Brian:
36' Chris - you must be referring to a "garboard" drain. If so, these are typically bronze, not brass or rubber although there are some newer ones of nylon. 1/2". Threaded fitting is from inside of the hull. They are not expensive or hard to replace, however, I suggest that you eliminate all other possibilities before you do. Is the bilge area with the drain compartmentalized or open all the way to the transom? If the area under the cockpit has limber holes to the engine compartment, it might be rain water. If you are able to dry the bilge and water reappears without running the engines, then you can eliminate a raw water system leak. You have eliminated drive and rudder shaft logs-what about through hulls? I had a sink drain through-hull that was cracked and leaked water into the cabinet, took forever to find it. Possible shower sump leak or any hose. Domestic water line leak between tank and pump? Try putting colored food dye around any through hulls or suspected areas and check latter for any signs of water running.
Gene