Local scouring induced by a rotating ship propeller

The objective of the present work is the study of the temporal development of the scour hole and of the consequent deposit mound induced by a rotating ship propeller on a mobile bed. Towards this end, an experimental campaign was conducted in the Laboratorio “Grandi Modelli Idraulici” of the Università della Calabria (Italy), using a scaled physical model of a propeller. The temporal development of the scour profile along the centreline was measured during each experimental test, revealing the presence of a small scour hole directly beneath the propeller and a main scour hole that expands downstream of it. At the same time, the scouring phenomenon causes the creation of a sediment mound that moves downwards. The formation of such characteristic regions was monitored in order to evaluate their dependency on the submergence depth of the propeller and also on the water depth. The results show that the temporal development of the overall maximum scour depth for all the investigated cases exhibits an exponential form. The temporal development of the overall maximum deposit height follows the same trend. A comparison between the experimental tests revealed that increasing the water depth for the same propeller submergence depth causes a longer and deeper scour hole. Basically, also the deposition mound tends to be higher. Instead, by keeping constant the water depth and reducing the propeller submergence depth a different trend is observed: the scour hole becomes deeper and shorter, while the deposition mound tends to be higher. The study also demonstrated that the normalized scour profiles generally fall inside a narrow band and, therefore, the scour holes can be considered affine in time. This affinity is preserved also for the deposition mound in the last phase of the scouring phenomenon.