A review on Simplified Modelling of Ship Manoeuvrability

International rules and recommendations increasingly demand for the assessment of manoeuvrability performances of a ship early at its design stage. Safety, of people, environment and goods are (obvious) reasons for the above, requirement. This calls for prediction tools which, in principle, should not, be based on experimental activity on ship models. Unfortunately, controllability does not depend on hydrodynamic characteristics of ship hull, system of propulsion and rudder separately, Rather, ship response to steering inputs is determined by the interacting behaviours of these three fundamental "ingredients". On this ground, free-running model tests are presently the most "robust" prediction technique but are also too expensive and, moreover, not available until the ship is completely defined. For surface ships, we are interested in unsteady simulation of non-symmetric motions with large amplitude, mainly in the horizontal plane. It can be expected that the corresponding flow field will be characterize by large vortical regions around and past the hull. Hydrodynamic loads will be dominated by such rotational effects, which are viscous in nature. It can be actually shown that force and moment acting on a bare hull can be exactly, expressed in terms of quantities depending on the vorticity field. A simple qualitative observation of vortex pattern past a turning ship is useful to put into evidence possible, difficulties in the simulation of manoeuvring motion. On the this ground, the numerical simulation of maneuvering ships requires the coupled solution of the equations of motion and the unsteady Navier-Stohes equations for flows bounded by a wavy free surface and moving solid surfaces. Even if simplified modeling of rudder actions can be introduced, the computational effort, for a surface ship would overcome costs and time requested by a suitable experimental investigation on maneuvering performances. At the end of the day, numerical fluid dynamics and computer resources are not mature for solving such complete problem with a high degree of confidence and reasonable cheap computational effort. Increasingly faster progress in this direction is continuously done and in next, decades the, "fluid-dynamic" simulation of ship maneuvers will be hopefully available. In the meanwhile, fluid dynamic models of variable complexity have been developed for solving some sub-problems, such as flow around bare hulls, propeller flows, rudder flows. In the following, we limit our discussion to the problem of simplified modeling of the hydrodynamic loads, and how to complement this effort by experimental tests and theoretical analysis.