Large Eddy Simulations (LES) are performed on a Vertical Axis Hydrokinetic Turbine (VAHT) at two different solidities, so as to enable a more complete physical description of the flow than for classic statistical calculations. To analyze the turbine performances, local quantities are defined to evaluate the contribution of the different turbine elements to the global VAHT power coefficient. For a deeper analysis of the major losses, the real turbine is also compared with an ideal turbine composed of only three infinite blades. It is observed that the ideal turbine with the lower solidity provides the best performance, but the losses due to the blade tips and the arms strongly increase for the real turbine at the same solidity. Consequently, for the considered real turbine, there is no clear gain to decrease the solidity. Simulations of the ideal turbine are performed for various solidities at their optimal Tip Speed Ratio (TSR) to study the evolution of optimal power coefficient as a function of solidity. A maximum power coefficient is obtained for a small value of the solidity. This is explained because the optimal TSR of this optimal solidity leads to angles of incidences on the blade which avoid a penalizing dynamic stall phenomenon but are high enough to produce an important positive torque The design of an efficient turbine has then to limit losses, to be able to use small solidity and then to avoid dynamic stall phenomenon by having a high optimal TSR.