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Re: Vented Loop Design?
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.