An encouraging project measuring the propeller inflow was done in 2014  by a group of researchers (Andre Kleinwächter, Katrin Hellwig-Rieck, Dr.-Ing. Eric Ebert, Robert Kostbade, Hans-Jürgen Heinke and Nils Damaschke) from the Universität Rostock and the Schiffbau-Versuchsanstalt Potsdam (Potsdam Model Basin SVA) using Particle Image Velocimetry (PIV).
PIV is a technique that uses a strong light source (typically a laser) to illuminate particles contained in a 2D plane of the flow. The reflected light is captured by a camera, after which the particle images are analysed to determine the velocity field in the part of the flow illuminated by the laser.
The Rostock Uni/SVA group made a series of portholes in the aft end of a Ro-Ro vessel and operated a PIV system through the portholes. This was likely the first successful PIV measurement at the ship scale, however, there were a few limitations that made this measurement a bit less than ideal. For example, the limited view from the windows did not allow making a large sector scan; the measurements needed to be made in small areas that were first averaged (producing one measurement point) and then combined to form a larger flow field. Furthermore, the measurements were not done during well-controlled sea trials (i.e. with reciprocal runs and careful measurement of all the relevant environmental conditions), making it difficult to use the results in CFD validations. Nevertheless, these measurements showed that PIV at full scale is possible which motivated us to develop our own full-scale PIV system.  

To be continued...