Originally Posted by VallelyJ
The duck bill is just a check valve. It's orientation isn't important; only the direction of flow (and that it's kept clean so that it doesn't stick shut). If the loop is the highest point on the line, and above the heeled waterline, the vacuum isn't going to know whether the plumbing is laying on its side or upright. If the hose starts to siphon seawater into the engine, the valve will break the vacuum.
If you aren't confident in the duckbill, or you won't be able to access it to check it every now and then, you can just get a 1/4" barb with a female threaded end that fits where the cap threads on, and put the 1/4" hose back on. I assume it exits through the cockpit coaming or someplace. Then you can leave the duckbill out.
That confirms what I believed, and did.
The issue for me is that I believe that the duckbill is designed to be installed at the absolute peak of the loop when the vented loop is vertical. Turning the vented loop on it's side effectively moves the duckbill to a point slightly less (1 or 2 mm) than the peak, because of the difference in the circumferences of the vent and the loop. My thought is that this may have prevented the vent from opening when it should.