It is not a good idea to add extra holes in the hull, but sometimes a system has to be changed to function properly.
The galley sink was originally designed to drain into a holding tank (this was per the European standards for certification). Not a great concept when it comes to food waste from doing the dishes (the tank could become more foul smelling then a black water holding tank).
So, a couple of years ago we changed out the drainage so that it went into a sump tank then via an automatic pump overboard. System worked great until the tank needed to be cleaned because of the gummy mess and microbes that accumulated. At times the automatic pump would pump the waste overboard, but would not shut off because the float switch failed from all of the gunk.
Thus to the title of this entry. We cut a one inch hole in the side of the hull abeam the sink, installed a SS thru hull fitting, a SS valve, and matted this to the sink plumbing. As a precaution to external wave action against the hull, a simple flapper style check valve was installed vertically as the last fitting before the sink drain.
The system is now simple and is working great. No more smell or maintenance and the system drains great.