Install a y-valve, not just a tee or wye fitting..
And while you're doing this, replumb your sink drain to drain out your head intake. It's the simplest, easiest cure for head odor due to trapped stagnant seawater in the head intake. You'll find a complete description of how it works in the Reference LIbrary in the Head Mistress forum--along with a bunch of other useful information. Also, make sure the new thru-hull and seacock is readily acessible so that it can be kept closed and easily be opened and then re-closed when the head is in use. Now would be a good time to re-route the holding tank vent too...Catalina puts 'em in rail stanchions, which just about guarantees ungodly odor every time the head is flushed--if not inside the boat, out the vent.
