The development of a numeric simulation for predicting ocean current turbine performance is presented in this paper along with performance predictions. This numeric model uses an unsteady blade element momentum (BEM) rotor model to calculate the rotor forces and seven degree-of-freedom (DOF) equations of motion to calculate the coupled effects between the rotor and the main body. For the results presented in this paper, this simulation is set to model a 20-kW experimental ocean current turbine, and performance predictions are made for environmental condition that it will likely operate when deployed in the Gulf Stream off Southeast Florida. This model predicts that this turbine will have a maximum rotor power coefficient of 0.45 and that the vertical current gradient will only minimally affect the system performance. This simulation is also used to quantify the cyclic loadings that will be induced for misalignments between the rotor axis and the incoming flow, and it predicts the system motions and the forces on the rotor when the system is operating in a wave field.