An experimental and numerical study of the free roll decay motion for a vessel of the Italian Navy is presented. Experimentally both sea trials and model scale tests have been carried out; numerical analysis has been performed by means of unsteady RANS simulations. The focus is on the analysis of the roll motion coefficients (damping and period of oscillations) at different Froude and Reynolds numbers. To this aim, experiments and numerical simulations were carried out at different Froude and Reynolds numbers. Sea trials were carried out on the Italian Navy ship Comandante Bettica in the Mediterranean Sea, close to the coast of Sicily; models experiments were performed in the towing tank at INSEAN. Computations were carried out by means of an inhouse unsteady RANS solver.
The analysis of the roll motion of a ship is of practical interest for both safety and comfort reasons. In this paper an experimental and numerical analysis of the roll decay for patrol boat of the Italian Navy is carried out. Full scale trials in the Mediterranean sea in cooperation with NSWCCD (Naval Surface Warfare Center, Carderock Division) and model scale experiments at the INSEAN towing tank have been performed. For a proper comparison, hull in fully appended configuration, (i.e. with the rudders, bilge keels, fins, and propeller apparatus, including struts, A-brackets and the propeller shaft) has been considered. To properly understand the effect of the rotating propeller on the roll damping, model scale experiments have been performed with and without the rotating propeller. Several Froude numbers have been considered, both in full and model scale, to highlight the effect of the ship speed on the roll damping. Numerical simulations have been carried out for three different Froude numbers; the steady flow around the vessel with a fixed heel angle, and the unsteady free roll decay of the vessel from an initial of heel angle of 10 degrees are computed.