from Ship Recognition Manual - The Federation, 2nd Edition, FASA

Notes - circa 2287:

     The Derf Class tender has been operational in Star Fleet for more than 25 years. When it entered service on Stardate 1/9807, the Derf Class marked a new concept in navigational beacon repair. Before its introduction, marker buoys and navigational beacons had to be retrieved and returned to a repair facility to be serviced. Derf Class tenders eliminated this need because they carried repair facilities on board.
     When a Derf arrives at a malfunctioning beacon's location, a shuttle uses a tractor beam on the beacon and tows it into the lower hull, which is the tender's main repair facility. The beacon is then placed on an assembly line and repaired robotically. When the work is finished, the shuttle tows the beacon back into the spacelanes, and the Derf moves on.
     Although the Derf is not designed as a fighting vessel, it is capable of aggressive defense. Most repair missions take place along the borders between the major powers, where the chances of encountering enemy ships is very high. Because of this high risk, the Derf is armed with medium-range phasers
     This protection does not prevent them from falling prey to enemy ships. On Stardate 2/0702, the USS Acropolis responded to signals from a malfunctioning marker buoy. As its shuttle neared the beacon, a Klingon warship appeared and opened fire before defensive action could be taken. The volley crippled the Acropolis' engines, and the tender was boarded and towed into Klingon territory.
     Intelligence later discovered that the beacon had been planted by Klingon operatives to entrap the repair tender. It is theorized that the Klingons gained technical information concerning robotics and repair techniques that they lacked, but it is not known just what gain this action brought them in the overall situation. Some analysts believe that study of the robotic repair systems will make it possible for Klingons to alter the functioning of navigation beacons robotically, creating potential havoc in border spacelanes.
     Of the 545 Derfs built, 16 Mk Is, 362 Mk Ills, and 68 Mk IVs remain in active service, and 38 Mk Is and 17 Mk Ills are in reserve fleets. Two Mk Is and 2 Mk Ills are used by Star Fleet Training Command; 12 Mk Is, 8 Mk Ills, and 1 Mk IV have been destroyed; 1 Mk III has been captured by the Klingons; 1 Mk I and 3 Mk Ills are listed as missing; 4 Mk Is, 4 Mk Ills, and 2 Mk IVs have been scrapped; and 2 Mk Is have been sold to the private sector.
     The Derf Class is built at Merak. The rate of production is 4 per year.