An experimental study of a 1/20th scale undulating membrane tidal energy converter (UMTEC) based on EEL Energy’s device is presented to assess the impact of the membrane's aspect ratio on its behaviour. The aim of this work is to study 3D effects on the machine to optimise the energy extraction. Five aspect ratios from 0.26 to 0.57 are tested at constant length, keeping other geometric parameters constant too. The mechanical energy of the membrane is conveyed above the free surface by a vertical cogwheel/chain assembly to be converted by an electric generator. After describing the experimental set-up, results are presented according to the reduced flow speed. Undulation dynamics and power conversion are analysed. This study highlights the effects of an undulation mode change on the undulation amplitude and frequency, independently from the aspect ratio. In addition, it shows that the UMTEC performances increase with the aspect ratio up to 0.5. Over this aspect ratio value, the efficiency of the device seems to stagnate at its highest value, suggesting that 3D effects become negligible.