hey mate. look, i cant fix the hull without rebuilding the entire thing and that would mean that you would have to start again, savvy? no, seriously, towards the bow the ship is perfect, but the stern should only be about 6 to 8 studs narrower than the mid-section. so, what i think you should do is cut it in half, delete the stern apart from the keel and the rudder, and just make it wider.
hope this helps
or you could just keep going with what you've got