One more thought, I've never liked the idea of the sink and a cockpit scupper being on the same drain line relying on a single shutoff point. It's the sink side I want to close, for a whole variety of reasons in the right/wrong conditions why I don't want the sink open, but do want the cockpit open, so put a simple ball valve on the side of the T that goes to the sink. Now your drain can be open, and sink is closed, and cockpit contents cant overflow into the sink no matter how bad it gets. If you are concerned about the wieight of the ball valve hanging off a T, use a marlon valve. Of note, I personally prefer bronze for all actual thru-hulls and seacocks - BTW, if you still have ball valves perched on thru-hull stems, time to replace with flanged seacocks that go flush against the hull, and while you're at it, replace the thru-hulls also (you'lll thank yourself later).
-David
Independence 31
Emerald