Anti-siphon valves

[ageless]

My 55c had dangerously brittle bilge pump hoses so my son and I set out to replace all of them the last two days. Each had one way valves that were shot, I’ve never been a fan so I decided on anti-siphon loops. We completed the two fwd engine room units today but these loops don’t seem to function as advertised. On starboard we ran a section of hose to aft of the water heater, mounted the loop, ran hose from there to the thru-hull. The loop is mounted 1.5’ above waterline, 8” above the thru Hull. When I engaged the pump to discharge water I poured in there it never discharged out of the thru Hull, it all drained right back into the bilge. Could it be the pump can’t pump the water as high as the loop is mounted? I can’t imagine the loop would function as well if it were below the level of the thru Hull. Thoughts?

[bostonhatteras]

Centrifugal pumps will allow any water in the hose that is prior to the loop to just drain back to the bilge. If it moved the water up the hose then most likely if you had enough water most would have been pushed past the loop. Centrifugal pumps cannot work with one way valves in the bilge side of the vented loop as far as I know. They need a zero pressure outlet to start working.

[Pascal]

Bilge pumps will easily push water up higher than a TH but whatever water is in the hose betwenn the pump and the high point will flow back. If the vilume of the bilge is big enough it is no problem.... if the volume of water flowing back is enough to raize the float switch enough to start the pump, you have a problem. In that case the only fix is a check valve. I don like them, especially the sprng loaded ones, but the bronze check valve with gravity flap work ok. I ve used them before.

[racclarkson@gmail.com]

Much as you dislike it, you need a check valve close after the discharge if you expect to have reasonably dry bilges. Your 1" or 1-1/4" hose holds a lot of water, and you are pumping up about 5' from the sump to the top of the loop--lots of volume. If your loop isn't vented, then you additionally could siphon back lots more water until it catches air from the overboard discharge.

Your pump will pump where you have it. But when you turn it off, what's uphill runs back. Enough and your float switch will be a perpetual motion machine--until it runs out of power.

Try and keep clean bilges and check valve issues will be minimal.

[Dan Mapes]

Same thoughts as above but adding the fact that some boats have flaps on the through hulls if they are just above the water line. The ones I have seen usually were under a clam shell so they did not get forced shut by water passing by the hull at speed.

[Chasemmc]

1" True union clear heavy duty plastic flap style check vaves run about $25 wholesale. I like clear as you can visually check for operation. The true union allow quick dismounting for periodic cleaning. Spring loaded don't last very long as spring is not stainless.

[Boatsb]

Many of the Johnson pumps I sell have check valves. I don't remember if the pumps come in 32 volt thougb.

[ageless]

Both of my engine room pumps are Rule 3700 32v, which have 1.5" discharge. Is it safe to say I could have done check valves without the anti-siphon loops?

Explain to me the function of the cap on top of the loop? Is it supposed to be closed, or left open slightly?

PVC glue schedule 40 on the ends to attach to discharge hose?

[Boatsb]

One does not replace the other.

Check valves can fail. If they are in a circuit where water can backflow in as a siphon you can sink the boat.

[Chasemmc]

Agreed, the check valve just prevents the hose water from refilling the bilge.