Stimulated by the energy efficiency indices introduced by the International Maritime Organization, the increased attention for the environment as well as economical pressures emphasize the relevance of accurate resistance estimation for ships. This leads to a particular interest for added resistance in waves, a resistance component that is currently hard to predict. One of the uncertainties lies within the phenomenon of bow wave breaking, which is expected to have a nonlinear, reducing impact on added resistance. A literature review reveals a lack of research done on this aspect. However, a better understanding of the phenomenon is key to improve the accuracy of added resistance estimates. If current methods are proved to overestimate added resistance, present estimations of required engine capacity would suffer the same bias. As a result, less powerful engines could then be installed which in turn would lead to more efficient ship designs.
Within this context, this thesis aims to gain new insights into added resistance by studying how it is affected by bow wave breaking. To reach this objective, experiments combining two fields, namely experimental hydrodynamics and optical measurements through stereo vision, are performed at the Delft University of Technology. The ship model no. 523 of the Delft Systematic Deadrise Series, a hard chine planing hull, is towed through calm water and regular head sea conditions at a constant speed. The speed and wave conditions are selected such that they gradually cause the bow wave to break. The ship model's speed is varied between a Froude number of F_r = 0.15 and F_r = 0.30. The incoming wave steepness is varied between a wave height to length ratio of H/λ = 1/60 and  H/λ = 1/30 for short, intermediate, and long wave conditions, corresponding to a wavelength to ship length ratio of λ/L_pp = 0.5,  λ/L_pp = 1.1 and λ/L_pp = 2.0, respectively.
During these experimental runs, the breaking of the bow wave is evaluated through visual observations, the added resistance is measured using resistance tests, and the relative wave elevation is assessed using a newly developed waterline detection method. Using the fact that the hull is semi-transparent, this latter method employs stereo imaging through cameras that are placed inside the ship hull. The waterline can then be traced using a Canny edge detection algorithm which is based on the intensity gradient at the free surface. By solving the stereo correspondence problem, the hull can be reconstructed and the detected waterline projected onto 3D coordinates.
The analysis of the experimental results shows that the linear approach, in which added resistance is proportional to incoming wave amplitude squared, does not hold. With increasing steepness, the added resistance coefficient decreases. Contrary to expectations, no clear correlation between the added resistance coefficient curve and the onset of bow wave breaking is revealed. However, the decrease in added resistance coefficient is strongest for conditions where the bow wave breaks most violently, which indicates an effect of bow wave breaking. The intensity of breaking could not be quantified by these experiments and therefore it is suggested to extend this study by applying CFD methods.
The relation between the added resistance and the incoming wave amplitude squared does not consider nonlinear phenomena (e.g. induced by green water and wave breaking). It would thus not take into account a possible decrease in relative wave elevation due to bow wave breaking and this is hypothesized to be the source of added resistance overestimation. This hypothesis is studied by reconstructing the relative wave elevation from optical measurements. Bow wave breaking seems to specifically affect the maximum relative free surface elevation with respect to the undisturbed waterline while its minimum elevation is characterized by the disappearance of the stationary bow wave for intermediate wave conditions, F_r ≥ 0.2. Considering the complexity of added resistance which is influenced by different factors, bow wave breaking effects are difficult to isolate. Nonetheless, the analysis of the experimental results led to the introduction of an alternative added resistance coefficient which nondimensionalizes the added resistance by the relative wave amplitude squared. This coefficient takes into account different factors affecting added resistance, e.g. the combined effect of ship speed, incoming waves, and bow wave breaking, and shows a constant trend compared with the common added resistance coefficient. These findings highlight the importance of an accurate relative wave height estimation.