I can see a problem with the vented loop idea. A pump won't be able to suck out all the water
in the input side. Once the pump is shut off, water in the hose will back flow to it's own level. That could be above the level of the sink drain. I had this problem with a bilge pump
. The pump would suck up the water in the bilge
but when it shut off, water remaining in the hose would backflow to the bilge
retripping the auto switch which caused the pump to constantly cycle. Had to put a check valve in the line to stop the problem. Relocating the pump in relation to the thru-hull might of solved
the problem though I didn't get that far. My plumbing
knowledge doesn't go much past 'payday is Friday' so others may have a better solution.
The easiest solution, if it will fit, would be a bucket that the sink drains into. You'd have to empty it when it's full so monitoring it and dumping it would be a bit of a hassle but no problems with pumps jamming or what crap goes down the drain.
Our old boat had the sink drain just above the waterline. On the tack with the sink on the downside, the motion of the boat would force water up the drain, into the sink and overflow onto the counter. We solved
that problem with a cork in the drain.