rudder (n.) Look up rudder at Dictionary.com
mid-15c. alteration of Middle English rother, from Old English roðor "paddle, oar," from Proto-Germanic *rothru- (cognates: Old Frisian roðer, Middle Low German roder, Middle Dutch roeder, Dutch roer, Old High German ruodar, German Ruder "oar"), from *ro- "steer" (see row (v.)) + suffix -þra, used to form neutral names of tools.

Meaning "broad, flat piece of wood attached to the stern of a boat and guided by a tiller for use in steering" is from c.1300. For shift of -th- to -d- compare burden (n.1), murder (n.); simultaneous but opposite to the movement that turned -d- to -th- in father (n.), etc.