American Ship Hull numbers
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Hull number is a serial identification number given to a boat or ship. For the military, a lower number implies an older vessel. For civilian use, the HIN is used to trace the boat's history. The precise usage varies by country and type.
For civilian craft manufactured in the United States, the hull number is given to the vessel when it is built and forms part of the hull identification number, which uniquely identifies the vessel and must be permanently affixed to the hull in at least two places. A Hull Identification Number (HIN) is a unique set of 12 characters, similar to the Vehicle Identification Number which is found on automobiles. In 1972, The US Coast Guard was asked to create a standardized format for HIN's to allow for better tracking of accidents and history of boats. This HIN format is as follows: The first 3 characters consist of the MIC (Manufacturers Index Code) and should only be letters. The following 5 characters are the unique serial number assigned by the Manufacturer, and can be a series of letters and/or numbers with the exception of the letters O, I, and Q (they can be easily mistaken). The last 4 characters determine the model and certification year of the boat.  The HIN may be found on the aft of the vessel in the uppermost right corner. Also, the HIN may be stated on the title, registration, and insurance documents.
United States military
The United States Navy, United States Coast Guard, and United States National Oceanic and Atmospheric Administration employ hull numbers in conjunction with a hull classification symbol to uniquely identify vessels and to aid identification. A particular combination of hull classification and hull number is never reused and therefore provides a means to uniquely identify a particular ship. For example, there have been at least eight vessels named USS Enterprise, but CV-6 uniquely identifies the World War II aircraft carrier from all others. For convenience, the combined designation, which is painted on the sides of the hulls, is frequently called the "hull number".
The U.S. Navy sometimes ignores the sequence of hull numbering. For example, the Navy built the last Los Angeles-class nuclear submarine as SSN-773. Next the Navy built the three Seawolf class submarines SSN-21 through SSN-23. Then the Navy later resumed the original sequence of hull numbers with the USS Virginia SSN-774 for its next class of nuclear attack submarines.
This change in numbering was done because the Seawolf class was to have a radical new and large design for the continuation of the Cold War into the 21st Century, but cost overruns combined with the end of the Cold War, and the resulting reduction of the Navy's construction budget resulted in only three of these boats being constructed: the USS Seawolf (SSN-21), the USS Connecticut (SSN-22), and USS Jimmy Carter (SSN-23).
Also, whenever warships are constructed in American shipyards for foreign navies, any hull numbers used to identify the ships during their construction are never reused by the U.S. Navy. For example the Perth class guided missile destroyers that were built for the Royal Australian Navy in Bay City, Michigan were given the hull numbers DDG-25, DDG-26, and DDG-27; but these hull numbers were not assigned to any American destroyers after the Australian Navy had changed those to its own identification numbers.
Several other new warships have been constructed in American shipyards for countries such as West Germany and Taiwan. Guided-missile frigates were constructed in Portugal under military-assistance aid packages were given the hull umbers DEG 7 through 11.
When a naval vessel is modified for use as a different type of ship, it is often assigned a new hull number along with its new classification. Often the actual number remains the same while the hull classification changes. For example, a heavy cruiser (CA) that was converted into a guided missile cruiser became a CG and its number was changed. This happened with the USS Albany (CA-123), the USS Chicago (CA-136), and the USS Columbus (CA-74), which became, respectively, the CG-10, the CG-11, and the CG-12.
Also, during World War II, nine Cleveland-class light cruisers (CL) were converted to light aircraft carriers (CVL), with different numbers.
During the 1970s, the guided missile frigates that were then redesignated as guided missile cruisers had their designations changed from "DLG" to "CG"; in this case, they kept their previous numbers. Some other guided-missile frigates were redesignated as guided-missile destroyers (DDG) and given new numbers.