[Illustration: Destroyer Escort (62 built) This smaller, more quickly built version of the destroyer was designed to protect allied shipping convoys from German U-boats, freeing destroyers for other duties.] New Careers for Old Ships During the long era of wooden sailing ships, when naval technology changed only gradually over the decades, a warship’s service lasted as long as the materials from which it was built. But as the pace of change quickened in the mid-19th century with the advent of steam propulsion and iron hulls, a vessel quickly grew obsolete without continual incorporation of the latest technology. This state of affairs, which intensified in the 20th century, provided Charlestown Navy Yard with a new role after World War II: lengthening or transforming the careers of old ships, otherwise destined for mothballs, through modernization and conversion. Modernization meant updating old electrical, propulsion, or weapons systems or performing structural surgery without altering the vessel’s function. This ranged from installing a sonar dome on the bottom of the hull to dismantling the entire superstructure and building a new one. The process normally took several months. Conversion, which could take years, involved major alteration of a vessel to prepare it for a different tactical mission. A typical example would be the conversion of a conventional scouting, escorting, and submarine-fighting destroyer to a radar picket destroyer, whose role was to provide mid-ocean radar warning. A notable postwar task undertaken by the yard was the 1956 conversion of the destroyer _Gyatt_ into the world’s first guided missile destroyer. Basically, the vessel’s aft five-inch guns were replaced with a twin missile launcher. But the ship had to be significantly altered to perform its new function. The yard designed automated systems that first affixed a booster charge to the missile and then moved it from the air-conditioned belowdecks magazine to the launcher. The decks and superstructure had to be reinforced to withstand the tremendous pressure and temperature of a launch. A system of ducts and blowout plates was installed to minimize damage and injury in the event of a premature explosion. Innovative retractable fins at midships helped stabilize the vessel for firing. With these and other changes, the yard remade _Gyatt_ into a sophisticated missile-firing machine. [Illustration: _In the late 1950s the Navy began installing sonar equipment in bow domes. Bow domes reduced hull resistance and were less susceptible to bubble noise. The Charlestown yard, already a leader in sonar technology, performed a prototype dome installation in 1958. To install a dome, workers first cut away part of the old bow, then fitted the prefabricated dome_ (next page, on U.S.S. _Willis A. Lee_ in 1961).]