A study of the hydrodynamics of ACV hulls with particular emphasis on plough-in

Since the major calm-water capsizes occurred in 1965/66, much experimental work has been done to establish better operational margins of safety. The general approach has been to establish well defined limits of manoeuvrability based upon available model and full-scale data. Mathematical modelling of the ACV motion was used as a secondary approach because the expressions involved are high in non-linearities involving aerodynamic and hydrodynamic force terms of similar orders of magnitude. In this study, a numerical technique for the solution of ACV non-linear equations is proposed and a two degree-of- freedom model is built up using the digital simulation language, SLAM. The simulation involved the use uf the technique of storing values of the various non-linear functions over a defined regime and then using these to provide updated inputs as the craft changed its state. An ACV overturn sequence is studied by developing, simulating and testing of equations describing the roll and sideslip motion of the craft. In particular, the equations take into account stiffness and damping forces associated with both the hard structure and the craft cushion system; inertial coupling effects due to craft deceleration are also incorporated; induced trim effects due to the position of the cushion wave system below the craft is modelled and suitable phase lag is employed depending upon the deceleration of the craft. A basic configuration craft is chosen based upon critical design parameters such as hull depth, skirt depth, VCG height and hull angle of inclination. Extensive numerical testing of this configuration is carried out involving a systematic method of variation of the critical design parameters. Results indicate that it is good philosophy to design ACV hulls with planing capability applied to all faces of the hard structure. The results also allow a set of ranges to be established for the critical design parameters, which, if adhered to will minimise the possibility of capsize for a craft configuration of the type chosen for the study.