The influence of the bow shape of inland ships on the resistance

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Abstract

Due to financial and environmental considerations there is a need to reduce the fuel consumption in the inland shipping sector. This can be realised by improving the hull design. This research focusses on the bow shape of inland ships. The bow shape with the lowest resistance is not by definition the best bow shape, since other factors such as load capacity play a role as well. Therefore it is researched what the influence of the bow shape is on the resistance. The designer could use this knowledge for finding an adequate trade-off between the resistance and the load capacity. The main research question is: What is the influence of bow shape parameters on the resistance of an inland ship? The research is limited to the ship type Groot Rijnschip, bulbous bows are not included and a reference stern shape is selected that remains unchanged during the research. First is has been investigated which parameters describe the bow. Eight parameters are defined: the bow length, the ship volume, the waterline entrance angle, the rake, the waterplane coefficient, the slope of the sectional area curve at two points in the bow and the ratio between to waterplane areas in the bow. A set of 133 ships with different bow shapes is generated, in which these parameters are varied in a random way. Subsequently it has been determined which parameters are relevant for sailing in restricted water. The sailing speed and water depth are selected for this. The wave making resistance is expected to be the resistance component that is influenced most by the bow shape. Therefore the wave making resistance is calculated with the potential code RAPID for all 133 bows at eight combinations of Froude numbers and water depths. The values for the wave making resistance that are returned by RAPID are suitable for comparing the different bow shapes, but not for predicting the absolute value of the resistance. Since the calculations are carried out at low Froude numbers, the results are subjected to relative large numerical uncertainties. Linear regression models are fitted to the results to evaluate which bow shape parameters have the largest influence on the wave making resistance. These parameters turn out to be the bow length, the ship volume and the two points that describe the slope of the sectional area curve. The way in which the parameters influence the wave making resistance is described in the models, which answers the main research questions. However, there are several restrictions to the models: they are not able to predict the absolute value of the wave making resistance and the coefficients in the models are strongly influenced by outliers in the data obtained from RAPID. The original assumption that the viscous resistance does not change significantly has been checked with several viscous double body calculations in PARNASSOS. The results support the assumption. For practical applications, the ratio between the wave making resistance and the ship volume should be as low as possible. This results in bows shapes with a steep increase of volume at the front part of the bow and a smooth transition between the bow and the parallel midship. For further research it is recommended to obtain a better estimate of the absolute value of the resistance. In this way it can be quantified what the influence of the bow shape on the wave making resistance means for the total ship resistance. If this more reliable data is obtained, it can be combined with data from the stern shape. In this way a detailed, adequate resistance prediction method for inland ships can be generated.