Reliability and Accuracy of Ship Powering Performance Extrapolation

The aim of this paper is to discuss factors that affect the reliability and accuracy of the ship powering prediction process from model experiments and to conclude with suggestions as to how the extrapolation process can be improved. Uncertainty exists in the experiments that are done to provide data for the ship powering performance prediction process and in the extrapolation method itself. Uncertainty analysis was used to address the sensitivity of the extrapolation process to uncertainty in the inputs and as a result to focus on those factors that contribute most to uncertainty in the results for a ship powering estimate. Questions are raised about the ship powering performance prediction process including: can experimental uncertainty be reduced; is it necessary to calculate wake fraction and relative rotative efficiency; how can uncertainty of the analysis method be minimised; how can frictional resistance of a ship model be most accurately subtracted from the total resistance; how sensitive is the powering estimate to uncertainty in the correlation allowance. Suggestions for improvements are made. The approach adopted is not prescriptive, but one of questioning standard practice in order to get practitioners to reconsider their analysis methods with a view to achieving improvements in accuracy.