Sink drains

[Axel]

I have a question for the huddled masses yearning to be free, or perhaps it's better addressed to those who have already built their boats and are free. Whichever. If you have what you think is the right answer, please share it.

In my layout, I am limiting through-hulls to a single salt water intake for the galley and head. I will have 3, perhaps 4 sinks. I intend to route them straight to holes in the hull. I prefer a straight shot in whatever configuration to limit clogging and make it easy to clear them when they occur. The 2 kitchen sinks will ideally empty through a common drain. Is it better to place the drain above or below the waterline?

In a T38 the space between the height of the bottom of the sink and the waterline is quite small so openings would have to be close the the waterline. If above, I would guess that passing waves could potentially drive green water up the drain and into the sinks with some force.  Does a standard plumbing 'P' trap prevent this or do i need locking plugs in the sinks? If so, where do you get locking plugs/stoppers or is there some cleverer solution I have not heard about?

 I am less attracted to the idea of doing the drain outlets below the waterline. I imaging gunk could sit long enough to create odors and so on if set up this way. Plus, the holes would be below the waterline. However I will do so if it is the most effective place for them. 

Oh, and if anyone has a recommended water tank breather valve that seals for liquid when upside down, please add that information. I have found them in big sizes for fuel cells and imagine they will work but if theres a purpose designed valve, please share.

Comments and advice please and thank you in advance,

Alex

[Björn]

The sinks on my T38 drain right in the water as well, even with quite some weather, where we got completely soaked in the cockpit, there came no water through the sink drains (however I had put some paper towels directly in the hole).
In summary, just continue as per plan and keep it simple.

[Axel]

Wow, Bingo! Both questions answered in 2 posts. Fantastic and thank you. Drains per original plan it is with the added security of the screw-in plugs.