American Ship Hull numbers 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 . Civilian use . 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 submarine s . 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 warship s 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 destroyer s 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 destroyer s 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 frigate s 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 . Hull numbers have been used to identify armored tank s for the U . S . Army and the U . S . Marine Corps , and other military services , also .