<P> The U.S. Navy's system of alpha - numeric ship designators, and its associated hull numbers, have been for several decades a unique method of categorizing ships of all types: combatants, auxiliaries and district craft . Though considerably changed in detail and expanded over the years, this system remains essentially the same as when formally implemented in 1920 . It is a very useful tool for organizing and keeping track of naval vessels, and also provides the basis for the identification numbers painted on the bows (and frequently the sterns) of most U.S. Navy ships . </P> <P> The ship designator and hull number system's roots extend back to the late 1880s, when ship type serial numbers were assigned to most of the new - construction warships of the emerging "Steel Navy". During the course of the next thirty years, these same numbers were combined with filing codes used by the Navy's clerks to create an informal version of the system that was put in place in 1920 . Limited usage of ship numbers goes back even earlier, most notably to the "Jeffersonian Gunboats" of the early 1800s and the "Tinclad" river gunboats of the Civil War Mississippi Squadron . </P> <P> It is important to understand that hull number letter prefixes are not acronyms, and should not be carelessly treated as abbreviations of ship type classifications . Thus, "DD" does not stand for anything more than "Destroyer". "SS" simply means "Submarine". And "FF" is the post-1975 type code for "Frigate ." </P> <P> The hull classification codes for ships in active duty in the United States Navy are governed under Secretary of the Navy Instruction 5030.8 B (SECNAVINST 5030.8 B). </P>

What does dd stand for in the navy