An experimental study of the performance and optimisation of a prototype novel drag-driven vertical axis tidal stream turbine is presented. The drag turbine has several unique advantages, including simple blade design, deployable in shallow waters and potential denser array spacing. Performance optimisation was conducted in the hydraulics flume at Cardiff University (CU), where the turbine reached C(p)max/lambda = 0.132/0.441 for its 90 degrees phase angle configuration. The CU turbine was then tested using the wider and deeper hydraulics flume at IFREMER, France. Testing at IFREMER reduced the blockage factor from 17% at CU down to 1%; into the range of unblocked conditions. Testing in an unblocked environment, under similar flow conditions, reduced the peak efficiency of the CU turbine by 43% to C(p)max/lambda = 0.067/0.346. Finally the CU turbine was compared to the performance of a Savonius turbine. The design of the Savonius was based on a literature review. The CU turbine showed inferior efficiency values compared to the performance of the Savonius. The Savonius reached C(p)max/lambda = 0.098/0.962 in unblocked conditions, 46% greater than C(p)max of the drag turbine. (C) 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).