Stepped hull forms are hulls with a vertical step in the hull bottom to improve performance at top speed. Stepped hulls are well documented anecdotally and scientifically to improve calm water performance at high speeds, but commonly demonstrate dangerous and unexpected dynamic instabilities during initial trials. These hulls also operate practically in waves, but their performance characteristics in waves are not well understood and rarely evaluated prior to full scale trials. To expand the scientific understanding of stepped hull performance, a systematic set of experiments and Reynolds Averaged Navier Stokes (RANS) computational fluid dynamics (CFD) simulations are used to evaluate the calm water performance, dynamic stability, and regular head wave response of two stepped hull models. Calm water experiments on two stepped hull models at varying displacement, longitudinal center of gravity location and forward speed offer data which can be used in the design to interpolate hull performance throughout expected operating conditions. CFD simulations in calm water are validated using the experimental results and numerical modeling approaches for stepped hull simulations are recommended. The calm water dynamic stability of both stepped hulls is investigated experimentally and numerically and procedures to evaluate the dynamic stability using both approaches are recommended. The performance of both stepped hulls in regular head waves is studied through experiments, which are used to validate CFD simulations of the hull in regular waves. System identification is used on five calm water CFD simulations to identify a reduced order model for the prediction of stepped hull response in waves. / Doctor of Philosophy / Boats designed specifically for high-speed operations, like military patrol craft, can often achieve higher top speeds when small vertical steps are added to the bottom of the hull. When a step is introduced, the back portion of the hull bottom is raised above the forward portion of the hull bottom to allow air underneath the hull at high speeds. When designed properly, a stepped hull can be safely operated by experienced and unexperienced users at higher speeds than conventional hulls. However, when the steps are not designed well a boat can be dangerous to operate, suffering from unexpected phenomena like end swapping (where the boat violently rotates from bow to stern) when beginning a turn. Unfortunately, it is currently difficult to predict how a stepped hull will behave during the early stages of the design. Builders often rely on full-scale prototype trials to assess the performance and safety of their designs, which is an expensive process. In this work, experimental tests (on small scale models) and computer simulations were performed on two stepped hull models in calm water and in waves to establish techniques to predict performance and safety without a full-scale prototype. The experiments were used to compare the performance of two geometries and assess the accuracy of computer simulation predictions. The computer simulations were found to predict the stepped hull performance accurately enough to be used in design prior to prototyping. Procedures are recommended to evaluate the safety of a stepped hull through experiments and computer simulations. Evaluation of hull safety through computer simulations or experiments offers the opportunity to reduce danger to test personnel during full scale trials and minimize the need for expensive design modifications after construction of the first prototype. The performance of the two hull designs in waves were studied experimentally, through computer simulation, and through a simplified method informed by computer simulations. The development of a simplified method to evaluate stepped hull performance in waves offers the opportunity for designers to consider performance in waves earlier in the design for a lower cost than previously available.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/113915 |
| Date | 22 February 2023 |
| Creators | Husser, Nicholas Alexander |
| Contributors | Aerospace and Ocean Engineering, Brizzolara, Stefano, Woolsey, Craig A., Coutier-Delgosha, Olivier, Gilbert, Christine Marie, Morabito, Michael Gorts |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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