This study investigated how power and efficiency in Archimedes screw turbines (AST) is impacted by varying parameters including; length, inner and outer diameter, pitch and number of flights. An index was created that determined how important some variables were over others by testing a number of laboratory sized screws and comparing the resulting power and efficiencies. A MATLAB AST performance model was implemented that included simplified power losses from internal hydraulic friction and outlet submersion to predict experimental results. New experimental techniques were developed and used to investigate fill height and overflow leakage in ASTs. The experiments showed screw rotation speed had an important effect on overflow, however current models are quasi-static and do not include rotation. A revised equation was developed to predict overflow in operating screws that includes the effect of screw rotation speed, and was shown to more accurately predict overflow in laboratory-scale screws.