that's great. more than i could do with it. rhino 5 doesn't like it much. the techniques i was thinking of don't work with the way it's setup. it looks like you are really close though. but like i said i would make all those cross-sections one curve instead of two. then i think your loft will work. the thick center portion you can make separate with an extrude. if you want you could join them together after. i'm not that great with cad either. so i understand. it can take a lot of trial and error.

i should add that if you make the thick portion from an extrude and want to join it to the hull, you'll want an overlap. you can trim the thick portion with the hull then join them. if they don't overlap then you will have problems trying to join stuff usually.