Source : Webster's Revised Unabridged Dictionary (1913)

Rudderstock \Rud"der*stock`\, n. (Naut.)
   The main part or blade of the rudder, which is connected by
   hinges, or the like, with the sternpost of a vessel.

Source : WordNet®

rudderstock
     n : a vertical post at the forward edge of a rudder that enables
         the rudder to pivot [syn: {rudderpost}]