The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rudderstock
    n 1: a vertical post at the forward edge of a rudder that
         enables the rudder to pivot [syn: rudderpost,
         rudderstock]