In article ,
Roger Long wrote:
"Jonathan Ganz" wrote

I took a look at the WM site and found an example of a vented loop. Looks
like it might fit, but I'm not sure where yet. It can either be within a
foot or so, or it would have to be at least 3 feet away in the engine
space. I'll have to check it out.


The vent has to be above the discharge and waterline for there to be any
point to it. "within a foot or so" would be OK if you just mean the lateral
distance and not the hose length. The loop should be just as high in the
boat as you can get it, ideally, just under the deck. If your bilge
discharge is high enough above the waterline, you don't really need a vented
loop except to keep surveyors happy. My bilge discharges are just ahead of
the transom and nearly a foot above the water line so I haven't added vents
to the lines which loop even higher under the cockpit coaming. It's
something I'll probably do before going very far off shore though. If the
boat was low in the water due to flooding and the pumps stopped, a back
siphon could be created.

This is an area that I need to investigate. I was down at the boat and took some time to look at the situation that develops. It looks like there's enough back-pressure on the line that it ends up jamming the flapper against the body of the valve enough to cause it to jam (not every time, but often enough, obviously). When I get in there with a screwdriver and force it open, the water that's trapped comes shooting out with a fair amount of force.

The discharge is well above the waterline - at least 18" or so, which would mean the loop (as you point out) would have to go higher. It's not a problem at the transom... plenty of room to do that via the stern laz. I definitely need to have something to prevent backflow. We typically get following seas, even coming back from someplace local. Come back through the the Gate, as I did last Sunday, and you definitely get a big following sea.

I'm wondering if there'll be so much backflow that the pump will continue to cycle. What do you think about a vented loop near the transom as you suggest, plus a check valve near the pump? Seems like this would prevent the flooding/back siphon and give the pump a break.