The Collaborative International Dictionary of English v.0.48:

flat-bottom \flat-bottom\ flat-bottomed \flat-bottomed\adj.
   1. having a flat bottom. a flat-bottom boat
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

flat-bottom \flat"-bot`tom\ flat-bottomed \flat"-bot`tomed\, a.
   Having an even lower surface or bottom; as, a flat-bottomed
   boat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flat-bottomed
    adj 1: having a flat bottom; "a flat-bottomed boat" [syn: flat-
           bottomed, flat-bottom]