Boats manufactured or imported on or after August 1, 1984, also have a duplicate secondary HIN affixed somewhere on an unexposed location inside the boat or beneath a fitting or item of hardware. The purpose is to help authorities identify your boat if a thief or vandals remove or damage the primary HIN on the transom. It is illegal for anyone (manufacturer, dealer, distributor, or owner) to alter or remove a HIN without the express written authorization of the Commandant, U.S. Coast Guard.
The regulations prescribe the format of the HIN. The first three characters are a MIC (Manufacturer Identification Code) assigned by the Coast Guard to the manufacturer or the person importing the boat; characters four through eight are a serial number assigned by the manufacturer; the last four characters indicate the month and year the boat was built, and the model year. Prior to August 1, 1984, the manufacturer had the option of expressing this in the form of a model year designation.
The Coast Guard maintains a searchable database of MICs.