[jergofish]

Molarboater, if it was my boat I'd forget about the vented loop and go the second route, a new through-hull above the water line. As you said, you can use current fitting for a raw water washdown.

The centrifugal impellor pumps are good in that they can pump vast quantities of water but have a failing that they cannot pump too much of a head or in other words, can't lift the water very high. A little research and you can find a graph on what the loss is per foot of lift. However, if your exit through-hull is only six inches above the water line and has a loop that extends up another foot, the looped area does not count as lift once the pump has overcome the intitial climb since a syphoning action occurs. The only problem is once the pump shuts down, all the water between the pump and the top of the loop will probably drain back into the bilge.

As far as backflow on a pump, impellor pumps do not prevent back flow but you could put a check valve in the line that would prevent backflow. I don't recommend it since it's one more place where a blockage could occur and shut your system down.