This is when an existing design is extensively modified or an existing design is altered from its original purpose to some other purpose. Examples of this are when a troller, trawler, or gillnetter is altered from a fishing vessel to a yacht or a small freight boat to a yacht. This requires a new superstructure, relocation of the engine and tanks, bulkheads, deck framing and interior arrangement. The only parts of the original design that do not change are the lines and offsets and part of the construction plan. The preparation of these, when feasible, is about 30% to 50% less than a custom design since most of the calculations and many of the drawings can be used without alterations.