Bilge siphon

[DannyS]

I have had a couple of interesting moments with my electric bilge pump siphoning water back into the boat after pumping out the bilge. Here's the scenario...After motoring a couple of hours on a little to no wind day, my wife went below to make some lunch and shouted to inform me of water pouring in. Pretty alarming as we were 15 miles offshore. I throttled back to a slow idle and the water stopped. After scouring the boat for a leak, I settled on a siphon as the answer for the water, and figured that it had to be either the manual or electric bilge pump since the through hulls are underwater as the boat squats while under power. After turning the boat around and aborting our trip (since I could not fully convince myself or my wife of my theory), I tried to confirm the siphon by throttling back up but could not get it to come back.Back at the marina, I checked over the entire boat, hoses, through hulls and found no evidence of a leak. A couple of weeks went by and this past weekend we had a crew of people aboard and all of their gear and I noticed that the boat was squatting again, this time not under power but because of the weight of the extra crew and poorly distributed gear. The siphon came back and I confirmed that it was the electric bilge pump by unhooking it from the plastic strainer and raising it up to see a stream of water pouring into the bilge. So now with my suspicion confirmed, I want to get the intermittant siphoning to stop so what I'm thinking I need is to place a vented loop into the high spot in the hose. As it is now, there is a loop well above waterline but somehow this is not enough to keep the siphon from starting. I thought of placing a check valve in the line but know it is not recommended even though plenty of people do it anyway. Anyone else have a similar experience? Am I on the right track?

 

[Don S/V ILLusion]

suggestion

Given all traditional fixes for this such as a vented loop or check valve can and will eventually fail, the only sure way I can think of to preclude a recurrence is to raise the discharge sufficiently above the waterline so it can't be submerged on a heel. Any bilge discharge that low is a poor design mistake.

 

[DannyS]

That's an option

I thought of that and I agree that the thru hulls are perhaps a bit close to waterline but it was my understanding that the high point in the line (the unvented loop) should have taken care of the siphoning. Does the fact that the loop is directly above the thru hull then turn and make the long run down to the bilge pump have anything to do with it? My thinking is that once the pump pushes water out and quits, the backwash down the long "leg" of the hose is enough to pull the water up the short "leg" and over the loop thus starting the siphon.

 

[Nice N Easy]

Siphon

Your correct in your assumption that once the water starts flowing, the loop will not make any difference. It will still siphon. The only sure fire, never fail way to cure this is to raise the discharge from the pump, so it is not below the water line no matter what. A vented loop, or a check valve will fix the problem, until it fails. Maybe tomorrow, maybe never, but you can't be sure. On my boat, the bilge pump discharge is through the transom, up fairly high. Never a problem with that set up.

 

[Bill Roosa]

Vent valve maintenance

Well it sounds to me like you have a workable solution already installed. The problem is the vent valve at the top of the siphon loop. It should unscrew and the little piece of rubber inside should allow air to enter but not leave. After cleaning test it to make sure. This is one of those yearly maintenance things where you go around the boat cleaning all the vented loops one day in December things. It has probably just got salted up and is not passing air.

 

[Bill Roosa]

After instillation

have some one pour a bucket of water into the bilge and with your ear by the vent listen for the "vacuum to break" It should just be a little sucking sound as the air enters right after the pump turns off. Sometimes it is a pulsating sound.

 

[Peggie Hall HeadMistress]

It's not a siphon...

It's an effect known as "ram water"...and no loop, vented or otherwise, can prevent it. It's what happens when a thru-hull is below waterline and water is pushed through the hose by the pressure of the water against the hull when the boat is underway.The ONLY cure in this case: move the thru-hull to a point on the hull that will remain above waterline at any angle of heel.

 

[Tim R.]

How about this

Close the seacock when in this situation?Tim R.

 

[Phil]

Check Valve

My boat came from the manufacturer with a check valve about 12" from the pump. It would appear its design is just for breaking a back flow. You might want to rethink a backflow valve.

 

[Don S/V ILLusion]

Danny

Just to reiterate, forget the backflow valve - it's just a matter of time until it fails and you won't know when. Despite the apparent history of no problem, it sounds like it is a problem now so past history is irrelevant. Replacing it with a float-switch activated pump won't change anything. If you don't want to move the discharge thruhull location to a better place as others have also suggested, about the only thing I can think of is to shut the power off except when you can be watching it to make sure it isn't sucking water back into the boat. Not an ideal solution, particularly when no one is aboard, but it appears to me as the only alternative to raising the discharge.

 

[DannyS]

while sitting at the slip...

I don't worry about leaving the boat with the bilge pump on since the thru hulls are out of the water while just sitting at the slip with no one aboard and no extra gear. I did make sure of that before I left last weekend.In typing all of this out, I keep thinking of little things to try to manage the problem. I'm thinking there's a magic level of water in the bilge that creates the right circumstances for this to occur. If I can remember to mop out the bilge before getting underway, I might be able to mitigate the problem before it has a chance to start.Just to explore the avenue of moving the thru hull higher, any suggestions on what to do with the existing thru hull?I know the best situation would be to remove and re glass the hole, but would running a hose off the thru hull and tying it up under the combing with a plug suffice?

 

[GuyT]

Check valve

I have to have a check valve on my bilge because of the water head and length of hose. I have 1-1/8 hose about 10 ft long which contains quite a bit of water. If the pump shut off and it all the water came back, the pump would turn back on again. Mine is 12 inches after the pump.

 

[Don S/V ILLusion]

seacock

would be my choice

 

[Phil]

Shut Off Power

Shutting off the power to the bildge sounds like a much better idea than a backflow check valve. At least that way if the boat sinks due to something else starting to leak you'll know it wasn't the bildge siphoning water back in.