SWATH vertical motions with emphasis on fixed fins control
The SWATH ship has been claimed as one of the advanced high performance vessels which can provide good seakeeping characteristics as well as maintaining high speed in rough seas. Despite the considerable amount of research and development carried out in the last fifteen years, there is still a lack of design data in the open literature concerning many of the specialised aspects of SWATH design. Two of these areas are the motion characteristics of hulls which are operating fairly close to the water surface and the design of active control systems to reduce static trim and motions in waves. This study is an investigation, both theoretically and experimentally using a model, into SWATH motion characteristics in the vertical plane. It aims to have an understanding of the seakeeping behaviour with and without the effect of fins in waves. The computer program for the motion prediction involves the computation of the hydrodynamic coefficients of the equations of motion on the practical range of frequencies, depth of submergences and column widths. The effects of these factors on the sectional hydrodynamic coefficients are discussed and are curve-fitted into approximate formulae in order to save computer time. The total (three dimensional) hydrodynamic coefficients are integrated stripwise, taking into account the forward speed and viscous effects. Analytic methods for the wave induced exciting forces were formulated and obtained by two approaches; the modified Morison's formula and the strip theory. The sectional Froude-Krylov force, caused by the undisturbed incident wave pressure and a diffraction component resulting from the distortion of the wave train by the presence of the hull integrated over the mean immersed surface of the hull section. Phase differences of the sectional forces are considered during the integration procedure. The forward speed and viscous effects are included together. In addition, a series of laboratory tests in calm water and waves as well as theoretical studies aimed at the design of vertical-plane control surface (fins), which would keep the SWATH ship on a near level trim at speed in calm water and reducing the inherently low level of motion in wave have been carried out. The forces generated by fins are composed of inertia effects and viscous induced lift and cross-flow drag. Since the fins are attached to the hull, the lift-curve slope were corrected by the fin-body effect. Only after fins are considered in the study, the combination of the forward fins are believed could be summed linearly. However, the downwash effect on the after fins by the forward fins are not able to be included. Since the exciting and restoring forces of a SWATH involved are smaller than those of the comparable monohull, adequate control forces can be generated for a SWATH at speed by reasonably sized fins. The good agreement of the comparisons of the analytical calculations and the experimental measurements confirms the accuracy of the study.