The overall objective of this numerical investigation consists of predicting and optimizing the energy-generation performance of an in-stream turbine undergoing preliminary design. This turbine is unconventional as its energy extraction surfaces are composed of twin, counter-rotating, unconfined screws aligned at an oblique angle with respect to the water current. First, a cost-effective computational fluid dynamics (CFD) model of the water flow in the vicinity of the turbine was implemented in a commercial package. This CFD model was benchmarked by comparing predicted power coefficient values with available, empirically-obtained, data provided by the client. During this benchmarking exercise, it was determined that a rotating (sliding) mesh approach was necessary to obtain accurate predictions. The CFD model was then used to predict power coefficient values over a range of tip-speed ratios at the design free-stream water velocity for the proposed (initial) screw design. Lastly, simulations were performed to investigate the performance of additional screw designs in the hope of optimizing the energygeneration performance while respecting the provided design constraints.