Place the drain well below the water line. As long as the sink is above the WL it will drain, but if the drain is too high, water gets pushed back up as the boat rolls and you are using the sink (seacock open). The drawback is that you'll have more "sitting" water in the pipe.

 

It is much better to locate it on the waterline. "Sitting water" is generally debris and block the drain if it is below the waterline. If you drain to the waterline everything will flow outside. As long as the sink is close to the center, the water will not push back even if the boat rolls or heels under sail.

 

I see merit in the waterline argument, any debris could impinge operation of the seacock, I would like the sink to drain instantly. More input from anyone out there, more the better!

 

Celenoglu has it backwards. Don't have your drain at the waterline if you want it to drain properly and clear any debris. If it's at the waterline, it's more likely that the discharge is horizontal which will lead to clogs and odors! You want I to drain as straight down as possible to keep the drain clear.

Also with the drain at the waterline, on one tack it will be well clear of the water and likely not drain at all!

 

GREETINGS EARTHLINGS : you can make a manifold so you do not have to put another hole in the boat make sure you can clean the out let and the intake from inside the boat while keeping all the water outside the boat. AS ALWAYS GO SAFE.

 

If the waterline is above the seacock, then the debris that float will be above the seacock, and the debris that sink will pass out through the bottom. Should have a really big seacock, like 1.5 inches.

 

Thru hull deeper the better. Less likely to be effected by heel.No elbows ,horizontals or tight bends in hose. Make sure hose is securely clamped up so that when the sink bib breaks it doesn't fall in the bilge . This fitting has to be above the WL for obvious reasons. Ask me how I learn this stuff.(toilet plunger may one day be a handy friend in the galley)

 

On my boat, both sink drains go straight down to a thru hull. There might be a reason for this.