Originally Posted by JohnRPollard
This topic comes up occasionally, and at first it seems like such an obvious solution that everyone begins to wonder why all builders don't set-up their systems like this at the factory. But they don't typically do it this way, probably for a variety of reasons.
One issue is that sink drain seacocks/thru-hulls are usually much larger than the head/engine intake sea-cocks. So there could be a concern about constricting the flow, especially if any debris goes down the drain. If that drain/intake gets clogged by sink debris, it could take the toilet partially out of commission.
Also, drain thru-hulls usually are not as far below the waterline as intake thru-hulls. If your head intake thru-hull is a foot or two below the waterline, you could easily end-up with a lot of "standing water" in your sink drain. This may lead to as much or more stink than you are trying to cure by giving the head a fresh water rinse.
One more concerning is healing. If your head intake/sink drain is closer to the centerline of the boat, and the sink is outboard, the sink can easily end-up below the waterline, or even below the intake/drain when the boat is healed. So if you open the thru-hull to flush the toilet, you had better have a way to prevent the water from gravity draining up, into, and out of the sink.
So think it through a bit before taking the plunge. We've had good results with simply adding water to the head (we have the same shower arrangement as Christyleigh, but a cup of water from the sink works almost as well).
Good thoughts to consider, John. In my situation, my head intake is closer to center line than the sink drain seacock. The head intake is also underneath the vberth -- while access is not difficult, it is not convenient (need to move a vberth cushion and raise a panel. My plan is to not even open the head seacock (unless fresh water stores are low), so the chance of any backflow is unlikely or impossible. Fair warning regarding sink debris, but the head sink is used pretty much only for hand/face washing.
Unless I'm missing something, I think the concerns you raise are more applicable to those who use the same thru-hull for sink drain and head intake.