Longitudinal cut method revisited: A survey on main error sources

Some of the main error sources in wave pattern resistance determination were investigated. The experimental data obtained at the Italian Ship Model Basin (longitudinal wave cuts concerned with the steady motion of the Series 60 model and a hard-chine catamaran) were analyzed. It was found that, within the range of Froude numbers tested (0.225 <= Fr <= 0.345 for the Series 60 and 0.5 <= Fr <= 1 for the catamaran) two sources of uncertainty play a significant role: (i) the presence of a wave pattern generated by the air pressure disturbance, related to the carriage motion, and (ii) the unsteadiness of the free-surface flow (precision error). The importance of these effects increases, of course, with the model speed. The propagation of experimental errors in the wave resistance determination by the longitudinal cut method was next examined: within the elaboration of measured wave cuts experimental uncertainties are shown to be significantly damped. The wave resistance coefficient can be obtained therefore, with reasonable accuracy, from the measurement uncertainty point of view. Moreover, the errors related to wave cut truncation, as well as to probe transverse location, typical of the longitudinal cut method, were estimated. Systematic tests were performed by means of a numerical approach, which allows one to compare the wave resistance evaluated by the longitudinal cut method (applied in this case to the computed wave pattern) with the value obtained by pressure integration on the hull. As a result, the longitudinal cut method can be applied without introducing any severe limitation for the ratio b/L (tank width over model length), provided the wave cuts are measured at a proper transverse distance.