Sometimes even a maintenance facility needs maintenance, and after a four-month overhaul, the Neponset Rivers’s floating drydock will be ready to start working on vessels next month. The drydock, named Dynamic, is 76 years old — the oldest active vessel in the Navy’s fleet after the sailing frigate USS Constitution which is on display a few miles away on Massachusetts North Shore. But Dynamic is a lot busier than Old Ironsides. “Dynamic has been operating a long time and remains a crucial asset to our sailors. We look forward to having her operational again,” said Dynamic’s commanding officer, Lt. Gaston Hatfield. “We have several landing craft units that need emergent repairs,” Hatfield said. “We need Dynamic to get them back up and running so they can perform their mission.” It handles work on Landing Craft Utility boats, patrol craft, Mine Countermeasure minesweepers, the smaller Minehunter Coastal ships designed to work in shallow waters close to sho...