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successful optimization procedure. Based on this, the following points are raised.

  • How far can the “automation” of an optimization code go, what is the responsibility of the code user and the designer?

  • The optimized hull forms, shown in Figs 7 and 9, suggest that the draft constraint has been implemented only at the ship's centerplane. The depicted “tunnel” bow sections cannot be justified both from the physical and the practical point of view.

  • The authors are optimizing the hull forms for the least total resistance. This is basically correct. However, it seems essential, before setting up the optimization procedure, to carefully consider the individual contributions of the viscous and wave components to the total resistance of the studied hull. In addition, with reference to propulsive aspects, the hull efficiency (wake and thrust deduction factors) of the stern should be taken into account, e.g., through specific geometric constraints. The rather U-shaped stern body suggests that possibly the studies will have not only increased viscous resistance, but possibly also inferior propulsive efficiency.

  1. Resistance Algorithms. The authors show great courage to present the rather disappointing results of Fig 12, where the measured resistance of the original hull is below the corresponding one of the optimized hull. The above figure would greatly gain in clarity if the authors would add the computational results for the total resistance and its components for the original and the optimized hull. In any case, the conclusion is that the used resistance algorithms are partly problematic and the authors discuss several aspects for the shown discrepancies in their paper. Considering the complicated flow around a ship hull form, weaknesses in the employed resistance algorithms are to a certain degree acceptable. However, for a hull form optimization procedure, it is possibly of less importance the accurate prediction of the absolute value of the resistance and its components. Rathermore, it is the prediction of the relative value of resistance between hull alternatives that is essential. In the studies case, it seems that the theoretical predictions for the resistance of the optimized hull ore more “optimistic” than those for the original hull. This is the point that should be clarified, e.g., by checking the predictions for the individual resistance components step by step.

The authors discuss carefully many aspects and possible shortcomings of their potential (wave resistance) and viscous flow solution. The discussors agree with the authors that the shortcomings of the employed κ-ε model and the neglect of the influence of the free surface on the viscous calculations are the main points to be addressed in future work.

AUTHORS' REPLY

Thank you for discussing several aspects on hull form optimization. As you point out, the intention of the present optimization was a local form optimization, but the changes allowed were probably too large to be considered as local variations.

The authors agree that it is necessary to include more aspects on hull performance in the optimization both in the objective function and as constraints. But an optimization system of this kind can only serve as one tool among others for the designer since many areas of hydrodynamic hull performance as well as practical considerations must be taken into account in the decision of the final design. Some of the considerations are probably difficult to include in an automatic optimization system.

Our experience is that the resistance algorithms work well for slender hulls like the Series 60. The problem in this case was that the physics of the flow around the stern was not well captured due to the neglected displacement effects in the wave prediction. The resistance algorithms may be more problematic for the full hull forms.

We agree that ranking is more important than absolute resistance values for numerical computations and optimization. But in cases where the total resistance is obtained as a summation of several resistance components also, the absolute value of the change of each component is important if the ranking is different for the components.



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