The modeling and sea trials of the Unmanned Trimaran Vehicles (UTSV) are presented in this paper. For control design, six DOF dynamic modeling of autonomous Trimarans is presented. The detailed mathematical modeling is intended to provide the appropriate framework for a control design which explicitly addressed the six DOF UTV dynamics. A control system was developed based on backstepping to guide the vessel to track desired trajectories based on the input of the sensors. Physical experiments were conducted to determine the maneuvering property of UTV.