Source : Webster's Revised Unabridged Dictionary (1913)

Flatboat \Flat"boat`\, n.
   A boat with a flat bottom and square ends; -- used for the
   transportation of bulky freight, especially in shallow
   waters.

Source : WordNet®

flatboat
     n : a flatbottom boat for carrying heavy loads (especially on
         canals) [syn: {barge}, {hoy}, {lighter}]