This paper presents the results of testing a horizontal axis tidal turbine in a towing tank. The Queen’s University Belfast turbine has 1.5m diameter and was installed in the CNRINSEAN towing tank which is 13.5m x 6.5m x 460m. With these towing tests it was possible to assess the performance of a tidal turbine under steady idealised flow conditions with chord Reynolds numbers of 49–85 × 104 . The effect of submergence on power performance and electrical output was assessed. Three different depths of centreline submergence were investigated: 0.5, 1 and 1.17 rotor diameters (D). The power coefficient at different tip speed ratios was assessed for these scenarios At 0.5D submergence (surface piercing) the electrical power coefficient was reduced by 4% and maximum power output reduced by 9%, compared to 1D. Wave generation at the surface from the passing blade tips as well as bubble entrainment was observed during these experiments. At the larger submergence, 1.17D, a reduction in electrical power coefficient and maximum power output was also seen with 2% and 3% respectively. The rotor was also tested upstream and downstream of the support strut. For the configuration, where the rotor was upstream of the strut, a small but consistence decrease in electrical power performance and maximum power output was observed.