The consortium responsible for the next stage of development of the OWEL wave energy converter will construct and test a large scale, pre-commercial demonstrator. It is expected that this will be installed at Wave Hub during 2013 and grid connected for a testing period lasting around 12 months. This paper reports on the preliminary design work being undertaken in the development of the marine demonstration device. This concentrates primarily on producing a fully costed design by detailing the hydraulic design and aspects of stability as well as providing insight into various design features such as the power take-off, naval architecture, moorings and control. The design is being largely informed by the results of a 15 month research project funded by the South West Regional Development Agency (SWRDA) in which the performance was determined and a detailed techno-economic model for a large scale OWEL device was generated.