bilge anti-siphon loop

[cooper999]

While I wait to address engine issues, (of course) I've stumbled upon another issue. While checking my fuel pickup down in the lazarette, I noticed that the hose coming from our electric bilge pump (the red hose in the picture) essentially plunges straight down from the through hull and doesn't have an anti-siphon loop. There is just about a foot of freeboard between the through hull and water level when we're not underway. However, when there or four of us in the cockpit, going 5-7 knots, the stern dips such that the waterline will often partially or fully cover the through hull. Methinks the absence of an anti-siphon loop may explain why we'll get water in the bilge sometimes while underway.

Is this (or something else related) a plausible reason for water in the bilge? Does anyone else, particularly E30+ owners, have a similar arrangement with the electric bilge pump hose through hull, but with an anti-siphon loop? Without one, and not getting any water?

I swear, every day a new discovery!

 

[bigd14]

Check valve, loop, or both?

I have the same issue. Wondering if a check valve or a loop, or both is best. I put a check valve in already since its a loooong hose run. I can easily put a high loop in as well.

Thanks.

 

[Frank Langer]

Our bilge pump on our 1984 E30+ is also done as you describe without a loop or check valve, but I don't think that's the cause of water in the bilge as it doesn't stay submerged for any length of time, and it's mostly above the water line. I think it's ok as is from the factory but will be interested to see what others think.
Frank

 

[Christian Williams]

I was convinced by Maine Sail's (R.C. Collins') thinking in opposition to in-line check valves on bilge hoses.

http://www.sailnet.com/forums/gear-maintenance/93937-bilge-pump-non-return-valve.html

I've removed two that were completely clogged--one with human hair from the shower sump. And they were pretty inaccessible.

However, it's easy to install a vented antisiphon loop, even if there currently isn't enough hose.

I don't think Ericson looped bilge hoses. Apparently it is not traditional practice. On my two boats the bilge hoses ran straight to the stern throughhull.

True, check valves for the familiar Rule submersible bilge pumps are very popular. However, consider that such pumps are often/usually not self-priming. If a check valve successfully prevents hose run-back, it causes the pump to loose its prime. When that happens, the pump runs noisily, makes small bubbles--and doesn't pump anything. Bilge water continues to rise. That phenomenon once caused me to think I was sinking and it took me months to figure out what was actually going on.

So--antisiphon loop, if fitted with submersible pumps.

Diaphragm pumps don't need one, they contain two check valves in the pump itself.

Can an unlooped, uncheck-valved bilge hose siphon water back into the bilge and sink the boat?

I don't know. I kinda doubt it. Siphons are hard to maintain and most hoses don't terminate deep under the waterline.

 

[Kenneth K]

I had the same experience as Christian, and removed the one-way check-valves from my bilge lines.

My experience on "self-priming" and Rule submersible bilge pumps:

I originally liked the idea of a one-way check valve (mine was located in the bilge, about 2" downstream of the Rule bilge pump). Without it, every time I turned off the pump, 20 ft of 3/4" bilge hose would drain from the line back into the bilge--frustrating. The check valve was effective at stopping the backflow of water from the hose back into the bilge. Success! Or so I thought; until the second time I went to drain the bilge.

On the second attempt (bilge level is high enough to submerge the pump and the bilge line is already full of trapped water due to the check valve) the pump just spins but pumps no water. Why? The pump is submerged, so it should be adequately "primed." However (my theory only here), the pump blades are designed to be effective when fluid is moving past them at a given rate (think 500 gph, or something close to the pump's rating). The blades are not effective, though, at getting static water moving against the back-pressure created by the trapped water behind the check valve. If I shook the pump hard enough (creating additional pressure at the pump inlet), or if I released a some of the trapped water behind the check-valve, the pump would start moving water again. Once the water is moving through the pump, it has no problem overcoming the check valve. The problem is only in the initial startup or "priming" of the pump.

So, a check valve is likely not the answer to your problem.

 

[toddster]

Dunno - my two bilge pumps came to me with check valves installed. They've worked fine, so I haven't messed with them. The one on the Rule pump is about a foot upstream, so there is a small amount of back flow. As noted, the one on the manual pump seems to serve no purpose, since the pump itself consists of two check valves. Except that it's maybe eight feet downstream from the pump - that's a considerable amount of water to back flow, which it will do as soon as the siphon is broken at the pick-up. However, that hose will shorten up considerably when I get around to reinstalling the manual pump so that it can be reached from the helm. (Where vacant holes from old engine gauges now reside.)

 

[Christian Williams]

Regarding vented loops with Rule-type pumps: consider the head issue.

In short, the loop can't be too high because such pumps can't lift water very high:

http://www.sailnet.com/forums/948599-post27.html is the source of these comments:

I work with centrifugal pump design every day. What's missing here is our basic tool, the pump curve supplied by the manufacturer. A centrifugal pump always runs on the pump curve, for every head condition along the curve there is a corresponding flow rate. The left end of the curve is the maximum head at zero flow, called shutoff. If you look at a pump curve you'll gain a whole new appreciation for the difficulty in applying centrifugal pumps.

You can learn a whole lot about your bilge pump by putting an open hose on it and running it in a shallow pan. Start with the open end down low and watch how much flow you get. Slowly raise the open end and watch how fast the flow rate falls off. At the height water stops coming out of the hose you have reached shutoff head. Measure the flow into a bucket and draw the curve for the pump. Some pumps have relatively flat curves downward to the right. A small amount of additional head pushes the pump back on the curve to the left and flow falls off very quickly. I attached a pump curve just to show what they include, curves for different impellors, efficiency points, horsepower curves etc.

I see here talk about having a loop well above the heeled water line. The fact is you want the loop as little above the waterline as will gaurantee you won't back flood when heeled. Any more greatly reduces the flow that you can actually pump. You might notice they rate bilge pump flow at about 2 feet of head. Lifting water 3 feet can cut the flow in half or more, and you might not get any flow at 4 feet!

There is also talk about venting the loop, to prevent a back siphon. This is true, however a forward siphon from a loop well above the water line to a discharge point above the water line actually will increase the pump flow a lot. So a tall vented loop can defeat your pump as well. When you do the test above trying raising the middle of the hose way up in the air with the open end near the ground. You'll see that the pump will move a lot of water way up high as long as the siphon on the downhill side pulls the water along. Drill a tiny hole in the top of that loop and the flow will fall way off.

A really important point missed here is discharge hose size on the pump. When you have only 2 feet of head to work with you really must avoid any friction losses. So jumping a 3/4" pump discharge up to 1" or even 1-1/4" can make a big difference, especially if it must run more than a couple of feet.

The comment about a small hole in the hose right at the pump discharge is a good one. In the instructions no one reads for commercial submersible pumps there is always a statement about drilling a 1/8" hole to prevent air locking. A bubble of air forms in the pump housing and the impeller can't grab enough water to build pressure enough to push water up the pipe. In a 44 story building in Manhattan I had to pull two pumps out of 18 foot deep tanks to drill the two holes that no one thought was important because those pumps air locked all the time!

 

[Glyn Judson]

Bilge hose safety.

All, I fitted a belt driven diaphragm pump to the bilge pick up hose that effectively prevents water from back flowing as can be seen in the attached image. Aside from the bronze strainer on the bottom of the pick up hose in the bilge I also added a fine mesh strainer between there and the pump in order to prevent and debris from gaming a flapper valve open, thereby rendering the pump useless. It all works like a charm and has for a decade or more. Glyn Judson, E31 hull #55, Marina del Rey CA

 

[bigd14]

Great info, thank you gentlemen. I guess I should know by now that there are as many opinions on the subject as there are boatowners. It appears that the best solution appears to be a diaphragm pump, with no check valve or high loop needed. I recall seeing several at a used gear store. I think I will check them out and if any of them look reasonably functional I will see if there is a rebuild kit available for it.

 

[cooper999]

Indeed-thanks to everyone for chiming in. What seems to have maybe been the culprit in my scenario was a cracked fitting at the transom, which wouldn’t manifest itself unless we were underway with a crowd (relatively speaking) in the cockpit, dipping the stern and electric pump outlet below the waterline as described above. Since addressed, though haven’t been back out to test. I’ll update to let you know how it goes.