The Collaborative International Dictionary of English v.0.48:
Flatboat \Flat"boat`\, n. A boat with a flat bottom and square ends; -- used for the transportation of bulky freight, especially in shallow waters. [1913 Webster] flat-bottomWordNet (r) 3.0 (2006):
flatboat n 1: a flatbottom boat for carrying heavy loads (especially on canals) [syn: barge, flatboat, hoy, lighter]