I haven't seen any obvious leaking on the bottoms of the hulls. I suspect its either the rudders, pylons, or the join between the deck and the sides of the hulls, because the boat did spend about a week and a half moored in the water, but did not fill with water. However, when I have 3 or 4 people, which is regular, it gets a lot, and the rudders are almost entirely submerged the entire time. When I've sailed solo it hasn't been bad.
I think the bottoms of the hulls are ok, I would think that I would have noticed by now if they were worn, but I could be wrong
It is indeed true that there is an aluminum plate that is drilled and tapped for the rudder gudgeons to attach to. I have to admit that I am not so sure about the drain plugs but one has to assume that it is so.
I have to ask though, if you are getting that much water in the hull, can you not see where it is draining out before you pull the drain plug? have you looked at the bottoms of the hulls to see if they are worn through or cracked?
hope this helped at least a little.