. When done pumping the water in the hose returns to the bilge and the water on the aft side of the loop returns to the sea. Traditionally many are built with a stern bilge pump exit and a loop of hose against the inside of the transom, this loop can often go clear up to top of the cowling, probably 3 ft above waterline or more. It's my preferred method, not always possible though.
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Old 08-04-2016, 13:43   #29
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Re: Anti Siphon Loop -- Bilge Pump

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A big loop does have some advantage... being that the water incoming has to raise up and over the loop to start a siphon. That wont occur much. When done pumping the water in the hose returns to the bilge and the water on the aft side of the loop returns to the sea. Traditionally many are built with a stern bilge pump exit and a loop of hose against the inside of the transom, this loop can often go clear up to top of the cowling, probably 3 ft above waterline or more. It's my preferred method, not always possible though.
While I agree the high loop would prevent water flowing down hill into the bilge (not a siphon), as soon as the bilge pump turns on and fills the hose with water a siphon will start if the discharge is underwater regardless of the height of the loop. Perhaps your boat is dry and your bilge pump rarely runs. Then you may be okay. But if you think the water splits flow directions when you say "When done pumping the water in the hose returns to the bilge and the water on the aft side of the loop returns to the sea" then you are mistaken. This can not physically happen without a vented loop as a vacuum would form, hence the vent.
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Old 08-04-2016, 14:09   #30
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Re: Anti Siphon Loop -- Bilge Pump

Yes, you are correct of course if the exit fitting stays submerged, most often they dip in and out. details, details! Aint boating fun?
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