Anti siphon loop vent failure

[Saltyhog]

Originally Posted by delmarrey

It seems to work on mine just fine. When I shut off the motor the air left in the exhaust rises back up the down-side of the loop allowing it to go up into the tube that feeds the PSS and drains back to sea level.

I have to do it this way b/c sea level is right at the top of my motor, other wise I'd be getting feed back from my stern tube.

Maybe I'm missing something here, but I would think that the vented part of a vented loop must be open to the atmosphere in order to act as a siphon break. In any event, Delmarrey-- that is one *VERY* clean engine room!

[Cheechako]

Originally Posted by Sailmonkey

Except then it's no longer a siphon break. One must introduce air into a fluid system to break a siphon!!!

Yeah, no longer seems to be a siphon break that way? Of course, many boats are fine without a break... until that one time when the motor gets filled!
Looking back, less than half my boats had a vented loop in the exhaust elbow feed line. In many cases there was not enough room above the water line in the engine compartment to do it and the hose run to do it would have been very long. I think I was lucky for sure.
I've always liked the idea of just a small tube in lieu of the actual valve on the vented loop.... especially if it dumps into something like a cockpit drain where you can monitor it.
It just seems that everytime I service a vented loop valve, I'm not sure they are actually free and working....

[delmarrey]

Originally Posted by Saltyhog

Maybe I'm missing something here, but I would think that the vented part of a vented loop must be open to the atmosphere in order to act as a siphon break. In any event, Delmarrey-- that is one *VERY* clean engine room!

I guess I should have mentioned that the loop valve was removed. It's pretty simple, when the pump stops pushing water threw, gravity takes over.

What air there is in the exhaust rises, and the water falls, leaving no water in the down-side of the loop and exhaust, and seeking sea level in the PSS feed tube. A cut-a-way view would be great about now.

Mine is very similar to the center section of this picture.

[Nicholson58]

Originally Posted by Dulcesuenos

Most actual vents are just above waterline. Mine are way above it. Two people here who had their vent fail had lots of water ingress. Wondering if that happens if the vents are 10" above wl. Did i word it erong?

Only an improper vacuum / antisiphon vent is near the water line. Search this in any knowledge forum or consult any surveyor. It should be as close to the boat long centerline and as high as possible. Its sole purpose is to vent when the source for water is lost. This is so that water will not reverse siphon back into the boat. Thast is why it is call a siphon breaker. Picture a siphon hose. It can move water in either direction and as long as it remains functional, it is a hole in your boat. The vent's function is to slash a hole in the top of the siphon hose the instant that it is not needed. This prevents water flow from reversing and filling your boat. For the vent to be at water level or below, it will never break the flow. Higher is better because that will provide a more positive termination of the siphon when conditions warrent it to stop.

[sww914]

My boat's PO did a lot of work to install a vented loop in the wrong place. It was only about 2" above the waterline. When it got windy in the harbor it would allow water to flow in. If it was windy for long enough it would fill the exhaust up until it reached the exhaust valves and then it filled the engine up with water. I didn't have time to relocate the whole thing several feet away where I could get it high enough so I removed the loop and replaced it with a ball valve. Yes, you have to remember to open it and close it each time you use the engine but it won't fail and take out the engine if I'm away and it gets windy.
Eventually I'll spend the time and money to do it right but for now this is not fool proof but it's fail proof.
On a side note, when I bought the boat the old engine was full of salt water and he let it sit like that for 8 years.

[Nicholson58]

Originally Posted by sww914

My boat's PO did a lot of work to install a vented loop in the wrong place. It was only about 2" above the waterline. When it got windy in the harbor it would allow water to flow in. If it was windy for long enough it would fill the exhaust up until it reached the exhaust valves and then it filled the engine up with water. I didn't have time to relocate the whole thing several feet away where I could get it high enough so I removed the loop and replaced it with a ball valve. Yes, you have to remember to open it and close it each time you use the engine but it won't fail and take out the engine if I'm away and it gets windy.
Eventually I'll spend the time and money to do it right but for now this is not fool proof but it's fail proof.
On a side note, when I bought the boat the old engine was full of salt water and he let it sit like that for 8 years.

Ours has a hight vented siphon breaker but also a sea cock to isolate the engine exhaust and prevent following seas from wrecking the engine. Keeps the birds out in the winter.