In response to the two major energy challenges of climate change and security of supply, there is increased interest in the extraction of energy from renewable sources. Marine renewables, including tidal stream power show great potential, however the economics of these technologies is currently less favourable than land based renewable energy technologies due to the challenge of working in the marine environment. A tidal stream turbine is being developed which will allow the extraction of energy from sites at which the flow speed is slower than those currently considered to be suitable for development, allowing the economic extraction of energy from a greater proportion of the tidal stream resource. The proposed system uses the boundary layer control techniques commonly found in the high-lift systems of aircraft to increase the lift force acting on the turbine blades in slow moving currents. In order to aid the design of the system an analytical model of the turbine is being developed. Previously, vertical axis tidal turbines have been designed with the aid of analytical models developed for the Darrieus wind turbine. A variety of models have been proposed for the Darrieus turbine, most of which are capable of giving good predictions of time averaged quantities, such as the power coefficient. Even the most advanced of these models, however, cannot predict with a high level of accuracy the instantaneous forces acting on the turbine blades. In addition, aerodynamic models may not be applicable to tidal turbines due to the significant differences between wind and tidal turbines, such as the presence of a free surface.