Scouring processes due to the navigation and manoeuvring of ships can generate stability problems to harbour’s structures. At the same time, the eroded material is settled in other areas, reducing the water level. Theoretical equations used are far from reality and only work for a single propeller. With the experiments performed at the Marine Engineering Laboratory (LIM/UPC) in LaBassA flume, new formulas are proposed to work with them to analyse the erosion effect in seabed provoked by twin propellers working at different revolutions under de scenario of unconfined conditions. The data quality is tested to examine if a new experimental system to analyse the scouring effects can be used in further experiments. A new formulation that relates erosion variables with the revolutions of the propellers system is added trough MATLAB self-made scripts.