I'd tee it in high. I've seen demonstrations on home remodeling shows on TV where they show how you need air behind water for a drain. But I'd think you'd want to get the bubble out close to the drain, not down by the seacock.

All of this is theory on my part; I hate plumbing with a passion.

We had a camper trailer once and the sink would not drain if you put a hose on the outlet. If you took the hose off and just let it run out the fitting on the side of the trailer it would empty right away. But hardly a drop would come out of the hose. I think it must be a similar thing as Gary describes????

Are you sure your drain line is clear? Particularly if you leave the valve open, you might have stuff growing up in the line a bit. I have a small plumbers helper which I need to plunge on one side now and then when draining seems to slow down. It works if you keep your palm over the other sink drain.

Here's a pic I found on the web that illustrates the vent concept. In the house it's basically the stack that goes through the roof.

I use a small plunger, put the drain plug in one of the sinks. Then put the plunger over the drain hole in the other sink and partially fill with hot water. When ready start plunging vigorously. Then repeat on the other side. Lastly, use the foot pump on the fridge to flush that line. Sometime I use a little pinesol just to freshen up the drain. Do not use anything caustic.
John Meyer
Hilbre

With my history, I have run into slow draining due to an air block a few times when I come back from sailing. Using the Refer Foot Pump a few cycles has cleared the air block.

What I have done for maintenance on the sink drain lines. (to each their own) I have used [U]a very small amount[/U] (1/2 of a cap-full of a small bottle) of bleach over night.

Just before bed - close the thru-hull, add very small amount of bleach in each drain, fill both sides with hot water just up to the sink drains, and let it soak overnight. In the morning open the thru-hull and flush the lines with fresh water.

Just my thoughts...

Gary, I think that the purpose of the vent in a house is to allow sewer gases to escape, not to enhance the flow of the sewage...but then, I am only an amateur plumber. In any event, I doubt that such a vent would help drainage of the C36 sink drain. In our boats, the drain is only about two feet long. My theory: The drain hose is of a small diameter, so food particles and other crud can easily add even more resistance to the flow.

Jack, your idea sounds like a good one. Whatever grows on our hull also grows in the sink drain hose; then add to that a mixture of food particles and other crud.

Larry, I saw a demonstration on "Holmes on Homes" where he built a little sink with drain and a vent/stack. He blocked the vent and the sink drained slowly and gurgled. He filled it up again, unblocked the vent and the sink drained in a quarter of the time. Convinced me. I believe you're correct that the stack also vents sewer gas, but it also functions to improve draining of liquids.

I'm certainly no plumber. My grandfather was a plumber, but he wouldn't even teach my Dad anything, told him you want to be anything BUT a plumber. I pretty much agree.