If a hull is extra heavy, it's likely that the styrofoam flotation blocks have become waterlogged. I've had it happen to me and heard from another guy who it happened to. The only cure is to install an inspection port and let the hull bake in the sun for weeks. The other guy I mentioned put in two inspection ports and dug out all the foam. I think he said that he took around 40 lbs from each hull.
You don't think of styrofoam as being able to hold water, but I've seen more than one covered styrofoam lid from a hot tub that weighed over 100 lbs.

Jack