Scouring effects induced by a ship propeller jet on a mobile bed.

The traditional marine propulsion system used to move a ship is constituted by an electric motor turning a propeller (sometimes two propellers), generally manufactured with more than three blades. Such a device produces a turbulent jet which impacts the seabed and banks of harbour basins or navigation channels up to a distance of several propeller diameters from them. Thus, the ship propeller jet can cause unwanted processes like scouring, downstream sediment deposition, drift of sediments, degradation of the structural stability of protection structures, as outlined by many authors (e.g., Abramowicz-Gerigk, 2010; Lam et al., 2011; Whitehouse, 1998; Yew et al., 2017). Such problems and damages become more significant, also in terms of repair costs, if continued mooring and unmooring of ships occur always at the same position (e.g., Hamill, 2015; Qurrain, 1994; Sumer and Fredsøe, 2002) and when the ships are loaded and the tide is at its lowest. However, the impacts on the bed depend on many factors, which may be different in most situations. For all these reasons, the investigation of the effects of the ship propeller jet have received continuous worldwide attention by civil, maritime/hydraulic and naval engineers, in order to understand the mechanism of bed scouring, to propose possible countermeasures and, therefore, to limit the operating costs for the necessary repair works. In particular, the aims of the proposed research derive from the limited experimental studies concerning a detailed investigation of the scouring effects induced by a ship propeller jet on a mobile bed, in terms of eroded and deposited volumes, maximum scour depth, maximum scour length and width and maximum deposit height.