Here's another approach. When I bought Tarwathie, the sink in the head drained to the toilet bowl, and the galley sink drained to the bilge.
The head sink solution is fine. The amount of water used in the sink is small compared to water used for flushing. Thus, it does not add significantly to the holding tank's storage.
We found that draining the galley sink to the bilge, is nasty. Kitchen grease gathered in the bilge and made it foul and smelly. However it did have the advantage it still drains, and no water backs up when sailing on starboard tack. We have a deep sink and sea level is only 1 or 2 inches below the sink bottom with no heel.
I assume that you all know that Westsails are designed to sail west, and that when circumnavigating east-to-west, most of the journey is done on on port tack. On port tack, the sink drain works fine. I always figured that the galley sink drain and the positioning of the pilot berths is what makes the design optimized for "west sail"
I converted the galley sink to use it's own seacock under the sink, but I routed the drain through a foot pump. The foot pump prevents sea water from backing up, even under sail, and it drains the sink effectively in all conditions. The Whale Gusher foot pump needs to be cleaned out every year or two. I think I got the tip to use the foot pump here on the WOA forum.
I still have the plumbing to drain the galley sink to the bilge. I can use that as a backup in case I ever need to close that sea cock.