The 2013 Great Lakes shipping season began on March 2 with the sailing of the tug/barge unit Prentiss Brown / St. Marys Conquest . The vessel, operated by Port City Marine Services, departed its winter lay-up berth in Milwaukee and sailed for Charlevoix, Mich., where it loaded 9,200 tons of cement for delivery to Chicago, according to the Lake Carriers’ Association. Next to get underway was the tug/barge unit Dorothy Ann / Pathfinder . The vessel, one of 10 operated by...