The depth of ships’ drafts, or the distance between the waterline and the deepest point of the boat, are now limited to 41 feet upriver of the port of Baton Rouge, the nation’s eighth-largest by tonnage, the U.S. Coast Guard said late Friday. That’s down from 45 feet, a level at which vessels are encountering problems as spreading drought in the U.S. midwest dries up the waterway, a critical freight artery.