Drug cartels rely on trucks, small planes, ships and speed boats to smuggle narcotics into the United States. And in recent years, they’re building increasingly sophisticated submarines, some of which can travel submerged.  These drug-running subs are proving to be an issue for the Navy and the Coast Guard, a top official acknowledged Tuesday. “It’s a very expensive proposition to try and find them, follow them, detect them as they work through the maritime environment,” Gen. Douglas Fraser, the Air Force general who heads U.S. Southern Command, said at a Senate hearing when asked about the subject. In July, Coast…