Experimental and Numerical Analysis of the Roll Decay Motion for a Patrol Boat

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 a 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. Numerical studies of the motion with six degrees of freedom of a ship are usually performed by means of liner potential theory; therefore, viscous related phenomena are intrinsically neglected, i.e. separations and vortical structures are in general not taken into account or modelled by means of zero thickness vortex layers shed from prescribed separation lines (usually coincident with geometrical singularity). Methods based on this theory give a satisfactory prediction of vertical motions, i.e. surge, heave and pitch, and, depending on the geometry of the body, of sway and yaw motions. In any case small amplitude motions have to be considered. On the contrary, such techniques fail when applied to the analysis of the roll motion. In this case the hydrodynamics is highly non linear, because viscous effects, flow separation and vortex shedding phenomena as well as lift damping contribution, are important. In this case methods based on the unsteady Reynolds Averaged Navier Stokes equations (URANSE), can contribute to improve the prediction of the roll motion of a ship.