Aerodynamic models are typically used to model tidal turbines but the differences between wind and tidal turbines are such that these models may not accurately represent the performance of a tidal turbine. Aerodynamic models assume free flow whereas in a tidal channel the channel boundaries are likely to have an effect on turbine performance. Models for flows with three different boundary conditions have been developed in order to demonstrate how significant the effect of the boundaries is. The first model is the classic aerodynamic model, the second considers flow in which all the boundaries are rigid and the third considers a channel where the free surface can deform. The effect of channel area to turbine area on performance has been investigated using these models. The results from the models indicate that the boundary conditions assumed in the model have a significant effect on the predicted performance of the turbine. The difference between the model results increases as the tip speed ratio increases and as the ratio of channel area to turbine area is reduced. The model results suggest that for realistically sized devices in actual tidal channels, aerodynamic models will not provide an accurate prediction of the performance.