I have a 16′ mud boat that has a double bottom its about two inches deep that has drain cocks to add water to what exactly is that for. should it be…. Looking for double hull boat plans ? here you can find the latest products in different kinds of double hull boat plans. we provide 19 for you about double hull boat. A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer.