In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

An experimental investigation of the propeller wake in a cavitation tunnel has been performed using particle image velocimetry (PIV). A PIV image analysis algorithm implementing iterative window offset and stretching has been adopted. The main characteristics of the propeller wake at different loading conditions have been pointed out by using phase sampling techniques. Longitudinal evolution of the blade viscous wake, developing from the blade surface boundary layers, of the trailing vortex sheets, due to the radial gradient of the bound circulation, is discussed up to 1 diameter downstream. In the near field the effects of turbulent diffusion and viscous dissipation, which cause a rapid space broadening of the velocity gradients in the trailing edge wake, are also examined. In the far wake the development of the slipstream instability and the breakdown of the hub and tip vortices are outlined.