Presently, there is no well-established methodology for predicting a-priori the power production of a proposed tidal turbine farm. Such a method must account for the spatial variability of velocity due to bathymetry and turbine wakes. This paper presents a simulation method designed to predict reduced turbine power output in the wake of an upstream turbine, which can be applied to large tidal farms containing many turbines. The method uses Reynolds Averaged Navier-Stokes simulations to predict the flowfield throughout the farm, and was validated using published experiments of a 0.7m diameter rotor. Experimental data collected with a single rotor were first used to develop a rotor performance model. Then data from tandem-rotor experiments were used to validate the ability of the simulations to predict the performance of the downstream turbine, located directly in the wake of the first turbine. Obtaining an accurate match to the experimental power required accurate prediction of the wake downstream of the first rotor. The method predicted the downstream rotor power quite well, with some discrepancies from the experiments. The proposed method is computationally efficient and can be applied to large tidal farms with many turbines.