By Sabrina Matteucci
NURC SWATH-USV Concept design and hydrodynamic optimization of an innovative SWAT-Hull form by CFD methods. Brizzolara, S.; Curtin, T.; Bovio, M.; Vernengo, G. NURC-SP-2011-001. June 2011.
The report presents the main characteristics and functions of an innovative platform which has been conceived and designed to extend the operational capabilities of current Unmanned Surface Vehicles (USV) in terms of platform stability in waves and of powering requirement at relatively high speed. The main idea which governs the project is the realization of a small autonomous surface unit (6m in overall wetted length) capable of undertaking several tasks in the marine environment even in moderate rough sea conditions. The designed vessel has mainly the ability to locate, recover and launch other members of the autonomous fleet (like AUVs or other underwater devices) and at the same time could carry out a surveillance service of the surrounding areas. To manage with these tasks the vehicle is build to provide a fairly good autonomy which is needed to face with intermediate range missions (100 nautical miles). The choice of a SWATH, Small Waterplane Area Twin Hull form has been motivated by its excellent properties of seakeeping quality, which is combined with a non conventional low resistance underwater hull shape which is able to reduce to a minimum the resistance of the vessel especially at higher speeds. The excellent seakeeping performance are due to the small area of the waterline figure which determine a low reactivity to waves exciting forces and moments. The result is a incomparably stable platform when compared to equivalent conventional monohull or catamaran platform solutions. Moreover, the innovative geometry of the immersed part of the hull lowers the advance resistance thanks to a positive interference effect between the generated wave trains. To obtain the most efficient profile of the underwater bodies a systematic optimization with an automatic procedure have been arranged for the purpose. This is based on a parametric definition of the geometry, a CFD solver and a differential evolution global minimization algorithm. Many experimental data have been verified to test the ability of the software to predict resistance. Simulations have been made both in the air and in the water to compare softwares result with original one and to verify its goodness. As expected all the CFD computations have demonstrated the well superior efficiency of the developed unconventional SWATH technology with respect to current alternatives of hull typologies. The report, after presenting the main characteristics of the vessel and its hydrodynamic properties, describes more in details the concept design following the usual items of the design spiral used by naval architects. So hull form design, hydrostatic properties, main structural layout and dimensioning, weight estimation, intact stability, powering requirement and propulsion layout, auxiliary systems, i.e. active stabilization systems and AUV launching and recovering systems, in our case.