Siphon Break

[Poche]

Would you install a siphon break for a bilge discharge with the thru hull a couple inches below the water line and the hose loop about three feet above?
That's for sitting in a boat slip unattended, not under way.
The boat hasn't had one for 58 years but I'm re-plumbing so the thought came up.
If the thru hull was deeper I would install one but at the waterline I can't imagine a siphon back into the boat.

[coopec43]

Originally Posted by Poche

Would you install a siphon break for a bilge discharge with the thru hull a couple inches below the water line and the hose loop about three feet above?
That's for sitting in a boat slip unattended, not under way.
The boat hasn't had one for 58 years but I'm re-plumbing so the thought came up.
If the thru hull was deeper I would install one but at the waterline I can't imagine a siphon back into the boat.

Maybe I don't understand your question?

No way in the world would I discharge bilge pump water below the waterline.





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[Benz]

Seems like if the siphon intake is physically higher than the place it would drain to, then a syphon will occur regardless of whether its inches or miles below the waterline. Perhaps the only thing stopping a syphon now is a checkvalve in the bilge pump. Scary.
Move the outlet to a healthy distance above the waterline.
Do it quickly.

[Poche]

Originally Posted by Benz

Seems like if the siphon intake is physically higher than the place it would drain to, then a syphon will occur regardless of whether its inches or miles below the waterline. Perhaps the only thing stopping a syphon now is a checkvalve in the bilge pump. Scary.
Move the outlet to a healthy distance above the waterline.
Do it quickly.

No, the loop is three feet higher and there is no check valve. Been like this for 58 years.

[Bill Seal]

Unless there's a way to introduce air into the top of the loop, the loop won't do squat to stop the water from siphoning back into the bilge. That's the way a siphon works.

[Sailmonkey]

Originally Posted by Poche

No, the loop is three feet higher and there is no check valve. Been like this for 58 years.

That’s 58 years of luck.

A siphon will form if the entire outlet of the pipe is below the waterline, doesn’t matter how far.

But, the bilge pump needs to be lower than the outlet for a siphon to form. I can’t imagine a scenario where this occurs, but it’s possible.

The only reason I can think you’ve not formed a siphon yet is that maybe the hose diameter is such an the hose angle is such that water and air exchange at the same rate on the pump side negating the ability of t he standing water in the hose to lift a column of water from the outside over the loop.

Again, just blind luck if this is working this way.

Install an anti siphon valve (vent valve) at the top of the loop. It’s cheap insurance, it’s the right way to do this job, and there’s really no downside.

[BlackHeron]

We have a siphon break and the bilge pump output is usually above the waterline unless the boat is heeled over hard on a port tack. If the bilge pump outlet ever has any possibility of being below the waterline then a siphon break is a no-brainer. Sooner or later it is going to bite you in the ask.