The hull number is actually the set of numbers in the middle of the "HIN" or Hull Idnetification Number. If I recall correctly, for a J/30 it begins TPY030 to denote Tillotson Pearson Yachts and a J/30. Then the next set of numbers is the hull number, then finally it's the month and year of construction. In the beginning of the run the month and year were done in numbers, like "0980", then later they changed to a letter code for the month. That all adds to up a HIN for Mondial which is TPY0302150980.