Longitudinal cut method revisited: A survey on main error sources

Some of the main error sources in wave pattern resistance determination wer e investigated. The experimental data obtained at the Italian Ship Model Ba sin (longitudinal wave cuts concerned with the steady motion of the Series 60 model and a hard-chine catamaran) were analyzed. It was found that, with in the range of Froude numbers tested (0.225 less than or equal to Fr less than or equal to 0.345 for the Series 60 and 0.5 less than or equal to Fr l ess than or equal to 1 for the catamaran) two sources of uncertainty playa significant role: (i) the presence of a wave pattern generated by the air p ressure disturbance, related to the carriage motion, and (ii) the unsteadin ess of the free-surface flow (precision error). The importance of these eff ects increases, of course, with the model speed. The propagation of experim ental errors in the wave resistance determination by the longitudinal cut m ethod was next examined: within the elaboration of measured wave cuts exper imental uncertainties are shown to be significantly damped. The wave resist ance 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 th e longitudinal cut method, were estimated. Systematic tests were performed by means of a numerical approach, which allows one to compare the wave resi stance evaluated by the longitudinal cut method (applied in this case to th e 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 distanc e.