HYDRODYNAMIC PERFORMANCES OF SMALL SIZE SWATH CRAFT

The good seakeeping characteristics of SWATH hull form are very interesting for small working craft and pleasure boats. Intrinsic limitations as the low values of weight per inch of immersion and transversal and longitudinal instability, can be acceptable and successfully managed when the mission profile does not ask for significant load variation and shift. The exploitation of SWATH concept is limited by the craft size, but if main dimensions allow enough static stability, this configuration appears very promising. SWATH behaviour in rough sea at zero and low speed have led to consider this hull form within the small craft design research program in progress at University of Naples Federico II. The design of small size SWATH working/pleasure craft has to begin from the consideration of strut waterplane areas that are the key factor to get acceptable static and dynamic stability. Displacement has to be reduced as most as possible to increase static stability, as shown by last design trends. The results of CFD analysis concerning SWATH resistance and propulsion, aspects are presented. A numerical evaluation of the hull-propeller interactions is performed, through simulations of self-propulsion tests with a simplified method (Actuator Disk model) to discretize the propeller effect. The effective wake coefficient, the thrust deduction fraction and hull efficiency are provided. To validate CFD resistance results a comparison with experimental tests performed by Authors is reported. The presented work highlights different hydrodynamic aspects, comments advantages and critical issues of SWATH concept and reports detailed CFD modelling procedure with the aim to provide a reference for SWATH small craft design.