[Bushwacker]

If you're concerned about self-bailing at rest, Gillie's right, the scuppers out the back with the ping-pong ball check valves work well, although debris can keep the balls from sealing.

However, if you're worried about quickly removing a large volume of water from taking a wave over the bow in what may be a boat with low freeboard (maybe a flats boat?!), I think the suction in the vertical SeaCraft type scuppers with the wedge forward of the exit will beat gravity every time! The SeaCraft scuppers are about 1" ID; going larger would also help. And if you go with vertical scuppers, make sure you can get rubber plugs for them, because you need the plugs unless your deck is several inches above the waterline!