This work presents an experimental study of the wake hydrodynamics of a three-blade contra-rotating propeller system (CRP) at advance ratio J = 0.7, 1 and 1.3. The study is carried out in a cavitation tunnel by employing a mixed hardware-software phase-locked acquisition and processing scheme. Flow measurements are thus phase-averaged based on both the forward and aft propeller angular position, so that the effect of their relative position on the flow features is investigated. Average and fluctuating velocity fields along with vorticity are presented and discussed. Strong interactions take place between the forward and aft propellers’ trailing vorticity with regular patterns of vortex reconnection and break-up which depend on the advance coefficient. Flow visualizations, carried out by inducing cavitation in the vortex cores, show that complex though organized patterns of helical coaxial vorticity occur within the wake.