A 40-foot boat caught fire in Admiral Hill Marina in Chelsea Wednesday after an explosion in the engine blew some floorboards and a section of the deck off, officials said. The Chelsea Fire Department responded to the scene at about 3:30 p.m., said Deputy Fire Chief John Quatieri. The boat was docked in the marina when it caught fire. There were two employees on the boat at the time of the incident, but no injuries were reported, Quatieri said. It was not immediately clear how the fire started in the engine, he added.