We present experimental and numerical simulation studies of a Horizontal Axis Hydrokinetic Turbine (HAHT) based on the DOE Reference Model 1. The performance and wake of a small array of three turbines was analyzed with measurements conducted on a 45:1 scale physical model of the DOE RM1 rotor. The influence of the Reynolds number dissimilarity between the laboratory model and the full scale is described. The details of the rotor flow field and wake evolution are analyzed from numerical solution of the RANS equations solved around a computational model of the full scale geometry. The influence of turbulent closure models is considered, as is the use of high resolution simulations in lower fidelity models for fast turn around, low computational cost, numerical design efforts.