This study investigates the modifications of the turbulent boundary layer that develops on the suction side of a NACA0015 hydrofoil when a stable partial cavity takes place near the leading edge of the foil. The velocity field measured in non cavitating conditions has been compared with its equivalent in cavitating conditions. A particular focus has been put on the evolution of the logarithmic law of the velocity profile and on the modification of the global parameters that can precise both the position of the laminar-turbulent transition and the detachment of the boundary layer. The wall friction has been estimated both by use of a numerical procedure and by treatment of the experimental results. This comparison is encouraging and gives confidence in the proposed methodology. The results have shown that the vapour phase modify the boundary layer thickness and enhance the exchanges with the external layer that lead to an increase of the velocity close to the wall. This phenomenon induces a stabilisation of the boundary layer and delays its separation.