Source : Webster's Revised Unabridged Dictionary (1913)

Fin keel \Fin keel\ (Naut.)
   A projection downward from the keel of a yacht, resembling in
   shape the fin of a fish, though often with a cigar-shaped
   bulb of lead at the bottom, and generally made of metal. Its
   use is to ballast the boat and also to enable her to sail
   close to the wind and to make the least possible leeway by
   offering great resistance to lateral motion through the
   water.

Source : WordNet®

fin keel
     n : a metal plate projecting from the keel of a shallow vessel
         to give it greater lateral stability