View non-flash version
www.sname.org/sname/mt April 2013 to craft operating in the fully planing regime with volumet- ric Froude numbers above 3.0. Volumetric Froude number is dened by the following equation: Existing tools that are useful for prediction of hydro- dynamic performance (resistance, sinkage, and trim as a function of speed) run the gamut from empirical formu- lations, which have their basis in analysis of experimental data, to computational uid dynamics codes, which include all components in the governing equations. Strictly speak- ing, an empirical approach is only applicable for hull forms that fall within the parameters of the experimental database from which the approach was der ived. Because the empirical methodology was generated from analysis of actual data and the results from the approach are consistent with the origi- nal data set, it is logical that results for hulls that fall within the parameters of the original data set are physically valid. It is not unusual for empirical equations to be used for predictions for hull forms that fall just outside the equa- tionsĀ range of applicability; however, these results must be used with caution. On the other end of the spectrum, physics-based numerical codes that include all the terms in the equations that govern the non-linear behavior of a planing hull in a seaway should, in theory, cover all possible hydrodynamic conditions. However, the procedure used in any given numerical code to transform the governing equa- tions into mathematical problems for which solutions can be calculated requires verication and validation to ensure the computed results are accurate. is verication and validation involves comparison of computed results with experimental data. Assuming a favorable correlation of computed results and experimental data is obtained, the physical results from the numerical code can then be con- sidered accurate, but only for the range of parameters for which the computed results have been favorably compared with experimental data. Using existing tools What happens, then, when there are no existing, proven methodologies that are applicable for a new design? When a new piece of equipment, such as a propulsor, or a new hull concept, appears to show promise for improving high-speed performance in a seaway, what are the avenues available for evaluating that equipment or idea before incorporating it in a production line? Alternately, when a novel concept shows potential for improving the performance or behavior of an existing planing hull, in what ways can the eective- ness of that concept be evaluated before implementing it on an entire class of craft? In either case, the challenge becomes one of using the tools that do exist to derive some level of insight that can guide the development eort. ree of the tools that can be used for this purpose are physics-based numerical calculations, model testing, and prototyping. Physics-based numerical calculations that account for the non-linearities present when operating at volumetric Froude numbers above 3 represent a possible avenue for evaluating performance trends of novel, high-speed hull forms. For example, if a numerical calculation is known to provide reasonable performance results for simpler geome- tries in the desired performance regime, then assessments of output data trends for additional hull form concepts operat- ing in the same regime can be made. However, because there are no experimental data to assess the physical accuracy of the computed results, it is advisable to regard numerical results with extreme caution. Results are directly aected by the quality of the mathematical hull representation and gridding process that are a prerequisite for running the calcu- lations, and accurate hull representation and gridding often takes signicantly more time and eort than anticipated, especially if multiple geometries are to be explored. Model testing is another means of evaluating perfor- mance of both atypical hull forms and hull form details. At rst glance, it might appear that testing models will provide all the information necessary for hull form design and/or optimization. However, in order for model scale results to be useful, signicant time must be devoted to dening model test objectives and determining how to collect the desired information. In the case of small craft operating at volumet- ric Froude numbers above 3, particularly in waves, it can be very complicated to balance the scale ratio selection, phys- ical limitations of the towing tank facility and equipment, and physical limitations on instrumentation size while still achieving the desired experimental data. Consideration WHAT HAPPENS WHEN there are no existing, proven methodologies that are applicable for a new design? = volumetric Froude number = craft velocity, ft/sec = acceleration due to gravity, ft/sec 2 = volume of displacement, ft 3where: